Plant Nutrition

There appear to be at least two ways by which plants monitor their nutrient supply: directly through localized changes in nutrient concentration in the external soil solution, or indirectly through changes in the internal nutrient status of the plant itself. The direct pathway has the advantage that it can allow the plant to respond to short-term changes in nutrient availability and can in effect provide the roots with spatial information about the distribution of nutrients within the soil profile, allowing it to concentrate its developmental responses to that region of the soil where they will be of most benefit for nutrient acquisition.

The indirect pathway has the advantage that it enables the plant to integrate its nutritional signals with those coming from range of other physiological processes (such as photosynthesis).

Soybean can fix atmospheric N₂ by their root nodules associated with soil bacteria, bradyrhizobia. In addition, soybean can absorb inorganic nitrogen, such as nitrate and ammonia from soil or fertilizer. Usually a high yield of soybean was obtained in a field with high soil fertility. By supplying a constant but low concentration of nitrogen either from soil or organic manure, soybean growth will occur without depressing nodulation and nitrogen fixation activity. However, it is well known that a high concentration of mineral N depresses nodule formation and nitrogen fixation activity. Phosphorus deficiency cause stunted growth, dark green coloration of the leaves, necrotic spots on the leaves, and a purple color to the leaves occurring first on older leaves.

Aluminum appears to interfere to different degrees in the uptake and use of Ca. The petiole collapse appeared to be a secondary effect of Al injury, and the presence of the symptom was not required for yield reduction by Al.

The rate and time of acquisition varied among nutrients and were associated with specific vegetative or reproductive growth periods. Nearly 75% of K uptake occurred before the onset of seed filling compared to the uptake of N, P, S, Mg,Ca, Zn, B, Mn, and Cu, which were more evenly distributed during vegetative and seed-filling growth phases.