How Do Plants Get Nutrients in the Soil in a Conventional Farming System?

Plants need an adequate supply of nutrients — particularly nitrogen, phosphorus, and potassium — to grow well. Ideally, these nutrients should be available in the proper quantity and at the time the plant can use them. This ideal timing, if complied with, will help farmers avoid supplying an excess of nutrients that plants cannot use anyway and may become contaminants in the environment instead.

Nitrogen, phosphorus and potassium are the nutrient elements most needed in large amounts by plants; however, they are not available in adequate amounts in the soil. Nitrogen is important for plants because it is a component of proteins and chlorophyll, the active pigment in photosynthesis; it is a constituent of nucleic acids and coenzymes that catalyse cell reactions. Phosphorus is also found in proteins, coenzymes, and nucleic acids; it is critical in metabolism and chemical energy generation and utilisation in the cells. Whilst its role is not clearly defined as a component of the various chemical compounds that make up the plant, potassium is important in the physiological mechanisms that regulate plant processes, particularly the all important processes of photosynthesis and carbohydrate translocation.

In conventional farming systems, nitrogen, phosphorus and potassium are supplied to the soil by application of inorganic fertilisers at levels recommended by soil testing technicians. The caveat is that variable conditions in the soil and the climate affect the rate of uptake or loss of nutrients in ways not yet fully understood. The ability to forecast factors that influence the storage, cycling, availability and uptake of nutrients is still relatively inadequate. This makes it difficult to predict the proper, environmentally safe levels of nutrients. Consequently, the application recommendations that farmers receive may just as easily lead either to insufficient or excessive fertilisation.

Working out the appropriate dosage amounts to apply may be tricky. Phosphorus fertiliser undergoes rapid conversion into less soluble compounds in acidic or alkaline soil, which then severely limits their availability for plant nutrition. Even if they are in available forms, the phosphorus may be tightly bound to organic soil compounds and clay, and remain locked in soil, inaccessible by plants. On the other hand, potassium and nitrogen (in its ammonium and nitrate forms) have greater solubility than phosphorus. Nitrate ions will leach readily into the soil, thus nutrient applications are susceptible to significant losses. Potassium and ammonium nitrogen are positively charged and are held on by negatively charged soil in the cation exchange, thus leaching will not occur in appreciable amounts except in sandy soils. Whilst there is understanding of the basic process, agriculture scientists need more information about nutrient cycling and nitrogen behaviour under various environmental conditions.

As a result of this difficulty, it is not surprising — and many studies have found — that recommended fertiliser doses worked out by some commercial soil testing laboratories consistently required far more fertiliser than was needed. Not only that, some farmers tend to apply greater amounts of nitrogen than recommended. However, with susceptibility to leaching and/or rapid conversion into insoluble forms, there is still no guarantee that the fertilisers will still be available to plants at the time plants have need for them.