Man cannot exist without soil.
Soil may be defined as a thin layer of earth crust that supports growth of land plants.
Soil is a living body full of activity rather than just an inert mixture of sand and clay.
Soils consist of minerals, organic materials and differ in morphology, physical make-up, chemical composition and biological characteristics.
Soil provides plants the physical support and meets their needs of water and nutrients.
Soils need to be scientifically managed forsustained crop production over long periods of time.
Soil properties decide the nature of conversion, conservation of nutrients, and how they are made available to the plants.
An ideal soil is made of mineral matter (45-50%), organic matter (1-5%), air (25%) and water (25%).
Major role of Organic matter
SOIL FERTILITY, PRODUCTIVITY AND HEALTH
Soil Fertility and Productivity
Management practices to improve soil fertility, productivity and health
Reduce pesticide use and provide habitat for beneficial organisms
Green manuring & Green leaf manuring
Green manuring is an efficient way of increasing organic matter in the soil. It is the practice of growing plants usually belonging to leguminous family and incorporating them into the soil after sufficient growth. An ideal green manure crop should be inexpensive to plant, easily to establish, produce succulent tops and roots rapidly, generate good ground cover quickly and capable of growing on poor soils. These crops should be tilled in to the ground at flowering stage. The most important green manure crops are Sunnhemp (Crotalaria juncea), Dhaincha (Sesbania aculeate), Clusterbeans (Cyamopsis tetragonoloba), Pillipesara (Phaseolus trilobus) and Sesbania rostrata (stem nodulating). Soil organic matter can also be increased by incorporating green leaves and twigs collected from trees, shrubs and herbs grown elsewhere. This is called green leaf manuring. Some important species suitable for the purpose are Neem (Azadirachta indica), Mahua (Madhuca longifolia), Gliricidia (Gliricidia sepium), Karanji (Pongamia glabra), Calotropis (Calotropis gigantea), Avise (Sesbania grandiflora), Subabul (Leucaena leucocephala). In addition to enhancing soil fertility, green manuring and green leaf manuring also improves soil structure, increases water holding capacity and reduces soil erosion. Growing green manure crops reduces weed proliferation and helps in reclamation of alkaline soils.
Method of preparation: (for 1 acre)
Water - 200 - 250 ltr.
Cow dung - 10 - 15 kg.
Cow urine - 3 - 4 ltr.
Jaggery - 1 - 2 kg.
Flour of pulses - 2 kg.
Soil - 2 - 3 handfuls
(Collect soil from the same field from under a tree or un-disturbed location).
Soils that cannot be economically used for crop production without adopting reclamation measures are called Problem Soils.
Acid soils have a pH of less than 6.5 and respond to liming. They are common in humid regions. Leaching of certain nutrients like Ca, Mg. etc., enhances development of soil acidity. Soils developed from acidic rocks like granite containing Silica are naturally acidic. Decomposition of organic matter in soil increases acidity due to production of a large number of organic and inorganic acids. Application of fertilizers like Ammonium Sulphate, Urea, Ammonium Nitrate also contribute to acidity. Increased concentrations of Aluminium, Manganese and Iron in acid soils adversely affect plant growth and yield. Plants suffer from Calcium, Magnesium and Potassium deficiency in acid soils. Rock Phosphate is an economical and better suited source of Phosphorus in acid soils. Muriate of Potash (MOP), which also tends to increase the pH is a better source of Potassium than Potassium Sulphate. A large number of beneficial microorganisms become inactive in acid soils thus slowing down mineralization and nitrification. It has toxic effect on root tissues and adversely affects the permeability. It affects the availability of Copper and Zinc and Phosphorus also gets immobilized. Plant diseases are more prevalent in acidic soils.
Reclamation of acid soils:
Acid soils can be reclaimed by adding metals like Calcium and Magnesium, a process called liming. The most common liming materials used are Lime stone [CaCO3], Quick lime [CaO] and Slaked lime [Ca(OH)2], Dolomite [CaMg(CO3)2].Quantity of liming material depends on the soil texture, with light soils requiring less than the heavy soils. Requirement of liming material also depends on the pH and it may range from about 2 MT/acre for soils of pH 5.5 to as high as 4.8 MT/acre for soils of pH 3.5. Lime should be applied based on soil test reports because excessive liming leads to Fe, Mn, Cu, Zn deficiency, decreased phosphate availability and Boron uptake. Lime should be applied at least one month before cropping and mixed with soil by ploughing. Apply lime in small quantities every year or once in two years. If the quantity is not more than 2 MT/acre, it can be applied in one dose.
The most common liming materials used are Lime stone [CaCO3], Quick lime [CaO] and Slaked lime [Ca(OH)2] and Dolomite [CaMg(CO3)2]. Dolomite is Calcium Magnesium bicarbonate [(CaMg(CO3)2]. When Magnesium is to be provided to a crop along with correcting the soil acidity, Dolomite is the right choice. Unlike Gypsum, which is added routinely in irrigated soils, Dolomite is to be used for correcting soil acidity. At higher soil pH values, the carbonate in limestone and dolomite will actually burn the crops, and even cause plant death in severe cases.
NOTE: Lime should never be applied with nitrogenous fertilizers and water soluble phosphatic fertilizers since it leads to loss of nitrogen in the form of Ammonia and decreases availability of Phosphorus.
Saline soils contain high percentages of chloride and sulphate of Calcium and Magnesium. The pH of the soils is less than 8.5 and Electric Conductivity is >4 decisiemens/m. The exchangeable Sodium is less than 15% of Base Exchange capacity of soils. The osmotic pressure due to high concentrations of soluble salts decreases water availability to plants, through exosmosis, resulting in stunted growth and retarded growth of leaves and stems. In extreme cases it leads to plasmolysis. Seed germination is also reduced. High concentrations of Sodium and Sulphate reduce absorption of Calcium leading to Ca deficiency. Saline soils are formed when salts that result from weathering are not fully leached especially in arid and semi-arid areas; due to upward movement of salts from lower depths during seasonal irrigation; due to irrigating with saline water or ingress of sea water.
Reclamation of Saline soils:
There are three ways of reclamation of saline soils – Eradication, Conversions and Control. Eradication involves repeated heavy flooding and draining of the soils to effectively leach out the soluble salts. The field should be made into smaller plots to hold irrigation water and flooded with good quality water up to 15-20 cm and puddled. The water is then removed through drainage channels. The process should be repeated 5-6 times. Leaching should be done before application of Gypsum. Scraping of surface salts also can be done as a method of eradication, but is not very efficient and is costly. Conversion involves using Gypsum to displace Sodium with Calcium from the exchangeable complex. Gypsum should be applied in powder form uniformly in the field and mixed by light ploughing of the upper 6 - 8 cm soil. Control method involves retardation of evaporation by mulching, deep chiseling of soil to open the impervious layer to improve water percolation and addition of organic matter.
Alakali soils are low in permeability of water and air, sticky when wet, hard on drying, and thus not congenial for plant growth. High percentage of Sodium decreases absorption of Calcium and Magnesium by the roots. Solubility of Zinc and Iron is affected. It may even cause lime induced Iron chlorosis. Plants show restricted root system and delayed flowering. Leaf burn is common due to excess of chloride and sodium. Alkali soils are of two kinds. Saline-alkali soils and Non-saline alkali soils.
These soils are both saline and alkaline and contain appreciable quantities of neutral soluble salts. While pH remains <8.5 and EC >4 decisiemens/m as in saline soils, unlike in saline soils the exchangeable Sodium is more than 15% of base exchange capacity of soils.
Non-saline alkali soils (Black alkali soils):
These soils do not contain large quantities of neutral soluble salts, but exchangeable Sodium is more than 15% of base exchange capacity of soils. Soil pH is >8.5 and EC <4 decisiemens/m. High Sodium content results in dispersal of clay and organic matter leading to close packing of soil particles and reduces pore space and aeration. Soil develops shallow cracks and surface soil becomes hard and compact. Water does not percolate quickly and remains muddy for a long time. Application of alkaline fertilizers like Sodium Nitrate, basic slag etc., also contribute to alkalinity.
Reclamation of Alkali soils:
Eradication: Leaching with excessive flooding and draining is done as in reclamation of saline soils after conversion of exchangeable Sodium into water soluble form.
