Influence of placement of ammoniated and non-ammoniated superphosphates on efficacy of the phosphate

The effects of various placements on the relative response of crops to the phosphate in nonammoniated and ammoniated superphosphate were studied in greenhouse pot cultures. In experiments at five locations, the state experiment stations of Alabama, Arkansas, Mississippi, and South Carolina, and the Plant Industry Station at Beltsville, Md., the effects of placement on response of millet, Sudan grass, and sorghum to nonammoniated superphosphate and superphosphate ammoniated to 2, 3, 4, or 5% under various conditions, were studied on six acid soils and one alkaline soil. The placements were (1) complete mixed fertilizer containing phosphate, nitrogen, and potash, mixed with all the soil, (2) the fertilizer mixed with 5% of the soil in a layer halfway down the pot; and (3) the fertilizer applied in a 3-inch circular layer 1 1/2 inches below the surface without mixing with the soil. In another experiment at Beltsville, millet was grown on four of the same soils used in the other experiments and on another acid soil. All the soils were fertilized with nonammoniated superphosphate or with ammoniated superphosphate (4%) prepared under conditions least conducive to phosphate reversion. Placements were the same as before except that the localized placement was a continuous band 0.5 inch wide 2 inches below the seed. Nitrogen and potash were applied separately in solution form. On the acid soils mixing with all the soil tended to be the least effective of the three methods of placement for both types of phosphate. The localized placements (unmixed layer and band) gave results with the nonammoniated superphosphate about equal to the method of mixing with 5% of the soil (mixed layer), with Sudan grass on Newtonia and Grady soils, and with millet on the Atwood soil. Results in these placements with the nonammoniated phosphate were slightly to markedly superior to the mixed layer placement with Sudan grass on Cecil soil and with millet on Newtonia, Grady, Cecil, and Sassafras soils, especially at the lower rates of application. The localized placements were inferior to the mixed layer placement on the Hartsells and Sunnyside soils with millet. The ammoniated superphosphates tended to give better results on the acid soils in the mixed layer placement than in the localized placements. The rate of application modified this effect somewhat in the millet tests on the Newtonia, Grady, Cecil, and Sassafras soils. The effect was more pronounced at the higher rates of ammoniation. It persisted even when the ammoniation conditions were least conducive to phosphate reversion. Observed placement effects arose mainly from the position of the phosphate since results obtained when the potash and nitrogen were applied separately were similar in trend to those obtained when all three nutrients were applied together. Results on the single alkaline soil tested were similar but less definite. The results reported in this paper point to the need for consideration of the placement factor in field comparisons of nonammoniated and ammoniated superphosphates, especially when more than 3% of ammonia have been added to the latter. A placement optimum for the nonammoniated superphosphate, or fertilizer containing it, may not be the best placement on a particular soil-crop combination for the ammoniated material.