Most people know what fertilizers are, as they are commonly seen in retail shops. A fertilizer is any material that supplies plants with the essential nutrients required for growth as well as optimal yield. It may be natural or synthetic (artificially manufactured) and can be organic or inorganic.
Organic fertilizers are natural materials with an origin tracing back to plants or animals. They include green manure, livestock manure, compost, household waste, crop residues, woodland litter, etc.
Inorganic fertilizers, also known as mineral fertilizers, usually come from mining of mineral deposits. They require some processing and include phosphate, lime, rock, potash, etc. They can also be manufactured industrially through chemical processes, an example being urea.
There are important differences between organic and inorganic fertilizers in terms of their properties, applications, and effectiveness. Organic fertilizers offer a feasible option as they are available on or near the farm at little or no cost.
For example, if a farmer has livestock, livestock manure can be used as a fertilizer free of cost. Inorganic fertilizers, however, involve costs of labor, transportation, handling, and the opportunity cost of the land used for producing them, making them very costly.
Fertilizer use and application are labor-intensive for organic fertilizers but not so for inorganic fertilizers. This makes it possible to devote time and effort to other tasks on the farm when using inorganic fertilizers.
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Role of Mineral Fertilizers in Crop Growth
Mineral fertilizers are materials, either natural or manufactured, containing nutrients essential to normal growth and development of plants. Fertilizers have become an integral part of the agricultural economy of developed countries, but their use in developing countries is a relatively recent occurrence.
Amongst the various agricultural inputs, fertilizers, perhaps next only to water, contribute the maximum to increasing agricultural production. It has been estimated that about 50 percent of the increase in agricultural production witnessed during the last decade in developing countries is attributable to fertilizer use.
All fertilizers are divided into organic and mineral fertilizers. Organic fertilizers are universal, containing all necessary nutritional elements for plants. For example, organic fertilizer like manure contains about 0.5% nitrogen, 0.6% potassium, 0.25% phosphorus, etc.
Mineral fertilizers contain many times more nutrients than organic fertilizers. For example, nitrogen fertilizers contain 15% nitrogen (sodium nitrate, NaNO3) to 82% (liquid ammonia). Phosphate fertilizers contain from 18% phosphorus (phosphate rock) to 40%.
Potassium fertilizers contain potassium from 16% to 62% (potassium chloride, KCl). Complex fertilizers contain nitrogen, phosphorus, and potassium. Some complex fertilizers also contain trace elements such as copper, boron, zinc, and manganese. Mineral fertilizers are stored in a dry, cool place, with a shelf life preferably not exceeding two years.
Mineral fertilizers are applied to the soil in early spring or autumn before soil preparation. During plant growth, fertilizers are also added (evenly scattered in dry form or watered).
Nutrient Benefits and Deficiencies
The lack of nitrogen in the soil causes leaves to lose color, become pale green, then turn yellow, halting the growth of new leaves, which remain underdeveloped. With a lack of potassium in the soil, seed germination decreases, plants become more susceptible to diseases, and lower leaves turn yellow at the edges, then brown, and die.
Phosphorus is needed to accelerate the development of the root system and fruit. A lack of phosphorus affects leaves, turning them grayish or reddish, with lower leaves turning yellow and dying.
Calcium contributes to the development of the root system. Sulfur plays a role in plant respiration processes; magnesium and iron, consumed in smaller quantities, are also required for normal development.
Magnesium and iron support the normal color of leaves. Other essential elements include boron, copper, manganese, and zinc. A lack of copper causes white spots on leaves, which curl, dry up, and halt plant growth. Copper also improves resistance to fungal diseases and increases fruiting.
To replenish copper reserves in the soil, copper sulfate (CuSO4·5H2O) is applied at 10–20 grams per 10 m². Manganese plays an important role in plant respiration and photosynthesis, increasing the yield of agricultural plants.
Mineral fertilizers containing manganese are used: fruit trees are sprayed before and after flowering with a solution containing 30 g of potassium permanganate per 1 liter of water, which can be combined with urea.
For vegetable crops, 60–100 g of potassium permanganate per 10 m² is used. Zinc supports the formation of organic acids from carbohydrates and prevents several plant diseases, especially in citrus.
Complex fertilizers containing a complete set of trace elements (boron, manganese, molybdenum, zinc, copper) and 11% nitrogen, phosphorus, and potassium are suitable for all soil types and used for main application.
Types of Phosphate Fertilizers
Phosphate fertilizers, the most common being superphosphate, precipitate, and phosphate rock, include simple superphosphate, a mixture of calcium dihydrogen phosphate Ca(H2PO4)2, calcium hydrophosphate CaHPO4, calcium phosphate Ca3(PO4)2, apatite, gypsum, and other impurities.
Simple superphosphate is obtained by processing phosphorite and apatite with sulfuric acid. Processing mineral phosphate with phosphoric acid yields double superphosphate, containing up to 50% phosphorus oxide (P2O5). Mixing phosphoric acid with lime produces precipitate CaHPO4·2H2O.
