Effect of deficient or toxic levels of nutrients in solution on foliar symptoms and mineral content of soybean leaves as measured by spectrographic methods

Studies of the chemical composition of soybeans, the emission-type spectrograph being employed for chemical assay, are reported herein. Soybeans were grown in sand cultures. Nutrient solutions which consisted of a complete solution; of toxic and deficient solutions with regard to magnesium, manganese, boron and zinc; of solutions lacking calcium, potassium, and phosphorus; and of solutions containing an excess (toxic) of copper were used. Leaves of the soybean plants were harvested by stalk position at midbloom stage and analyzed spectrographically for 10 elements. Certain toxicity and deficiency symptoms were developed on soybeans and these were described and compared with those observed by other investigators. By chemical analysis, it was found that the leaves of soybeans which received a complete nutrient solution contained strikingly different quantities of elements depending upon the stalk position on which they grew. For example, the contents of ash, calcium, magnesium and manganese decreased in concentration progressively in leaves from bottom to top of stalk; whereas potassium, phosphorus, copper, and nitrogen increased in this respect. The magnitude of these respective decreases and increases was found to be approximately 40 to 50% when expressed relative to concentrations in the lowermost leaves. Content of certain elements was increased markedly in the leaves of soybean plants receiving nutrient solutions which lack calcium, potassium, or phosphorus. Deficiency of calcium in nutrient solutions caused increases in magnesium, phosphorus, potassium, and boron; deficiency of potassium caused increases in calcium and phosphorus; whereas deficiency of phosphorus had its most marked effect on increasing the potassium content of leaves. Solutions deficient in potassium also resulted in leaves containing smaller concentrations of manganese, copper, iron, and boron. The deficiency of phosphorus in the solution seemed to have little effect upon the uptake of these micronutrients. A deficiency of magnesium increased the potassium content of the soybean leaves twofold, whereas magnesium in toxic amounts caused both the potassium and calcium contents of leaves to decrease to near trace amounts. The lack of magnesium had a definite inhibiting effect upon the uptake and translocation of phosphorus and boron, especially into the upper leaves. Deficiency of boron repressed the uptake of manganese and potassium, but favored uptake of calcium and magnesium. Manganese deficiency resulted in leaves high in phosphorus, potassium, and boron, but had little effect on uptake of other elements.