Although the soybean was introduced into the United States in 1804, it is only recently that its production has increased to the point where as a cash-grain crop it ranks fourth in the United States and second in the Midwest. It is perhaps owing to the relatively new status of the soybean as an important crop that the soil fertility requirements are not as yet well understood.

Notwithstanding the fact that soybean yields increase with the fertility level of the soil, experiments in the Midwest have shown in general that soybeans do not give the profitable response to direct application of fertilizer that is obtained with corn. The yield increases produced by direct application of fertilizers are comparatively small and unpredictable. This behavior may result from plant characteristics that have been classified as feeding power or it may result from a lack of knowledge of the plant in relation to its environment so that advantage is not taken of the proper means to bring about a profitable increase in yield from fertilizer application. Regarding' the latter point, Norman (20) has suggested that information on the nutritional needs of the plant during its various stages of growth might be of considerable value in the experimental approach to the soil fertility problems of the soybean. To obtain such information for soybeans grown in the field on two Iowa soils differing widely in fertility level was the primary object of the present investigation.