A soybean (Glycine Max. L. Merr.) variety classified as “tolerant” to very high levels of P fertility was paired and compared with a “very sensitive” variety in field experiments involving seven levels of P fertilizer (0, 16, 49, 147, 442, 1,330, and 3,980 kg P/ha). These experiments were carried out at two locations with three pairs of varieties over a 3-year period, and irrigation was included as a variable at one location for 2 years.
Yields of “tolerant” and “very sensitive” varieties were similar at the lower P levels, although responses by the “very sensitive” variety sometimes appeared slightly larger.
Yields of all varieties were depressed by the largest P application, but the yield of the “tolerant” member of each pair was always greater than that of the “very sensitive” variety. At this largest P addition, the average yield of the “very sensitive” varieties was only two-thirds of that of the “tolerant” varieties. These results were consistent with the “tolerance classification” that had been made previously by Howell and Bernard (4) from the behavior of the varieties in solution culture under greenhouse conditions.
Irrigation resulted in marked increases in yields of both “tolerant” and “very sensitive” varieties, but no interactions were found among irrigation, varieties, and levels of P fertilizer.
The observed relation between P-fertilizer additions and yield of soybeans indicated that the addition of the P-fertilizer in geometric rather than arithmetic increments would often be advantageous for such experiments involving a wide range of fertilizer additions.