Large quantities of Gypsum are applied to replace Sodium and leach it downward and out of reach of plant roots. Gypsum reacts with Sodium Carbonate and adsorbed Sodium to produce Sodium Sulphate, which dissolves in water and is leached away. Dosage of Gypsum ranges from 800 kg/ acre in soils of pH 7.4 to 7.8 to as high as 5.7 MT/acre for soils of pH above 9.0. Gypsum can be applied in one dose if the requirement is below 3 MT/acre; in two doses if it is between 3 and 5 MT and three doses if it exceeds 5 MT.
In extreme cases ground Sulphur is incorporated into the soil several weeks before planting of the crop. In case of soils of pH 8.0, up to 2.50 MT Sulphur and 5-7.5 MT of organic matter per acre, is required to reduce the pH to about 6.5.
Sulphuric acid may directly be used when alkalinity is very high. However, the method is expensive and hazardous.
Lime or Lime sulphur:
Calcium Carbonate replacing Na+ in soil colloid with Ca2+.
Ferrous Sulphate and soluble salts of Aluminium:
These bring down alkalinity by formation of Sulphuric acid resulting in production of leachable Calcium Sulphate.
Mollases (2 MT/acre) and press mud (1 MT/acre) are also effective in reclamation of alkali soils through production of organic acid.
Organic manures bring down the pH through production of carbonic acid in the soil, which solubilizes Calcium salts. Microbial activity in organic manure also stimulates oxidation of Sulphur to Sulphate form.
Mulching improves physical and microbial properties, prevents crust formation, disrupts capillary continuity, reduces water loss through evaporation and prevents accumulation of salts on soil surface.
Fertilizer management in Saline and Alkali soils
Fertilizers of acidic nature are advised for alkali soils and Ammoniacal fertlizers are useful in saline soils. Due to decreased availability and leaching of chemical fertilizers, generally 50% higher dose of Nitrogen and Phosphorus are recommended. Application of Zinc Sulphate and Gypsum is very essential in alkaline soils. Organic manures have been observed to be better than inorganic fertilizers in both saline and alkali soils. Dhaincha (Sesbania aculata) is an important source of green manure in reclamation of alkali soils. Green manuring with Dhaincha along with Gypsum is effective in restoring physical condition and enriching the soil in nitrogen and organic matter.
Gypsum - The versatile soil amendment
Gypsum is the most versatile soil amendment that has many uses especially in irrigated soils. Gypsum being Calcium Sulphate (CaSO4.2H2O) is a key ingredient for maintenance of irrigated soils that would gradually turn saline or alkaline. Calcium in Gypsum replaces Sodium in sodic soils and helps in leaching it as Sodium Sulphate. In addition to preventing and correcting sodicity, Gypsum affords stability of soil organic matter, stable soil aggregates, improved water penetration and helps rapid seed emergence. It helps in breaking up compact soil, especially in combination with organic amendments. Gypsum rapidly decreases the pH of sodic soils. Gypsum improves the ability of soil to drain and not become waterlogged due to a combination of high sodium, swelling clay, and excess water. Gypsum provides Calcium, which is a major mechanism that binds soil organic matter to clay giving stability to soil aggregates. In combination with lime, it decreases leaching losses of Potassium and Magnesium. Gypsum increases water-use efficiency by improving water infiltration rates, hydraulic conductivity and better water storage in the soil leading to deeper rooting and better wateruse efficiency. It provides Calcium that is essential for the biochemical mechanisms by which most plants nutrients are absorbed by roots. Calcium in Gypsum regulates the balance of micronutrients, such as Iron, Zinc, Manganese and Copper, in plants. Calcium prevents excess uptake of many of them; and once they are in the plant, Calcium keeps them from having adverse effects when their levels get high. Application of Gypsum is essential for good fruit quality, especially in very high pH soils where adequate Calcium is not available. Gypsum helps prevent blossom-end rot of Watermelon and Tomatoes and bitter pit in Apples. Gypsum is preferred over lime for management of scab in Potatoes grown in acid soils. Root rot of Avocado trees caused by Phytophthora is partially corrected by Gypsum and organics. Gypsum decreases volatilization loss of Ammonium Nitrogen from applications of Ammonia, Ammonium Nitrate, Urea, Ammonium Sulfate, or any of the Ammonium Phosphates.
Soil looses a considerable amount of its nutrients due to uptake by plants, through leaching and erosion. Continued maintenance of a high level of soil fertility is indispensable for sustained agricultural production. From time to time the inherent fertility of soil has to be evaluated. Soil test information along with knowledge of specific crop requirements, cropping history and physical characteristics of the soil help in determining the exact amounts of different nutrients and soil amendments, if any, needed for a certain crop or cropping sequence.
Soil sample should be properly collected and be representative of the area to be tested. Avoid drawing samples from old bunds, marshy spots, hedges, areas previously occupied by compost heaps, etc. Do not sample in a field within three months of the application of lime, ash or fertilizer. Where crops have been planted collect samples between plants and lines. To get representative samples, collect soil from at least 10 to 15 places randomly spread over the field. Scrape away the surface of the randomly selected points to remove weeds and litter. Dig pits of 6” x 6” or a ‘V’ shaped pit of 1 ft. deep or collect samples of 0- 6”and 6-12” by scraping the sides of the pit. Make a composite sample by mixing samples from all pits thoroughly after breaking all lumps. Carry out successive quartering along with thorough mixing each time to prepare a final sample weighing about 500 gm. Dry samples in shade and pack them in clean cloth bags. Label each sample to provide the following information to help the chemist to schedule accurate fertilizer / amendment recommendations. Farmer’s name and address. Cropping history. Crop intended to be grown. Details of standing crop, if any. Date of last ploughing. Quantity of lime, Gypsum and fertilizers applied and date of last application. Whether green manuring is practiced, if so when.
Soil samples are analyzed for the following five soil properties.
Wherever facilities exist samples are also analyzed for micro nutrients, especially for Zinc, Iron, Boran, Manganese and Copper.
Soil test result interpretation
Soil test report usually has three main parts apart from the farmer and soil sample details. The first part indicates the actual test values in respect of pH, EC, OC, N, P, K, S and wherever facilities are available it also includes Zn, Fe, B, Mn and Cu. The rating of the soil as Low, Medium and High is also indicated against each component. The second and third parts contain recommendations on application of macro, secondary and micronutrients along with organic manure, biofertilizers and soil amendment required, if any. Conventionally, when the soils are classified as ‘Medium’ in respect of the nutrient status, it is recommended to apply fertilizers as per the recommendations for the particular crop.
Food - The supreme medicine:
Just like human beings, plants also need balanced nutrition for higher performance. Balanced nutrition gives the plants general health and vigor. Vigorous plants are less prone to insect and disease attack. Plant protection begins with plant nutrition. Hence, Plant nutrition and Plant protection should go hand in hand. Man's health is dependent on Plant's health. Plant's health is dependent on Soil's health. Let us conserve soil and its health.
Root, Leaf & Chlorophyll:
All crops are primarily plants. Only Plants have the ability to produce food on their own and provide food to rest of us directly or indirectly. Root, leaf and chlorophyll play major role in food production.
Roots are not just leg of the plant, but also mouth of the plant. They are the supplier of raw material like water and minerals to the leaf. Hence, larger the root system (deeper & wider) – quicker the absorption and supply of raw materials to the leaves. Deeper the root system more the drought resistance to the plants. Healthier the root – higher the efficiency of the root activity. Anything that can impair the root growth can also impair its function. Physical factors like soil texture, temperature, moisture and physical damage can affect root growth. Chemical factors like soil pH can impair the mineral absorption of the minerals. Biological factors like soil-borne insects, nematodes, pathogenic bacteria and fungi can cause disease and hence impair the root’s functions. Hence, it is important to ensure that the roots are healthy and functional.
Leaf is the kitchen of the plant where food is prepared. More the number of leaves and larger their size, more kitchen space to cook food faster. Anything that affects the leaf development will affect their efficiency. Damages due to foliar diseases caused by viruses, bacteria, fungi and insects drastically reduce their efficacy. Physical factors like very low or high temperatures, lack of sufficient water and nutrition can lead to poor leaf development. So help the plant to have more and larger leaves for better productivity
Chlorophyll is the cook in the kitchen. Only this green pigment of the plant has the ability to use solar energy required for photosynthesis. Larger the number of chlorophyll content per unit area, greener the leaves. Greener the leaf, faster the food production per unit time. Many nutrients like Nitrogen, Magnesium, Iron and Zinc are very essential elements for chlorophyll production. Through balanced nutrition help the plant to grow big and greener for higher yields.
