Part 1, Section 2: Soil Fertility Management
Soil Fertility Management
PLANT NUTRIENTS
Additional considerations
Some nutrients, such as nitrogen and phosphorus, are present in the soil in large amounts but are made available to plants only very slowly. Others, such as potassium, are both present in the soil in large amounts and readily available.
Nutrients also behave differently in the soil. One important behavior characteristic of a nutrient in soil is its relative mobility. This is illustrated in Figure 1.2-2.
In general, as nutrient mobility increases, fertilizer placement becomes less critical, but the potential for nutrient loss becomes greater. Thus, nitrogen placement relative to the plant is not very critical for uptake, but the potential for loss of nitrogen once it has been applied to the soil generally is very high, and little available nitrogen will accumulate in the soil.
Conversely, as nutrient mobility decreases, fertilizer placement becomes more critical, but the potential for nutrient loss lessens. At this extreme, phosphorus placement near the plant is very critical for uptake, because phosphorus does not move more than ¼ inch to get to a root.
The loss of phosphorus from soils usually requires erosion of the soil itself, however, so phosphorus therefore accumulates in the soil.
Some recent evidence indicates that, in spite of its low solubility, significant amounts of soluble phosphorus can be lost in runoff from fields with very high or excessive soil test phosphorus levels. Therefore, from an environmental perspective, it is best to manage phosphorus to avoid excessive soil test phosphorus levels if possible. The behavior of most micronutrients is similar to that of phosphorus. The other nutrients fall somewhere between these two extremes in mobility behavior.