Proper nutrition for crops is as important as for human beings. Malnutrition causes similar problems in plants as in human beings. Soil is a natural source of crop nutrition that needs to be replenished with nutrients consumed by crops.
With per capita arable land becoming ever scarcer, sustainable intensification is the prime option for meeting future food needs and protecting natural resources. Cropping intensification leads to enhanced flows of plant nutrients to crops and depletes the soil of its nutrients.
Nutrient depletion is a major and, often, hidden form of land degradation, with long-term consequences of yield loss and unsustainable agricultural production. Mineral fertilizer application contributes about 48 per cent of all plant nutrients that global crop production extracts. Lack of proper nutrients in soil is one reason that productivity of adjacent farms in Pakistan lacks by a fact of 10. Progressive farmers manage their soil in a way that ensures optimum productivity while uninformed farmers either overuse the soil to deplete it of all nutrients or add high, or wrong, nutrients (fertilizers) that also impede productivity.
Increasing agricultural production by improving plant nutrition management, together with a better use of other production factors, is a complex challenge. Agricultural intensification requires increased flows of plant nutrients to crops, a higher nutrient uptake and higher stocks of plant nutrients in soils.
This also results in a higher production of crop residues, manure and organic wastes. Excessive use of nutrients, inefficient management of cropping systems, and the inefficient use of residues and wastes result in loss of plant nutrients, which means an economic loss to the farmer. On the other hand, an inadequate, or insufficient, use of plant nutrients creates an insidious depletion of the stock of plant nutrients on the farm, which will also mean an economic loss for the farmer.
Environmental hazards can be created by applying too much nutrients compared to the uptake capacity of cropping systems, while the depletion of nutrient stocks is a major, but often hidden, form of environmental degradation.
Plant nutrition management depends largely on prevailing economic and social conditions. Farmers' decisions depend on their economic situation and their socio-economic environment, on their perception of economic signals and on their acceptance of risks.
Agriculture, inevitably, removes plant nutrients from the soil and from the farm. Consequently, if a farming system is to be sustainable, these nutrients have to be replaced by whatever sources are available. For the farmer, nutrient losses is money wasted.
In many developing countries, the loss of soil fertility from continual nutrient mining by crop removal without adequate replenishment, combined with imbalanced plant nutrition practices, poses a serious threat to agricultural production. It is already causing yield decreases as large as those caused by other forms of environmental degradation.
While recycling and transfer of nutrients from non-crop areas, crop residues and animal manure can partially make up for exports of nutrients by harvested products, the use of external sources such as mineral fertilizers is essential to meet crop requirements and to increase crop production in many farming systems. Intensification of agriculture by increasing the use of plant nutrients is limited by environmental hazards and economic constraints.
Increased attention is now being paid to developing Integrated Plant Nutrition Systems (IPNS) that maintain, or enhance, soil productivity through a balanced use of mineral fertilizers combined with organic sources of plant nutrients, including biological nitrogen fixation. IPNS is ecologically, socially and economically viable, and it can increase both soil productivity and crop yields. It focuses, first, on the seasonal or annual cropping system, rather than on an individual crop; secondly, on the management of plant nutrients in the whole farming system; and, thirdly, on the concept of village or community areas rather than individual fields.
Plants build up their biomass using water, carbon dioxide from the air, energy from sunlight and nutrients taken up from the soil and water. For optimum plant growth, nutrients must be available: as solutes in the soil water; in adequate and balanced amounts, corresponding to the instant demand of the crop; in a form which is accessible to the root system (except when provided through foliage).
Plants are supplied with nutrients mainly from soil reserves; mineral fertilizers; organic sources; atmospheric nitrogen through biological fixation; aerial deposition caused by wind and rain; irrigation, flood or groundwater, and sedimentation from runoff.
These are elements that are essential for plant growth and that are taken up from the soil or from water - irrigation, flood or groundwater - or are supplied via a hydroponics medium. The primary nutrients are nitrogen, phosphorus and potassium which are consumed in relatively large amounts. Three secondary nutrients are taken up in smaller quantities but are essential for plant growth: calcium, magnesium and sulphur. Micronutrients, or trace elements, are required in very small quantities but are often important for plant or animal metabolism. They include iron, zinc, manganese, boron, copper, molybdenum and chlorine.
Soils contain natural reserves of plant nutrients in quantities that depend on soil composition and the stage of weathering. These reserves are often in forms unavailable to plants, and only a minor portion is released each year through biological activity or chemical processes. This release is too slow to compensate for the removal of nutrients from agricultural production.
The quantities (or stock) of plant nutrients available for a crop are determined by the supply of nutrients to the crop from internal and external sources, the uptake of nutrients by the crop, and losses of plant nutrient to the environment. Thus, plant nutrient stock is constantly changing. The capacity of a soil to store easily-available plant nutrients is an important factor in plant nutrition management. Chemical analysis can provide an approximation of the nutrient stock, the accuracy of which is related to soil type, cropping conditions and crop species.
Fertilizers are mineral or organic substances, natural or manufactured, that are applied to soil, irrigation water or a hydroponics medium, to supply plants with nutrients. Mineral fertilizers have a higher plant nutrient content and a lower bulk than organic sources of plant nutrients.
Some micro-organisms are able to convert nitrogen in the air to ammonia for use as their nitrogen source. The conversion is made by bacteria living either on their own in the soil or, on a considerably larger scale, in symbiosis with leguminous plants or trees. Biological nitrogen fixation can be enhanced by inoculation with efficient strains of nitrogen-fixing micro-organisms, part of the fixed nitrogen being directly assimilated by plants.
Some nutrients are supplied in small quantities to the soil surface through aerial deposition. These include nitrates in rainwater, ammonia as a gas or dissolved in rainwater, sulphur in acid rain, salts and chlorine in marine spray and calcium in the form of dust.
Irrigation water, floodwater and groundwater also supply plant nutrients, either naturally or because fertilizers have been added to the irrigation water. While water often contains small amounts of nutrients, irrigation can also result in a loss of nutrients from leaching. Some of the nutrients provided by surface and groundwater originate from plant nutrient losses within the watershed.
Amendments are substances that are applied to the soil to correct a major constraint other than low nutrient content. Lime, for example, is used to remedy acidity; phosphates are used to reduce phosphorus fixation; gypsum is used to improve sodic (alkali) soils; and peat can be added to the surface layers to increase organic matter content.
The practice of allowing short fallow periods on arable land is a means of accumulating the natural supply of nutrients for a later cropping period. However, much less nutrients are gathered in a short fallow than in a long fallow period, of ten years or more, in perennial vegetation.