MANURES AND FERTILIZERS
Fertilizer is any material of natural or synthetic origin added to the soil to supply one or more plant nutrients. Fertilizers and Pesticides are the two majorsupplies for agriculture. Fertilizers and pesticides play the role of food and medicine for plants in conventional agriculture. Fertilizers are the most widely used materials. Based on the production process, they are broadly categorized into Organic manures, Bio-fertilizers and Chemical Fertilizers.
These are derived from animal, human and plant residues, which contain plant nutrients in complex organic forms. Manures have lower nutrient content per unit quantity, compared to fertilizers with high nutrient content. Manures have longer residual effect and improve soil physical, chemical and biological properties. Major sources of organic manures are:
Bulk organic manures
Bulk organic manures contain low concentrations of nutrients and are applied in large quantities. Farmyard manure (FYM), compost and green-manure are the most important and widely used bulk organic manures. Use of bulk organic manures has several advantages:
a. Farm yard manure (FYM):
Farmyard manure is the decomposed mixture of dung and urine of farm animals along with litter and left over material from roughages or fodder fed to the cattle. On an average well decomposed farmyard manure contains 0.5% N, 0.2% P2O5 and0.5% K2O.
Generally FYM is applied @ 4 to 8 MT/acre. The entire amount of nutrients present in farmyard manure is not available immediately. About 30% of Nitrogen, 60 to 70% of Phosphorus and 70% of Potassium are available to the first crop. Well rotten manure can be applied just before sowing. When the FYM is partially rotten it should be applied three to four weeks before sowing. The existing practice of leaving manure in small heaps scattered in the field for a very long period leads to loss of nutrients. These losses can be reduced by spreading the manure and incorporating by ploughing immediately after application.
b. Sheep and Goat Manure:
The droppings of sheep and goats contain higher nutrients than farmyard manure and compost. On an average, the manure contains 3% N, 1% P2O5 and 2% K2O. It is applied to the field in two ways:
c. Poultry Manure:
Poultry manure contains higher Nitrogen and Phosphorus compared to other bulk organic manures. The average nutrient content is 3.0% N, 2.6% P2O5 and 1.4% K2O. The excreta of birds ferment very quickly. If left exposed, 50% of its Nitrogen will be lost within 30 days.
Concentrated organic manures
Concentrated organic manures have higher nutrient content than bulk organic manures. The important concentrated organic manures are oilcakes, blood meal, fish manure, etc. These are also known as Organic Nitrogen Fertilizers. They are converted into readily usable Ammoniacal Nitrogen and Nitrate Nitrogen through bacterial action. Organic manures are therefore, relatively slow acting, but they supply available Nitrogen for a longer period.
a. Oil Cakes:
Solid portion left out after oil is extracted from oilseeds is dried as cake and used as manure. The oil cakes are of two types: Edible oil cakes, which can be safely fed to livestock; e.g., Groundnut cake, Coconut cake etc. Non edible oil cakes e.g., Castor cake, Neem cake, Mahua cake, etc. Both edible and non-edible oil cakes can be used as manures. Nutrients present in oil cakes, after mineralization, are available to crops 7 to 10 days after application. Oilcakes need to be well powdered before application for even distribution and quicker decomposition.
b. Animal based Concentrated Organic Manures:
Blood meal when dried and powdered can be used as manure. Meat dried and converted into meat meal is a good source of Nitrogen.
Role of Organic Matter in Soil Fertility
Organic matter forms a very small but important portion of soil. It is obtained from dead plant roots, crop residues, various organic manures like farmyard manure, compost and green manure, fungi, bacteria, worms and insects. Organic matter improves the physical condition of the soil, particularly the structure. Decaying organic matter acts as food for bacteria, fungi and other organisms. Presence of organic matter helps dissolve many soil minerals and renders them available to plants. It plays an important role in the nutrient supplying power of soil, as it has high cation exchange capacity (CEC). Increases the water holding capacity of the soil, particularly in sandy soils.Improves aeration and infiltration in heavy soils. Reduces loss of soil by water and wind erosion. Regulates soil temperature. Serves as an important source of certain plant of food elements (N, P, K, S, etc.). The buffering nature of the organic matter is advantageous in management of pesticide,herbicide and heavy metals residues.
Advantages of Organic Manures
Organic manures are better source of nutrient in balanced amounts than inorganic fertilizers, where soil is deficient in both macro and micronutrients. Organic based fertilizers supply micronutrients, increase soil moisture retention and reduce leaching of nutrients. In addition to increasing yield and fertilizing plants directly, organic manures improve the biodiversity (soil life) and long-term productivity of soil, and may prove a large depository for excess carbon dioxide. Organic matter increases the abundance of soil organisms by providing organic nutrients and micronutrients for organisms such as fungal mycorrhiza.
Disadvantages of Organic Manures
Bio-fertilizers are microbial inoculants capable of fixing and mobilizing important nutritional elements in the soil through biological processes. They include micro-organisms like bacteria, fungi and algae capable of fixing atmospheric Nitrogen or convert insoluble Phosphate and Potash in the soil into forms available to the plants. Soil microorganisms play an important role in soil processes that determine plant productivity. Some support plant growth indirectly, by reducing growth restricting conditions either through production of antagonistic substances or by inducing resistance against plant pathogens. The organic compounds, released by roots and bacteria, play an important role in the uptake of mineral nutrient. The hormones produced by the rhizosphere bacteria have direct beneficial effects on higher plants.
Advantages of Bio-fertilizers
Bio-fertilizers have definite advantage over chemical fertilizers. The bio-fertilizers effectively enrich the soil and cost less than chemical fertilizers that harm the environment and deplete nonrenewable energy sources. Bio-fertilizers provide certain growth promoting substances like hormones, vitamins, amino acids, etc., in addition to respective nutrient. Bio-fertilizers supply the respective nutrient continuously throughout the entire period of crop growth in the field under favorable conditions. Continuous use of chemical fertilizers adversely affects the soil structure whereas bio-fertilizers improve soil structure. Unlike chemical fertilizers that are toxic at higher doses, bio-fertilizers have no toxic effects.
Types of Bio-fertilizers
Nitrogen fixing bio-fertilizers:
Phosphorus solubilizing bio-fertilizers (PSB):
Potassium mobilizing bio-fertilizer:
Sulphur solubilizing bio-fertilizer:
Iron solubilizing bio-fertilizer:
Silicon solubilizing bio-fertilizers:
Vascular Arbuscular Mycorrhiza (VAM):
The bio-fertilizers are also available as consortia that are blend of Nitrogen fixers (Azotobacter and Azospirillium), Phosphate solubilizers and Potash mobilizers.
How do bio-fertilizers work?
Bio-fertilizers fix atmospheric Nitrogen in the soil and root nodules of legume crops and make it available to the plants. They solubilize the insoluble forms of Phosphates like Tricalcium, Iron and Aluminium Phosphates into available forms. They scavenge Phosphate from soil layers. They produce hormones and metabolites, which promote root growth. They decompose organic matter and help in mineralization. When applied to seed or soil, bio-fertilizers increase the availability of nutrients and improve the yield by 10 to 25% without adversely affecting the soil and environment.
CHEMICAL FERTILIZERS (Synthetic Fertilizers
Chemical fertilizers play an important role in increasing crop production. The main macronutrients present in inorganic fertilizers are Nitrogen, Phosphorus, and Potassium which influence vegetative and reproductive phases of plant growth. Fertilizers are available in both solid and liquid forms. Solid fertilizers are available as Powders, Crystals, Prills, Granules, Supergranules and Briquettes. Liquid fertilizers are applied directly or with irrigation water.
These supply only one primary plant nutrient, namely Nitrogen or Phosphorus or Potassium. e.g. Urea, Ammonium Sulphate, Potassium Chloride and Potassium Sulphate.
These are available generally in granular form, contain two or three primary plant nutrients of which two primary nutrients are in chemical combination. e.g. DiAmmonium Phosphate, NitroPhosphates and Ammonium Phosphate.
