Under irrigation, removal of the surface 10, 20, 30, and 41 cm of soil decreased N yields 40.6, 47.5, 58.7, and 64.7%, respectively. Respective decreases under limited moisture were 20.0, 36.3, 53.9, and 62.4%. Total N in the surface 15 cm of soil and N yields were closely related.
Relationships among drymatter yields of grain sorghum (Sorghum vulgare), associated N uptake, and N treatments (soil NO3-N + fertilizer N) were used to estimate maximum yields attainable and to determine the internal N requirement of grain sorghum. That requirement was almost a constant (1.0%) despite the widely different yield maxima attained under the contrasting moisture regimes.
Under irrigation, similar yield potentials were attained on all except the treatment with the greatest topsoil removal (41 cm). Undefined factors, other than N, P, or water, reduced the yield potential on that treatment. Under limited moisture, fertilizer treatments did not restore drymatter yields on any depth of soil removal. Differences are attributed to differences in water storage and use.
Efficiency of uptake of available N was not directly affected by topsoil removal but was affected by the N status of the plants. It remained relatively constant until the plant N requirement was met, and then declined.