The influence of phosphorus and nitrogen on millet and clover growing in soils affected by salinity

Summary The growth of foxtail millet and clover in soils of varying degrees of salinity (0.5 to 13 mmhos/cm), treated with nitrogen and phosphorus, was studied. Salinity levels were achieved by addition of sodium chloride. Nitrogen (10 to 60 ppm N) and phosphorus (6.4 to 44.8 ppm P) were added as NH₄NO₃ and H₃PO₄, respectively. The growth of millet decreased sharply with increase in soil salinity, when N-P treatments were not applied. The development of this plant altered under saline conditions, however, when nitrogen and phosphorus were added; various N-P combinations affected plant growth in saline soil differently. Phosphorus, when applied at relatively high rates, significantly improved plant growth. Increased rates of nitrogen in the N-P treatments generally had no significant effect on growth; it was reduced when the N/P ration was highest. Clover ceased to grow when the salinity of the soil exceeded 7 mmhos/cm and no N-P was added. Phosphorus enhanced the growth of clover, and at high rates of its application in the N-P combinations, clover grew even at the highest salinity level tested (13 mmhos/cm). Nitrogen increase had no marked effect on plant development. Comparing enhancement of growth at high salinity levels, clover was somewhat less affected by the N-P treatment than millet. The top/root ratio of clover generally increased with increase of phosphorus in the N-P combinations. This research (Parts I and II) was supported in part by a grant from the U.S. Department of Agriculture under P. L. 480.