Manganese, Iron, and Sulfur

The increased use of manganese sulfate as a fertilizer in the past few years has been striking. It is estimated that between 15,000 and 20,000 tons are now being used annually, most of which is used in Florida on citrus and vegetable crops.

The presence in the soil of excess lime restricts the availability of manganese. Oxidizing conditions, the quantity and quality of soil organisms, and other elements also affect the availability of manganese.

The occurrence of iron deficiency in soils of Hawaii high in both iron and manganese and the demonstration of an iron-manganese balance with crops in nutrient cultures indicates the need of a proper balance between the two elements. That this relationship may not hold with all plants is indicated by the fact that in leaves of healthy tung trees in commercial orchards there is a range in the total manganese content from 40 parts per million to 4,000 parts per million, with the iron content remaining approximately constant at 50 to 70 parts per million.

Iron is one of the most important of the minor elements. Except for the use of iron sulfate on pineapples in Hawaii, usually as a spray, and in some dry areas in the West containing excessive lime in the soil, comparatively little iron is added to supplement the regular fertilizers. Usually there is sufficient available iron in most soils to take care of the needs of the plant and, in addition, most fertilizers contain iron as an impurity.

The comparatively small amount of iron in the plant would indicate that it functions as a catalytic or enzymatic agent somewhat like copper, zinc, and manganese. The problem of lime-induced iron chlorosis has received considerable attention through the years, but so far no one theory has been proposed to explain all the facts.

Sulfur, like magnesium, has always been considered an essential element needed by the plant in fairly large amounts, but it is only in recent years that sulfur or its compounds have been used as fertilizer materials.

The sulfur needed by crop plants comes largely from the air, rainwater, and the decay of organic matter in the soil. Where fertilizer is used the sulfates present are important particularly in the low-analysis superphosphate that contains approximately 50 percent calcium sulfate.

Sulfur deficiency may be expected, therefore, in areas distant from industrial centers, especially where there is not much decomposing organic matter and where fertilizers containing little sulfates are used.

About 50,000 tons of sulfur were used as a fertilizer in 1944. Although most of this was used in California and other Western States to counteract soil alkalinity, there is abundant evidence to show its beneficial effect as a crop nutrient in sulfur-deficient areas. Added sulfur or sulfate has greatly increased yields of alfalfa in Montana, Minnesota, Washington, and Oregon. In Arkansas it has increased the yield of cotton, and in Florida the yield of peanuts.

THE AUTHOR

Matthew Drosdoff, a soil technologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering, has been in charge of soils work for the past 7 years in connection with investigations on tung trees. Stationed in Gainesville, Fla., in a region deficient in most of the minor elements, much of Dr. Drosdoff's work has been concerned with minor element nutrition.