Use of Nitrogen Fertilizers

by F. W. PARKER

WAR LIVES on nitrogen. All explosives except the atomic bomb are nitrogen compounds. War increases the need for nitrogen fertilizers for growing more food and fiber. To meet increased wartime needs, the United States more than doubled its capacity to produce fixed nitrogen compounds, by building 10 new synthetic nitrogen plants. As a result, our Army and Navy had plenty of ammunition, and American farmers used more fertilizer nitrogen than they had ever used before in fact, 10 times the 62 thousand tons they used in 1900. But even this record supply was not all they wanted or could well have used.

Nitrogen for fertilizers comes from natural organic materials like dried blood, tankage, and cottonseed meal, or from chemical compounds like ammonium sulfate, sodium nitrate, ammonia solutions, and ammonium nitrate. A sharp cut in the price of chemical nitrogen between 1925 and 1935 followed the development of synthetic nitrogen processes, stimulated the use of nitrogen, and caused a trend away from the higher priced natural organics.

Farmers in some States use a great deal of nitrogen. Others use very little. In Maine, Florida, and Ohio mixed fertilizers supply most of the nitrogen. In California, Mississippi, and South Carolina most of the nitrogen is used as nitrogen fertilizer materials unmixed with phosphate and potash. Seven States along the South Atlantic and Gulf coasts use 53 percent of the fertilizer nitrogen; Iowa and Minnesota and the great wheat-producing States use little nitrogen. Why?

Several factors are involved, among them the nitrogen content of the soil, rainfall, cropping or farming system, and the value of the crop per acre. In the Southeast, where rainfall and mean annual temperature are relatively high, the soils are low in nitrogen and farmers use a large quantity of fertilizers. In the Corn Belt the soils are higher in nitrogen, and less is applied as fertilizer.

Difference in rainfall is an important factor. Generally speaking, rainfall in the States west of Minnesota, Iowa, Missouri, and Arkansas is so low that water, rather than nitrogen, limits crop yields except on irrigated land. There is some evidence that nitrogen fertilizers may be profitably used for the production of grass seed and wheat under certain conditions in parts of the West, but for the most part the use of nitrogen is limited to irrigated crops.

A third factor is the farming system. In a good livestock system legumes are grown and much of the nitrogen is returned to the land in farm manure. In a cash crop system a higher proportion of the nitrogen is sold. In eight Southeastern States, 75 to 85 percent of the farm income is from the sale of crops. In the Corn Belt States only 20 to 40 percent of the farm income is from the sale of crops, whereas 60 to 80 percent is from livestock. Differences in farming systems as well as differences in soils and rainfall are important.

The value of the crop being grown also is a factor. High-value crops, such as citrus, tobacco, potatoes, and vegetables, usually are heavily fertilized. Cotton, one of the higher valued field crops, receives moderate fertilization. Lower valued crops generally are fertilized at lower rates.

Large quantities of nitrogen are required for high yields of most crops. A 60-bushel corn crop contains about 95 pounds of nitrogen, 57 pounds in the grain and 38 pounds in the stover. This nitrogen must come from the soil, legumes that have been turned under, manure, or fertilizers.

The time when the crop needs nitrogen corresponds with its rate of growth. Little nitrogen is needed in the seedling stage, but that little is highly essential. The demand is greater when growth is quite rapid. Usually this is in midsummer for spring-planted crops. Corn planted on May 22 in Ohio needed only 12 pounds of nitrogen before July 1. Between July 10 and August 10 the crop absorbed 81 pounds of nitrogen almost 60 percent of the nitrogen required for the 117-bushel crop. These figures indicate the corn crop needs most of its nitrogen during the 1 month of maximum growth.