These are mix of straight fertilizers that contain two or three primary plant nutrients.
1. Nitrogenous fertilizers
Nitrogenous fertilizers are most widely used among fertilizers since deficiency of Nitrogen in the soil is the foremost and crops respond to Nitrogen better than to other nutrient. They account for over 80% of fertilizers used, particularly Urea. It is extremely efficient in increasing the productivity of crops and the possibilities of its economic production are unlimited.
1.1. Ammoniacal fertilizers:
Ammoniacal fertilizers contain Nitrogen in the form of Ammonium or Ammonia. These are readily soluble in water and therefore readily available to crops. Except rice, all crops absorb Nitrogen in Nitrate form. These fertilizers are resistant to leaching loss, as the Ammonium ions get readily adsorbed on the colloidal complex of the soil.
a) Ammonium Sulphate [(NH4)2SO4]
A white salt completely soluble in water, which is advantageously used in paddy and jute cultivation. Contains about 20% Nitrogen and 24.0% Sulphur. Can be applied before or at the time of sowing or as top-dressing in growing crop. Once most widely used but presently less commonly used. It is especially valuable where both N and Sulfur (S) are required. It's high solubility provides versatility for a number of agricultural applications. Although there are other fertilizers that contain higher percentages of N, it provides an excellent source of S, which has numerous essential functions in plants, including protein synthesis.
Ammonium Sulphate is used primarily where there is a need for supplementing N and S. Because the N fraction is present in the Ammonium form, it is frequently used in flooded soils for rice production, where Nitrate-based fertilizers are a poor choice because of leaching losses. A solution containing dissolved Ammonium Sulphate is often added to post-emergence herbicide sprays to improve their efficacy, particularly when the water contains significant concentrations of Calcium, Magnesium or Sodium.High-purity grade of Ammonium Sulphate is often used for this purpose to avoid plugging spray nozzles.
b) Ammonium Chloride (NH4Cl):
It is a white salt containing 26.0% Nitrogen and 66% Chlorine. Ammoniacal Nitrogen is not lost by leaching, since NH4 is fixed by soil clay and humus complex. Ammonium Chloride fertilizer application is recommended for saline and alkaline soils. Chlorine present in Ammonium Chloride gives resistance to some pests and diseases. Ammonium Chloride is less hygroscopic than other fertilizers and thus has a fairly long shelf life. The chloride radical in Ammonium Chloride frees Phosphorous, Potash, Magnesium, Silica, Iron, etc., from the soil. It can be applied both as basal and top dressing. It is usually not recommended for crops like Tomato, Potato, Grapes, Tobacco, etc., which may be injured by Chlorine.
c) Anhydrous Ammonia (NH3):
It is a colorless and pungent gas containing 82.0% Nitrogen. It is inexpensive and can be applied directly to soil by injection using blade type applicator having tubes. It becomes liquid (Anhydrous Ammonia) under suitable conditions of temperature and pressure.
1.2. Nitrate Fertilizers:
They contain the Nitrogen in the form of NO3. These ions are easily lost by leaching because of the greater mobility of Nitrate ions in the soil. Continuous use of these fertilizers may reduce the soil acidity as these Nitrogenous fertilizers are basic in their residual effect.
a) Sodium Nitrate (Chile Salt Peter or Chilean Nitrate) [NaNO3]:
Sodium Nitrate is a white salt containing about 15.6% Nitrogen. It is soluble in water and available for the plants, without any chemical change in the soil. It is easily lost by leaching and denitrification. When large quantities of Sodium Nitrate are added year after year, the Nitrate ions are absorbed by crops and Sodium ions accumulate and affect the structure of the soil. Sodium Nitrate is particularly useful in acidic soils.
b) Calcium Nitrate [Ca(NO3)2]:
It is a white crystalline hygroscopic solid containing 15.5% Nitrogen and 19.5% Calcium. Calcium is useful for maintaining desirable soil pH. It's high solubility makes it an immediately available source of Nitrate and Calcium directly to soil through irrigation water, or as foliar applications. Nitrate moves freely with soil moisture and is immediately taken up by plant roots. Unlike many other common N fertilizers, Ca(NO3)2 does not acidify soils.
Calcium Nitrate is popular in agronomic situations where a readily soluble source of Nitrate or Calcium is needed. Broadcast application of Ca(NO3)2 is desirable because the risk of Ammonia volatilization is eliminated with its use. Ca(NO3)2 is also used to provide supplemental Ca for plant nutrition. Since Ca is not mobile in the plant it is important to apply Ca at critical growth stages. Solutions of Ca(NO3)2 are commonly added to irrigation water, foliar and fruit sprays to overcome such shortcomings that affect yield and/or quality (blossom end rots in vegetables), or to meet peak Ca demands during critical growth periods. Ca(NO3)2 is Chloride-free and can have an ameliorating effect under saline growing conditions, combating the negative effects of Na and Cl. Calcium is an important nutrition that gives rigidity to cell wall and offers resistance to many abiotic and biotic stresses. Ca(NO3)2 is widely used in intensive cropping systems that have a high focus on crop quality.
c) Potassium Nitrate (Salt Peter) [KNO3]:
Potassium Nitrate (KNO3) is a soluble source of two major essential plant nutrients. The purified salt contains 13.0% Nitrogen and 45% K2O or 36.4% Potassium. N is immediately available for plant uptake as Nitrate, requiring no additional microbial action and transformation in the soil. The Nitrogen of the Potassium Nitrate has the same properties and value as that of the Sodium Nitrate. Potassium Nitrate is commonly sold as water-soluble, crystalline material primarily intended for application with water or in a prilled form for soil application.
Growers of high value vegetable and orchard crops prefer to use a Nitrate-based source of nutrition in an effort to boost yield and quality. It is commonly used for high-value crops that benefit from Nitrate (NO3-) nutrition and a source of Potassium (K+) free of Chloride (Cl- ). Many crops have high K demands and can remove as much or more K than N at harvest.KNO3 is applied to the soil before the growing season or as a supplement during the growing season. Foliar application of K during fruit development can be advantageous, since it often coincides with high K demand, declining root activity and nutrient uptake. It is also commonly used for greenhouse plant production and hydroponic culture.
1.3. Ammoniacal and Nitrate Fertilizers:
These fertilizers contain Nitrogen in both Ammonium and Nitrate forms. Nitrates are available for rapid utilization by crops and the Ammonical form for gradual utilization.
a) Ammonium Nitrate (NH4NO3):
Ammonium Nitrate was the first solid Nitrogen (N) fertilizer produced in large scale. It is white, water soluble and hygroscopic crystalline salt containing 35% Nitrogen, one half in Nitrate and the other half in Ammonium form. The Nitrate form is mobile in the soil water and is immediately available for plant uptake. The Ammonium fraction is taken up if roots grow nearby or after it is converted to Nitrate by soil microorganisms during nitrification. In the Ammonium form, it cannot be easily leached from the soil. Since plant roots do not directly absorb the urea form of N to a large extent, Ammonium Nitrate is an efficient and immediate source of plant nutrition. This fertilizer is quick-acting, but highly hygroscopic and not fit for storage. It has an acidulating effect on the soil. It is dangerous in pure form because of explosion hazard.
b) Calcium Ammonium Nitrate (CAN) (5Ca(NO3)2 NH4NO3.10H2O:
Calcium Ammonium Nitrate is a fine free-flowing, light brown or grey granular fertilizer, containing 25% Nitrogen and 18.5% Calcium. It is almost neutral and can be safely applied even to acid soils. Half of its total Nitrogen is in the Ammoniacal form and half is in Nitrate form.
c) Ammonium Sulphate Nitrate [(NH4)2SO4 NH4NO3]:
It contains 26% Nitrogen, three fourths of which is in the Ammoniacal form and the rest (6.5%) as Nitrate. In addition to Nitrogen it contains 12.1% Sulphur.It is available in a white crystalline form or as dirty-white granules. It is readily soluble in water and quickacting. It's keeping quality is good and it is useful for all crops. It's acid effect on the soils is only one-half of that of Ammonium Sulphate. It can be applied before sowing, at sowing time or as top-dressing.
