Soil fertility

"Plants require at least 16 s for normal growth and for completion of their life cycle. Those used in the largest amounts, , and oxygen, are non-mineral elements supplied by air and water. The other 13 elements are taken up by plants only in mineral form from the soil or must be added as s. Plants need relatively large amounts of , , and . These nutrients are referred to as primary nutrients, and are the ones most frequently supplied to plants in fertilizers. The three secondary elements, , , and , are required in smaller amounts than the primary nutrients. Calcium and magnesium are usually supplied with liming materials, and sulfur with fertilizer materials. Contaminants in also supply 10 to 20 pounds of nitrogen and sulfur per acre each year, depending on local air quality. The micronutrients consist of seven essential elements: , , , iron, , , and . These elements occur in very small amounts in both soils and plants, but their role is equally as important as the primary or secondary nutrients. A deficiency of one or more of the micronutrients can lead to severe depression in growth, yield, and crop quality. Some soils do not contain sufficient amounts of these nutrients to meet the plant's requirements for rapid growth and good production. In such cases, supplemental micronutrient applications in the form of commercial fertilizers or must be made."

"Many tropical soils are poor in inorganic nutrients and rely on the recycling of nutrients from soil organic matter to maintain fertility. In undisturbed such nutrients are recycled via the ... ; , meanwhile, depends on the mineralization of organic nutrients from the plant remains ... or on (short-lived) inputs from ash ... . This dependence on organic nutrients in tropical soils has the result that tests of soil quality which only give isolated measures of inorganic nutrient status are unreliable ... , and that the effects of fertilization can be inconsistent because of leaching or fixation of inorganic nutrients. Here we attempt to quantify the role of organic matter in sustaining the fertility of soils from three different climate zones. We estimate rates of from ecological measurements and , and determine its relation to the and nutrient budgets. We find that agriculture without supplementary fertilization was economical for 65 years on temperate and for six years in a tropical semi-arid . An extremely nutrient-poor Amazonian soil showed no potential for agriculture beyond the three-year lifespan of the forest litter mat, once biological nutrient cycles were interrupted by slash-burning. These observations suggest that quantification of organic-matter cycling may provide an important guide to the agricultural potential of soils."

"Attention is again called to the fact that the is the original source of 98½% per cent of the materials found in the green plant; the carbohydrates, fats and being composed of elements supplied in the form of water and gas. These substances are furnished free of cost in humid climates, the supply being practically beyond control, and their use by the plant results in no impoverishment of the land. The subject of practical importance to the farmer is the supply of the other 1½% per cent of the plant, consisting of nitrogen and the ash elements which are derived directly from the solid particles of the soil. It has been shown that seven of these elements are essential to plant growth. Experience has proved that only three of these elements (i.e. , and ) are likely to become exhausted, or, in other words, that nothing is gained by adding to the soil any of the other elements of . This is due to the fact that the plant uses nitrogen, phosphoric acid and potash in rather larger quantities than the other elements, and that they exist in smaller quantities in the ground, and not because they are any more essential to vegetation. Occasionally soils are found that are actually deficient in , but in most cases lime is present in sufficient abundance for the growth of the plant."