agriculture

In total, plants need at least 16 elements, of which the most important are carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium. Plants obtain carbon from the atmosphere and hydrogen and oxygen from water; other nutrients are taken up from the soil. Although plants contain sodium, iodine, and cobalt, these are apparently not essential. This is also true of silicon and aluminum.

Overall chemical analyses indicate that the total supply of nutrients in soils is usually high in comparison with the requirements of crop plants. Much of this potential supply, however, is bound tightly in forms that are not released to crops fast enough to give satisfactory growth. Because of this, the farmer is interested in measuring the available nutrient supply as contrasted to the total nutrient supply. When the available supply of a given nutrient becomes depleted, its absence becomes a limiting factor in plant growth. Excessive quantities of some nutrients may cause a decrease in yield, however.

Determining nutrient needs

Determination of a crop’s nutrient needs is an essential aspect of fertilizer technology. The appearance of a growing crop may indicate a need for fertilizer, though in some plants the need for more or different nutrients may not be easily observable. If such a problem exists, its nature must be diagnosed, the degree of deficiency must be determined, and the amount and kind of fertilizer needed for a given yield must be found. There is no substitute for detailed examination of plants and soil conditions in the field, followed by simple fertilizer tests, quick tests of plant tissues, and analysis of soils and plants.

Sometimes plants show symptoms of poor nutrition. Chlorosis (general yellow or pale green colour), for example, indicates lack of sulfur and nitrogen. Iron deficiency produces white or pale yellow tissue. Symptoms can be misinterpreted, however. Plant disease can produce appearances resembling mineral deficiency, as can various organisms. Drought or improper cultivation or fertilizer application each may create deficiency symptoms.