1.4. Amide Fertilizers:
Amide fertilizers are readily soluble in water and easily decomposable in the soil. The amide form of Nitrogen is easily changed to Ammoniacal and then to Nitrate form in the soil.
a) Urea [CO(NH2)2]:
It is the most widely used solid N fertilizer in the world. It is a white crystalline substance readily soluble in water. It is the most concentrated solid Nitrogenous fertilizer, containing 46% Nitrogen. The high N content of urea makes it efficient to transport. The Nitrogen in urea is readily fixed in the soil in Ammoniacal form and is not lost in drainage. It may be applied at sowing or as topdressing. It absorbs moisture from the atmosphere and has to be kept in moisture proof containers. Urea is also commonly found in naturesince it is expelled in the urine of animals.
It is mostly mixed with soil or applied on the soil surface. It may be dissolved in water and applied to soil as a fluid, along with irrigation water, or sprayed onto plant foliage. Urea in foliar sprays is quickly absorbed by plant leaves. When urea contacts soil or plants, a naturally occurring enzyme (urease) begins to quickly convert urea back to NH3 in a process called hydrolysis. During this process, the N in urea is susceptible to undesirable gaseous losses as NH3. Plants can utilize small amounts of urea directly as a source of N, but they more commonly use the Ammonium and Nitrate that are produced after urea is transformed by urease and soil microorganisms. In case of flooded fields or calcareous soils, slow release Nitrogenous fertilizers like Sulphur coated urea, urea super granules, neem coated or neem blended urea are to be used, so that loss of Nitrogen can be minimized. Urea has good storage properties and causes minimal corrosion of application equipment. Avoid urea when the fertilizer will remain on the soil surface for prolonged periods of time.
b ) Urea Ammonium Nitrate (UAN) (CH8N4O4)
Among fluid N fertilizers, a solution of urea [CO(NH2)2] and ammonium nitrate [NH4NO3] containing between 28 and 32% N is the most popular. While 25% of total N in the form of NO3-is immediately available for plant uptake, 25% in NH4+ form is rapidly oxidized by soil bacteria to form NO3-. The remaining urea portion (50% of the total N) is hydrolyzed by soil enzymes to form NH4+, which is subsequently transformed to NO3- in most soil conditions. However, additional management may be required to avoid volatile losses. UAN is compatible with many other nutrients and agricultural chemicals, and is frequently mixed with solutions containing P, K, and other essential plant nutrients. UAN solutions are commonly injected into soil beneath the surface, sprayed onto the soil surface, added to irrigation water, or sprayed onto plant leaves as a source of foliar nutrition.
c) Calcium Cyanamide (CaCN2):
Calcium cyanamide or nitrolime contains 20.6% Nitrogen. It is a greyish white powdery material that decomposes in moist soil giving rise to Ammonia.
All forms of Urea have the disadvantage of undergoing considerable losses as Ammonia gas, if not incorporated into soil soon after application. The loss of Ammonia, termed volatilization, can be as high as 50%. Factors like surface application, inadequate irrigation following application, high temperatures, high soil pH and low cation exchange capacity of soil further increase loss of Ammonia. Urease inhibitors are compounds that when added to Urea slow the rate of Ammonia production, thereby reduce Ammonia loss to the atmosphere. These compounds that inhibit urease are chemically stable and are mixed with or coated onto urea-containing fertilizers. e.g. N-(n-Butyl) thiophosphoric triamide (NBTPT), Phosphoric triamide (NBPTO), Phenylphosphorodiamidate (PPD) and hydroquinone.
These are compounds that reduce the rate at which Ammonium is converted to nitrate, thus help reduce N losses through denitrification and leaching. They help in preserving N as Ammonium and delay nitrate production by depressing the activity of Nitrosomonas bacteria. The most commonly used nitrification inhibitors are 2-chloro-6-(trichloromethyl)-pyridine (Nitrapyrin), dicycandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP). These can be injected directly into the soil with anhydrous Ammonia (Nitrapyrin) or coated onto solid N fertilizers or mixed with manures. Their inhibitory effect will last from 25 to over 70 days. In general, nitrification inhibitors are more effective in sandy soils, or soil low in organic matter and exposed to low temperatures.
Coating fertilizers to ensure sustained release of nutrients is an effective but less expensive method when compared to the use of nitrification inhibitors. A variety of coatings have been applied to fertilizer particles to control their solubility in soil and rate of nutrient release offering multiple environmental, economic, and yield benefits. Elemental sulphur (S) was the first widely used fertilizer coating, where molten S was sprayed over urea granules. Later it was covered with thin layer of organic polymer. Presently, various resin-based polymers are used to coat fertilizer granules. Coatings are most commonly applied to granular or prilled nitrogen (N) fertilizer, but multi-nutrient fertilizers receive the coatings as well. Since urea contains the highest N of common soluble fertilizers, it is the base material for most coated fertilizers. It has been established that Neem oil serves as an effective inhibitor if coated on Urea, ensuring gradual release of urea and reduced loss due to leaching. Depending on the composition and thickness of the coating the duration of nutrient release can vary from several weeks to many months. Although coated fertilizers cost more than the uncoated materials, they significantly decrease loss of nutrients through leaching and gaseous losses, mitigate fertilizer burn, save on cost of fertilizer application by eliminating the need for multiple fertilizer applications and ensure uniform plant nutrition, better growth and improved plant performance.
2. Phosphatic fertilizers
Phosphatic fertilizers are chemical substances that contain the nutrient Phosphorus in absorbable form (Phosphate anions) or that yield after conversion in the soil. They should be basal placed in the root zone, because their poor mobility restricts them to the place of application.
a) Single Super Phosphate [Ca(H2PO4)2]
Single Super Phosphate (SSP) was the first commercial mineral fertilizer that led to the development of the modern plant nutrient industry. This material was once the most commonly used fertilizer, but other Phosphorus (P) fertilizers have largely replaced SSP because of its relatively low P content. It contains 16% P2O5 in available form(Phosphorus- 7 to 9%), 18 to 21% Calcium and 11 to 12% Sulphur and essential micronutrients in small proportions. In agronomic studies where SSP is demonstrated to be superior to other P fertilizers, it is usually due to the S and Ca that it contains. It is a grey ash like powder with good keeping or storage qualities. SSP helps to treat Sulphur deficiency in soils (40% of Indian soils are Sulphur deficient).
SSP is an excellent source of three plant nutrients. The presence of both P and Sulphur (S) in SSP can be an agronomic advantage where both of these nutrients are deficient. Continuous application of SSP reclaims Alkali soils. It improves root formation and ensures quality product output. When locally available, SSP has found widespread use for fertilizing pastures where both P and S are needed. Phosphatic fertilizers hardly move in the soil and hence they are to be placed in the root zone.
b) Triple Super Phosphate (TSP):
The concentrated super Phosphate is called as Triple Super Phosphate and it contains 46% P2O5. Technically, it is Calcium dihydrogen Phosphate or Monocalcium Phosphate, [Ca(H2PO4)2.H2O]. This fertilizer is suitable for all crops across all soils. In acid soils, it should be used in conjunction with organic manure. It can be applied before or at sowing or transplanting. It is an excellent source of P, but its use has declined as other P fertilizers have become more popular.
It has the highest P content for a dry fertilizer that does not contain N. Over 90% of the total P in TSP is water soluble, so it becomes rapidly available for plant uptake. As soil moisture dissolves the granule, the concentrated soil solution becomes acidic. TSP also contains 15% Calcium, providing an additional plant nutrient. A major use of TSP is in situations where several solid fertilizers are blended together for broadcasting on the soil surface or for application in a concentrated band beneath the surface. It is also desirable for fertilization of leguminous crops, such as Alfalfa or Beans, where no additional N fertilization is needed to supplement biological N fixation.
c) Rock Phosphate (RP):
Rock Phosphate is a naturally occurring mineral, inexpensive and best suited for acidic soils. Unlike water-soluble P fertilizers that quickly dissolve, RP dissolves slowly to gradually release nutrients, but the rate of dissolution may be too slow to support healthy plant growth in some soils. It is necessary to apply RP well in advance of the plant demand owing to its slow dissolution. Low Calcium and high organic matter in the soil tend to speed RP dissolution. It is usually not recommended when the soil pH exceeds 5.5. It should be broadcasted and incorporated with tillage to speed up reaction with the soil.RP also contains 5% Calcium carbonate. Because of its natural occurrence, it is recommended even in organic crop cultivation.
d) Phosphoric acid:
Application of Phosphorus to irrigation water may cause precipitation of Phosphate salts. Phosphoric acid and Mono Ammonium Phosphate are more suitable for fertigation. Phosphoric acid is inexpensive, provides highest amount of Phosphorus (52%) and cleans up the drip system.
e) Mono-Amonium Phosphate (MAP)[(NH4)(H2PO4)] and (f) Di-Ammonium Phosphate (DAP) [(NH4)2HPO4]
These are two widely used phosphate fertilizers. The characteristics and agricultural use of these important sources of phosphorous are discussed under Complex fertilizers.
