Studies in central and northern Illinois at 4 locations and 12 location‐years were conducted with 5 rates of N applied in the fall and as spring‐preplant. Sidedress N was also included at 1 of the 4 locations for 4 years. Relative efficiency of the times of application was calculated by dividing the corn (Zea mays L.) yield increase from a given rate of N added at one time by the yield increase from the same rate of N applied at another time. At the Carthage and Hartsburg locations the 3‐year average relative efficiencies of fall‐ versus spring‐applied N are about 0.8 and 0.9 (fall was 80 and 90% as effective as spring) at N rates of 67 and 134 kg/ha, respectively. Fall and spring N were about equally effective at 201 and 268 kg/ha of N. There was generally little yield response to N rates greater than 201 kg/ha at Carthage and Hartsburg. Fall and spring N gave similar corn yields for all rates of N at Urbana. For the 4‐year average at DeKalb, sidedress N was the most effective, spring N was intermediate, and fall‐applied N was the least effective. The difference between spring and sidedress N was less than that between fall and spring N. There was considerable year‐to‐year variation in relative efficiency. The importance of the time at which conditions suitable for N loss occur is discussed.

The objective of this research was to determine the effect on soybean (Glycine max (L.) Merrill) yields of N added at different rates by different times and methods of application, as direct and residual, and as inorganic and organic sources. A number of studies were conducted over a period of several years at 10 field locations in Illinois. Nitrogen at rates up to 360 kg/ha added for corn (Zea mays L.) the preceding year had no effect on soybean yields. Neither were soybean yields increased by organic sources of N such as manure or alfalfa (Medicago sativa L.), or by combinations of organic and inorganic sources. Fertilizer N added for soybean as plow‐down, disked‐in, and side‐dressed at early flowering and at pod filling did not increase yields. Nitrogen added for soybeans planted on four dates did not increase yields. High rates of N (1800 and 1440 kg/ha), broadcast and disked‐in in the spring, decreased yield due to germination and seedling injury. Considering all the studies, yields were significantly increased in only 3 out of 133 instances and these occurred at high, uneconomical rates of N fertilizer. It is concluded that N available to the plant is not the growth factor that presently limits soybean yields in Illinois.

Experiments were conducted on Zanesville, Elliott, and Muscatine soils to determine the relative efficiency, with respect to corn yields, of broadcast P as compared to banded P. The 16 fertilizer treatments consisted of 4 rates of banded P and 4 rates of broadcast P in factorial combinations. Yields from each of the 3 soils were used to calculate a multiple regression equation for each soil. The equation is of a quadratic form, and it expresses the relation between yield and rates of banded and broadcast P. The regression equations account for 77 to 88% of the variation in yield. The largest yield increases from added P were 1287, 1474, and 873 pounds of corn per acre for the Zanesville, Elliott, and Muscatine soils, respectively. With all the P either banded or broadcast, the relative efficiency of broadcast P ranged from 0.49 to 1.23 for the 3 soils For a given soil, the relative efficiency of broadcast P varied with rate. At 35.2 pounds of P added per acre, higher yields were obtained on Zanesville and Elliott soils if a combination of banded and broadcast application were used than if all the P was added either banded or broadcast. Banding all the P gave higher yields than the other application methods if 17.6 and 8.8 pounds of P per acre were added. Yields from the Muscatine soil were not dependent on whether all the P was drilled, broadcast, or applied by a combination of both methods.

Soil,leaf,and grain natural 15 N abundance was measured in corn (Zea mays L.) and soybean (Glycine max L.) plots which had received various rates of fertilizer N for 20 years. Soil 15 N abundance did not significantly reflect the amount of fertilizer N applied and should not be used to estimate fertilizer IM additions. Corn leaf and grain 15 N abundance reflected additions of fertilizer N only at low rates of fertilizer N applied. Soybean leaf 1S N abundance reflected a decrease in symbiotic N fixation with additional increments of fertilizer N applied. Additional Index Words: corn 15 N, isotopic fractionation, soybean 15 IM, symbolically fixed N.

No abstract