The phosphate fertilizers include:
- Ca3(PO4)2 – phosphate rock
- Bone meal
- Ca(H2PO4)2 + 2CaSO4 – simple superphosphate
- Ca(H2PO4)2 – double superphosphate
- CaHPO4·2H2O – precipitate
- NH4H2PO4 + (NH4)2HPO4 – mofos (complex fertilizer)
Types of Nitrogen Fertilizers
Nitrogen fertilizers include:
- NH3 – ammonia (contains the largest mass fraction of nitrogen)
- NH3·H2O – ammonia water
- CO(NH2)2 – urea
- NH4NO3 – ammonium nitrate
- KNO3 – potassium nitrate (Indian)
- NaNO3 – sodium nitrate (Chilean)
- Ca(NO3)2 – calcium nitrate (Norwegian)
Types of Potassium Fertilizers
Potassium fertilizers include:
- NaCl·KCl – silvinit
- KCl – sylvite (potassium salt)
- K2SO4 – potassium sulfate
- K2SB3 – potash, wood ash, peat ash
Role of Organic Fertilizers in Soil Fertility
Plants absorb water and minerals from the soil, essential for growth, flowering, crop yield, and other vital activities. Soil is a storehouse for organic and inorganic plant nutrients. Some soils are rich in organic and humus content, considered fertile and more productive, while others deficient in humus and minerals are less productive.
Soil is subjected to continuous nutrient depletion due to crop use, requiring the addition of mineral resources. Soil components are removed by living organisms and returned through death and decay. If the rate of removal exceeds the rate of addition, the soil becomes less fertile.
Minerals are lost due to crops, leaching, or soil erosion. Minerals are often removed from the top layer by rainwater. Regular cultivation of crops makes the soil less productive. In intensive cultivation, there are few chances for restoring lost nutrients until supplied externally.
Leguminous plants compensate for the loss of nitrogenous compounds. Manure and fertilizers must be supplemented to restore soil fertility.
The deficiency of mineral nutrients can be compensated through organic manures such as green manuring, compost, etc., or by applying chemical fertilizers from external sources.
Benefits of Organic Manures
The organic content of the soil, a good source of plant nutrients, contributes significantly to soil fertility. Organic manures improve soil fertility in the following ways:
- They modify physical properties, increasing soil granulation, permeability, and moisture-holding capacity.
- They provide food for soil microbes, enhancing microbial activities.
- Decomposition products of organic manures help bring mineral constituents of soil into solution.
- They improve physicochemical properties, such as cation exchange and buffering action.
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Types of Organic Manures
Organic manures are of several kinds, some discussed below:
1. Farmyard Manures
Solid and liquid excreta, such as dung and urine of farm animals, are termed farmyard manures. They are ready-made manures containing nitrogen, phosphorus, and potassium. Farmyard manures of different animals vary greatly in composition but are suitable for all soil types and crops.
When collected in fields under exposed conditions for months, farmyard manure loses considerable fertilizing value due to ammonia volatilization upon decomposition.
Therefore, manure should be protected from weather, and preparation should be carried out in trenches about a meter deep. When trenches are filled with dung, the surface is covered with cow dung-earth slurry. In about three months, the manure is ready for use.
2. Compost
Compost manure can be prepared from refuse materials such as straw, sugarcane refuse, rice hulls, forest litter, weeds, leaves, and kitchen wastes. It is prepared in pits, usually 6–8 inches long, 1½ to 2 inches wide, and one meter deep.
In the pits, a 30 cm thick layer of plant residues moistened with dung, urine, and water is formed, followed by a second 30 cm layer of mixed refuse moistened with slurry. The operation is repeated until the heap rises about 50 cm above ground level.
The top is covered with a thin layer of moist earth. After three months of decomposition, the material is mixed and covered again. After a couple of months, the manure is ready for use.
There are two types of composts:
- Farmyard compost, obtained from animal excreta and plant residues.
- Town compost, obtained by decomposition of kitchen wastes and garbage from towns and cities. Compost manures are rich in all plant nutrients.
3. Green Manures
Green manuring is the practice of growing, plowing, and mixing green crops with soil to improve fertility and productivity. Its effects on soils are similar to those of farmyard manures. It is a cheap and effective method to increase soil fertility, supplementing farmyard and other organic manures without significant cost.
Green manures add nitrogen and organic matter to the soil, improving crop yield. Through green manuring, mobilization of minerals, reduction of organic nutrient losses due to erosion, leaching, and percolation, and improvement in physical, chemical, and biological activities of the soil can be achieved.
Green manuring also improves soil aeration and drainage conditions. Both leguminous and non-leguminous crops are used for green manuring. In India, leguminous crops such as sannhemp (sanai), dhaincha, berseem, clover, Phaseolus mungo, and cowpea are generally used.
4. Sawdust
Sawdust can be used as bedding material to conserve animal urine or for making compost. It is a low-fertilizing material but richer than wheat straw in calcium.
5. Sewage
In modern sanitation systems, water is used for the removal of human excreta and other wastes. Sewage consists of two components: the solid part, called sludge, and the liquid part, called effluent or sewage water.
Sewage is rich in several plant nutrients and can be used for fertilizing crops by irrigating the soil directly with sewage water, but there is a danger of spreading human diseases.
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