3. Potassic fertilizers
Potassic fertilizers are chemical substances containing Potassium in absorbed form (K+). There are two Potassium fertilizers viz., Muriate of potash (KCl) and Sulphate of potash (K2SO4). They are water soluble and so are readily available to plants. The potassic fertilizers should be basal placed in the root zone, because their poor mobility restricts them to the place of application.
a) Potassium Chloride (KCl) or Muriate of Potash (MOP):
Potassium Chloride is a white or red, crystal containing 60.0% K2O. It is completely soluble in water and therefore readily available to the crops. The K+ will be retained on the negatively charged cation exchange sites of clay and organic matter. The Cl- portion will readily move with the water. It is not lost from the soil, as it is adsorbed on the colloidal surfaces. The Chlorine content is about 47.0%.
Potassium Chloride is the most widely used K fertilizer due to its relatively low cost and because it includes more K (50 to 52%) than most other sources (60 to 63% K2O and 45 to 47% Cl). Its Chlorine content (47%) is objectionable to some crops like tobacco, potato, etc., where quality is the consideration. It can be applied at sowing or before or after sowing. Potassium Chloride is often spread onto the soil surface prior to tillage and planting. It may also be applied in a concentrated band near the seed. Since dissolving fertilizer will increase the soluble salt concentration, KCl is placed to the side of the seed to avoid damaging the germinating plant. Pure grade of KCl can be dissolved for fluid fertilizers or applied through irrigation systems.
b) Potassium Sulphate (K2SO4):
This “potash” is commonly referred to as Sulphate of Potash or SOP. It is a white salt and contains 50% K2O and 17 to 18% Sulphur. It is soluble in water and therefore readily available to the crop. It does not produce any acidity or alkalinity in the soil. It is preferred for fertilization of crops like Tobacco, Potato etc., where quality is of prime importance. It is expensive because it is made by treating Potassium Chloride with Magnesium Sulphate.
SOP supplies a valuable source of S, required for protein synthesis and enzyme function. In crops where addition of Cl is to be avoided, K2SO4 makes a very suitable K source. Potassium Sulphate is only one-third as soluble as KCl, so it is not as commonly added through irrigation water unless there is a need for additional S. Fine particles (<0.015 mm) are used for making solutions for irrigation or foliar sprays. Foliar spray of K2SO4 is a convenient way to apply additional K and S to plants, supplementing the nutrients taken up from the soil. Leaf damage can occur if the concentration is too high.
4. Complex fertilizers
Complex or Compound fertilizers have two or more primary essential plant nutrients combined into each particle. This allows each granule to deliver a mixture of nutrients as it dissolves in the soil and avoids segregation of nutrient sources during transport or application. These offer advantages like convenience of application in the field and economic savings in meeting the crop nutritional needs. The transportation and application cost is relatively less when compared to straight fertilizers. When micronutrients are included in compound fertilizers, uniform distribution throughout the root zone can be achieved. Although several grades of complex fertilizers are available, it could be required to supplement them with straight fertilizers, when the proportion of nutrients does not meet the requirements of a particular crop. These fertilizers are recommended to be applied before planting or early in the growth period and later nitrogenous fertilizers are to be applied as required. Application of complex fertilizers works out more expensive as compared to straight fertilizers. Since they may not supply the major nutrients in a crop specific ratio, Hence, it is always better to use straight fertilizers.
These are combinations of Ammonium Phosphate, Monoammonium phosphate and Diammonium phosphate with or without potassium salt. Nitrophosphates contain Nitrogen and Phosphorus in several proportions. Nitrophosphate fertilizers vary depending on the combinations of nutrients used to make the final granule. Popular grades are:
b) Ammonium Phosphate:
The widely used ammonium phosphate fertilizers are:
i. Mono-Ammonium Phosphate (MAP)[(NH4)(H2PO4)]:
MAP contains 12% Nitrogen and 61% P2O5. It is a widely used source of N and P. It is water soluble and dissolves rapidly in soil if adequate moisture is present. The pH of the solution surrounding the granule is moder¬ately acidic, making MAP an especially desirable fertilizer in neutral and high pH soils. It should be applied in concentrated bands beneath the soil surface close to growing roots. It is also commonly applied by spreading across the field and mixing into the surface soil with tillage.
ii. Di-Ammonium Phosphate [DAP][(NH4)2HPO4]:
DAP is the world’s most widely used Phosphorus (P) fertilizer. The standard grade of DAP is 18-46-0, containing 18% N, 46% P2O5. The black colour and uniform sized granules contain micro nutrients such as Iron and important plant nutrients such as Sulphur, Calcium, Magnesium, Nitrogen and Phosphate. It has excellent handling and storage properties. It is recommended for basal application of fertilizers due to its slow releasing nature.
Note: When applying MAP as a foliar spray or through irrigation water, do not mix with Calcium or Magnesium fertilizers.
DAP is an excellent source of Phosphorus (P) and Nitrogen (N) for plant nutrition. It is highly soluble and thus dissolves quickly in soil to release plant-available Phosphate and Ammonium. A notable property of DAP is the alkaline pH that develops around the dissolving granule. As ammonium is released from dissolving DAP granules, volatile Ammonia can be harmful to seedlings and plant roots in immediate proximity. This potential damage is more common when the soil pH is greater than 7, a condition that commonly exists around the dissolving DAP granule. To prevent the possibility of seedling damage, care should be taken to avoid placing high concentrations of DAP near germinating seeds. The Ammonium present in DAP is an excellent N source and is gradually converted to nitrate by soil bacteria, resulting in a subsequent drop in pH. Therefore, the rise in soil pH surrounding DAP granules is a temporary effect. This initial rise in soil pH neighboring DAP can influence the micro-site reactions of Phosphate and soil organic matter.
iii. Ammonium Phosphate:
Contains 16% N and 20% P2O5.
iv. Urea Ammonium Phosphate (UAP):
UAP is available in grades like 28-28-0, 24-24-0 and 20-20-0.
a. Is an excellent liquid fertilizer that is widely used in agriculture.
b. The most common ammonium polyphosphate fertilizers have N-P2O5-K2O composition of 10-34-0 or 11-37-0.
vi. Ammonium Phosphate Sulphate[20:20:0:13]:
a. This special product contains 20% N,20% P2O5, and 13% S.
b. It is highly soluble in water and has greater mobility in soil.
c. It can be applied both as initial application and top dressing for supply of N, P2O5, and Sulphur nutrients.
c) N.P.K. Complex fertilizers
These are complex fertilizers containing N, P2O5 and K2O in various proportions.
SECONDARY MAJOR NUTRIENT FERTILIZERS
1. Magnesium fertilizers
These are chemical substances containing Magnesium in the form of Magnesium cations (Mg2+).
a) Magnesium Sulphate monohydrate (MgSO4.H2O):
Magnesium Sulphate is a naturally occurring mineral that contains 9.6% Mg and 13% S. It is mined from geologic marine deposits and provides a soluble source of both Mg and S for plant nutrition.
Magnesium Sulphate provides a highly concentrated form of two essential plant nutrients—Mg and S. Since Magnesium Sulphate applications have no major effect on soil pH, it can be used in all kinds of soils, irrespective of soil pH. It is commonly used prior to or during the growing season to meet the nutrient requirement of crops. Due to its high solubility it can be used to supply both Mg and S during peak periods of crop demand. Since Magnesium Sulphate is an earth mineral mined from naturally occurring deposits, it is permitted as an organic nutrient source by some organic certifying agencies. Magnesium Sulphate is totally soluble and suitable for both fertigation and foliar application.
