The availability of soil phosphate. IV.—the effect of nitrogen on the utilization of phosphate by Italian ryegrass

In pot and solution culture experiments with Italian ryegrass and oats and laboratory studies of soil, using Harlington medium loam and Buxted silty clay loam, repeated application of nitrogen as compared with a single addition was found significantly to promote the uptake of added fertilizer phosphate, but not that of difficultly‐soluble native phosphate, by ryegrass. In the very acid Buxted soil (pH 4.9), repeated use of physiologically basic nitrogen (NaNO3 and KNO3) more effectively increased the uptake of both native and added phosphates by ryegrass than did the physiologically acid nitrogen [(NH4)2SO4 and NH4C1]. In these cases the uptake of phosphate was positively related to that of nitrogen. The effect of nitrogen fertilizers in the slightly acid soil (pH 6.4) chiefly depended on their influence on soil pH, the uptake of phosphate being favoured by those increasing soil acidity. Comparison of various phosphate compounds containing ammonia showed that their efficiency in relation to the utilization of phosphate by ryegrass was closely associated with the ammonia content as well as with the uptake of nitrogen. Repeated addition of physiologically acid nitrogen decreased the acid‐extractable phosphate but increased the phosphate‐fixing capacity in the slightly acid soil, while physiologically basic nitrogen behaved in the reverse manner. None of the nitrogen treatments appreciably affected either the acid‐extractable phosphate or fixing capacity in the very acid soil, which had a low phosphate status and high fixing capacity. Results of solution culture with oat seedlings showed increased absorption of phosphate at pH 5.5–6.2 with maximum at pH 5.8–6.0. The intake of phosphate decreased sharply beyond the pH range 4.0–6.2.