Making sure diseased or disease-carrying plants are not introduced into the growing area reduces the disease potential. This discussion focuses on visual examination of the leaves. Nitrogen Nitrogen deficiency is seen by stunted plant growth and small, pale green or greenish yellow leaves. Eventually, older leaves develop red blotches or red edges. Additional nitrogen should be in the form of ammonium sulphate.
Bacteria convert nitrogen to ammonium, and other bacteria convert ammonium to nitrate.
These latter bacteria are less active at higher pH. High levels of organic matter and low pH can reduce the availability of nitrogen to plants by fixing the nitrogen in the microorganisms. Toxicity due to an excess of nitrogen shows as chlorosis and leaf drop of young leaves. This is due to nitrate forms of nitrogen. Nitrogen in the form of ammonium produces better color and more growth and is not as toxic. The chlorosis may be induced by an alkaline environment at the roots, and the leaf drop may be caused by an unavailability of trace elements at the high pH. Phosphorus The leaves of plants with phosphorus deficiency become dark green, sometimes almost black, and reddening later develops on the lower surfaces, especially along the midribs.