2. Calcium fertilizers
These are chemical substances containing the Calcium as absorbable Calcium cations (Ca2+). The raw material of Calcium fertilizers is lime found in nature.
a. Calcium Carbonate (CaCO3 ):
Calcium carbonate, the chief component of limestone, is a widely used amendment to neutralize soil acidity and to supply Calcium (Ca) for plant nutrition.
The term “lime” can refer to several products, but for agricultural use it generally refers to ground limestone.
Limestone/Calcite – Calcium carbonate (CaCO3) - mostly insoluble in water, but solubility increases in acid conditions and it contains a maximum of 40% Ca.
Dolomite – Calcium Magnesium carbonate [CaMg(CO3)2] - mostly insoluble in water, but solubility increases in acid conditions and it contains between 2 to 13% Mg.
Hydrated/Slaked lime – Calcium hydroxide [Ca(OH)2] - relatively insoluble in water; forms a solution of pH >12.
Burned lime/Quick lime – Calcium oxide (CaO) - reacts with water to form hydrated lime.
It is primarily used to raise the pH of acid soils and reduce the concentration of Aluminium (Al) in soil solution. Poor crop growth in acid soils is largely due to soluble Al, which is toxic to the root system of many plants. Lime will reduce soluble Al either by the reaction CaCO3 + H2O I Ca2+ + 2OH- + CO2 or Al3 + [soluble] + 3OH- I Al(OH)3 [insoluble]. Addition of Calcium Carbonate also supplies valuable Ca (and possibly Mg) for plant nutrition. Some secondary benefits of neutralizing soil acidity with agricultural lime include:
b) Calcium Nitrate [Ca (NO3)2]
Calcium Nitrate is a highly soluble source of Nitrate and Calcium, which can be added directly to soil, through irrigation water, or with foliar applications. Chemical formula:
Solid Ca (NO3)2•NH4NO3•10 H2O15.5% N, 18 to 22% Ca, <1.5% NH4
Liquid Ca(NO3)2•4H2O8 to 9% N, 11 to 12% Ca, <1% NH4
Calcium Nitrate is popular in agronomic situations where a readily soluble source of Nitrate or Calcium is needed. Nitrate moves freely with soil moisture and can be immediately taken up by plant roots. Unlike many other common N fertilizers, Ca(NO3)2 application does not acidify soils since there is no acidity producing nitrification of Ammonium occurring. Broadcast application of Ca(NO3)2 is desirable in some circumstances because the risk of Ammonia volatilization is eliminated. Ca(NO3)2 is also used to provide supplemental Ca to the crop. Solutions of Ca(NO3)2 are commonly added to irrigation water and to foliar and fruit sprays to overcome Ca deficiency that can affect yield and/or quality (Apple bitter pit), or to meet peak Ca demands during critical growth periods. Part of the popularity of Ca(NO3)2 also arises from its Chloride-free nature and it can have an ameliorating effect under saline growing conditions, combating the negative effects of Na and Cl-. Calcium is an important nutrition that gives rigidity to cell wall and offers resistance to many abiotic and biotic stresses. Ca(NO3)2 is widely used in intensive cropping systems that have a high focus on crop quality.
c) Calcium Chloride (CaCl2.6H2O):
It contains at least 15% Calcium. It is highly water soluble and can, therefore, be dissolved for application as a foliar nutrient. Effective in management of bitter pit disease in apple, spongy tissues in mango and fruit cracking in lemon. It increases the keeping quality of fruits.
Note: To avoid precipitating insoluble fertilizer salts, Ca(NO3)2 should not be mixed with soluble Phosphate or Sulphate fertilizers in nutrient solutions or for fertigation.
3. Sulphate fertilizers
These are chemical substances containing the nutrient Sulphur in the form of absorbable Sulphate anions (SO4)2-. The Sulphur requirements of plants are about two third of their Phosphorus requirements. Substantial Sulphur supplies occur as minor constituents of various N, P and K fertilizers. Fertilization with Sulphur becomes necessary with increasing removal from the soil with rising agricultural production especially in plants with high Sulphur requirements, like Mustard. Formulations containing 90% elemental sulphur along with swelling agent like Bentonite clay are available to ensure uniform soil application.
Thiosulfate (S2O32-) fertilizers are clear liquids that provide a source of Sulphur. They also contain other nutrients including Nitrogen, Potassium, Calcium or Magnesium. Thiosulfate quickly reacts to form tetrathionate, which is subsequently converted to sulphate, within 1-2 weeks and become available for plant uptake. Thiosulphates may be applied through surface irrigation, sprinklers, and drip irrigation. Many of them are used in foliar sprays to provide a rapid source of plant nutrition (not recommended with ATS). Commonly available thiosulphates include - Ammonium thiosulphate [(NH4)2S2O3;N 12%: S 26%], Potassium thiosulphate [K2S2O3; K2O 25%: S 17%], Calcium thiosulphate [CaS2O3; Ca 6%: S 10%] and Magnesium thiosulphate [MgS2O3; Mg 4%:S 10%].
The importance of fertilization of crops with micro-nutrients is increasing mainly because of greater removal from the soil, intensive liming of soil, intensive drainage of soil, higher use of Nitrogenous, phosphatic and potassic fertilizers, etc. There are seven essential micronutrients required by plants - Iron, Manganese, Zinc, Copper, Chlorine, Boron and Molybdenum.
a) Iron fertilizers:
These are generally water soluble substances, predominantly sprayed on crops as foliar nutrients. Plants absorb Iron in the form of Fe2+. Commonly used Iron fertilizers are:
b) Manganese fertilizers:
c) Zinc fertilizers:
Zinc Sulphate (ZnSO4.7H2O>)
Chelated Zinc (Zn - EDTA)
d) Copper fertilizers:
e) Boron fertilizers:
f) Molybdenum fertilizers:
It provides a readily available supply of Potassium, Magnesium and Sulphur to growing plants. Since three essential nutrients are contained within a single particle, it helps in uniform distribution of nutrients when it is spread in the field. It contains 23% water soluble Potassium (as K2O) and 11% Magnesium (as MgO). It provides an adequate supply of Sulphur and because it is in the sulphate form, it aids initial root growth, promotes seed production and vigorous plant growth. Potassium Schoenite is readily water soluble, has a neutral pH. It does not contribute to soil acidity or alkalinity, unlike sources of Magnesium, such as dolomite, which increases soil pH and elemental Sulphur or ammonium sulfate, which lower the soil pH. It is water soluble and suitable for fertigation or foliar spray application. Being Chloride free, it is an ideal source of K, Mg and S in cultivation of Chloride and salt-sensitive crops such as fruits, vegetables, Grapevines, Hops, Potatoes, Sunflowers, Sugarcanes, and Tobacco. It is generally recommended at the rate of 50 kg per acre for soil application and 5-8 gm/ltr as foliar sprays at 15 days intervals during flowering and fruiting stage.
Silicon fertilizers are available both in liquid and granular forms. Some liquid formulations contain Silica in combination with Boron and Potassium. Plants absorb Silicon as silicate, which gets deposited as Silica in the cell walls, improving structural rigidity, strength and erectness. Silicon is known to decrease susceptibility to abiotic stresses, diseases and insect attacks. Silicon helps crops to increase tolerance to Zinc deficiency and increases reproductive rate.
Methods of Fertilizer Application
The different methods of fertilizer application are as follows:
It refers to spreading fertilizers uniformly all over the field. Suitable for crops with dense stand and the plant roots permeate the whole volume of the soil. Large doses of fertilizers are applied and insoluble phosphatic fertilizers such as rock Phosphate are used
1.1. Broadcasting at sowing or planting (Basal application):
The main objective of broadcasting the fertilizers at sowing time is to uniformly distribute the fertilizer over the entire field and to mix it with soil.
1.2. Top dressing:
It is broadcasting of fertilizers particularly nitrogenous fertilizers in closely sown crops like paddy and wheat, with the objective of supplying Nitrogen in readily available form to growing plants.
Disadvantages of broadcasting:
It refers to the placing fertilizers in soil at a specific place with or without reference to the position of the seed. It is normally recommended when the quantity of fertilizers to be applied is small, development of the root system is poor, soil has a low level of fertility and to apply Phosphatic and Potassic fertilizers.
2.1 Plough sole placement:
In this method, fertilizer is placed at the bottom of the plough furrow in a continuous band during the process of ploughing. Every band is covered as the next furrow is turned. This method is suitable for areas where soil becomes quite dry up to few centimeters below the soil surface and soils have a heavy clay pan just below the plough sole layer.
2.2 Deep placement:
It is the placement of Ammoniacal Nitrogenous fertilizers in the reduction zone of soil particularly in paddy fields, where Ammoniacal Nitrogen remains available to the crop. This method ensures better distribution of fertilizer in the root zone soil and prevents loss of nutrients by run-off.
2.3 Localized placement:
It refers to the application of fertilizers into the soil close to the seed or plant in order to supply the nutrients in adequate amounts to the roots of growing plants. The common methods to place fertilizers close to the seed or plant are as follows:
In this method, fertilizer is applied at the time of sowing by means of a seed-cumfertilizer drill. This places fertilizer and the seed in the same row but at different depths. Although this method has been found suitable for the application of Phosphatic and Potassic fertilizers in cereal crops, sometimes germination of seeds and young plants may get damaged due to higher concentration of soluble salts.
2.3.2 Side dressing:
It refers to the spread of fertilizer in between the rows and around the plants. • Placement of Nitrogenous fertilizers by hand in between the rows of crops like Maize, Sugarcane, Cotton etc., to apply additional doses of Nitrogen to the growing crops. Placement of fertilizers around the trees like Mango,Apple, Grapes, Papaya, etc.
2.3.3 Band placement:
It refers to the placement of fertilizer in bands. Band placement is of two types.
220.127.116.11 Hill placement:
It is practiced for the application of fertilizers in orchards. In this method, fertilizers are placed close to the plant in bands on one or both sides of the plant. The length and depth of the band varies with the nature of the crop.
18.104.22.168 Row placement:
When the crops like Sugarcane, Potato, Cereals, etc., are sown close together in rows, the fertilizer is applied in continuous bands on one or both sides of the row, which is known as row placement.
2.3.4 Pellet application:
It refers to the placement of Nitrogenous fertilizer in the form of pellets 2.5 to 5 cm deep between the rows of the paddy crop. The fertilizer is mixed with the soil in the ratio of 1:10 and made into small pellets that are deposited in the mud of paddy fields.
Advantages of placement of fertilizers:
There is minimum contact between the soil and the fertilizer, and thus fixation of nutrients is greatly reduced. Weeds all over the field cannot make use of the fertilizers. Residual response of fertilizers is usually higher. Utilization of fertilizers by the plants is higher. Loss of Nitrogen by leaching is reduced. Being immobile, Phosphates are better utilized when placed.
COMMON METHODS OF APPLYING LIQUID FERTILIZERS
a) Starter solutions:
They are solutions of N, P2O5 and K2O in the ratio of 1:2:1 or 1:1:2 applied to young plants at the time of transplanting, particularly in vegetables. Starter solutions help in rapid establishment and quick growth of seedlings. Application of starter solutions requires additional manpower and also results in higher fixation of Phosphate.
b) Foliar application:
Several nutrient elements are readily absorbed by leaves when they are dissolved in water and sprayed on them. Foliar application is effective for the application of minor nutrients like Iron, Copper, Boron, Zinc and Manganese. Insecticides may also be applied along with fertilizers. The concentration of the spray solution has to be carefully regulated to avoid serious damage due to scorching of the leaves.
It is a method of fertilizer application through irrigation water by the drip system. Nutrients and water are supplied near the active root zone resulting in greater absorption by the crops. In this system fertilizer solution is distributed evenly. Efficiency is high (80-90%) due to increased availability of nutrients. In this method liquid fertilizer as well as water soluble fertilizers are used. Fertigation saves water, fertilizer, time, labour and energy substantially.
(i) N fertigation:
Urea, Ammonium Nitrate, Ammonium Sulphate, Calcium Ammonium Sulphate and Calcium Ammonium Nitrate are used as Nitrogenous fertilizers in fertigation. Urea is well suited for injection in micro irrigation system. It is highly soluble and dissolves in nonionic form, and does not react with other substances in the water. Urea does not cause precipitation problems.
(ii) P fertigation:
Application of Phosphorus to irrigation water may cause precipitation of Phosphate salts. Phosphoric acid and Mono-Ammonium Phosphate, Mono-Potassium Phosphate are more suitable for fertigation. Phosphoric acid is inexpensive, provides highest amount of Phosphorus and also helps clean up the drip system.
Application of K fertilizer does not cause any precipitation of salts. Potassium Nitrate, Potassium Chloride, Potassium Sulphate and Mono Potassium Phosphate are used in drip fertigation.
(iv) Micro nutrients:
Fe, Manganese, Zinc, Copper, Boron and Molybdenum can also be applied through drip fertigation. Fe, Mn, Zn, Cu, B, Mo are also supplied along with special fertilizers.
d) Injection into soil:
Liquid fertilizers for injection into the soil may be of either pressure or non-pressure type. Non-pressure solutions can be applied either on the surface or in furrows without appreciable loss of plant nutrients under most conditions. Anhydrous Ammonia must be placed in narrow furrows at a depth of 12-15 cm and covered immediately to prevent loss of Ammonia.
e) Aerial application:
In areas where ground application is not practicable, the fertilizer solutions are applied by aircraft particularly in hilly areas, forest lands, grass lands or sugarcane fields, etc.
Advantages of Using Chemical Fertilizers
Disadvantages of Chemical Fertilizers:
Integrated Nutrient Management
Integrated Nutrient Management refers to the maintenance of soil fertility and of plant nutrient supply at an optimum level for sustaining the desired productivity through optimization of the benefits from all possible sources of organic, inorganic and biological components in an integrated manner.
Mobilizing unavailable nutrients and adopting appropriate crop varieties, cultural practices and cropping system.
Super granules, coated urea, direct use of locally available rock PO4 in acid soils, Single Super Phosphate (SSP), MOP and micronutrient fertilizers.
By-products of farming and allied industries, FYM, droppings, crop waste, residues, sewage, sludge, industrial waste, etc.
Microbial inoculants to substitute 6 - 16 kg N/acre. (See under the chapter on SOIL for more details).
Law of the Minimum
‘Law of the minimum’ states that if one of the essential plant nutrients is deficient, plant growth will be poor even when all other essential nutrients are abundant.
Therefore a balanced supply of each and every nutrient at its optimum level is important to realize the complete potential of a particular crop.
The plant nutrients have to be supplied in recommended proportions using straight or compound fertilizers available commercially, to achieve the anticipated yields. Hence, it is required to know the proportion of each nutrient in the available fertilizer and calculate the quantities to be applied to achieve the required nutrient inputs.
In Tomato, to achieve a yield of 40 MT/ acre, the requirement of nutrients is 120 kg of Nitrogen (N), 80 kg of Phosphorus (P2 O5) and 150 kg of Potassium (K2O).
When one intends to use DAP as the source of Phosphorus and Urea as the source of Nitrogen, fertilizer calculation is done as below.
Step 1: Calculate the quantity of DAP required to supply 80 kg Phosphorus.
Step 2: Calculate the quantity of N supplied by 173 kg DAP
Step 3: Calculate the requirement of Urea to supply N.
Step 4: Calculate the requirement of Potassic fertilizers to supply 150 kg Potasium.
a. When Muriate of Potash (60% K2O) is used Quantity of MOP required is 100 x 150 60 = 250 kg.
b. When Sulphate of Potash (50% K2O) is used Quantity of SOP required is 100 x 150 50 = 300 kg
Thus the total requirement of fertilizers required to be applied is 173 kg DAP + 194 kg Urea + 250 kg MOP / 300 kg SOP per acre of tomato crop.
Similarly when Single Super Phosphate (SSP) is used as source of Phosphorus, calculation is done as follows:
Step 1: Calculate the quantity of SSP (16% P2O5) required to supply 80 Kg Phosphorus.
Step 2: Calculate the requirement of Urea (46% N) to supply 120 Kg of N
100 x 120 46 =260.87 Kg
Step 3: Calculation of Potassium (K2O) remains same as in the previous case unless other sources of Potassium like Potassium Nitrate or Mono Potassium Phosphate is used.
Thus the total fertilizers required to be applied is 500 Kg SSP + 260.87 Kg Urea + 250 Kg MOP or 300 Kg SOP per acre of tomato crop to achieve 40 tons yield.