Ammonium Orthophosphate and Ammonium Polyphosphate as Sources of Phosphorus for Potatoes

A field experiment was initiated in 1978 to compare the effects of granular diammonium phosphate (DAP) and liquid ammonium polyphosphate (APP) on growth, nutrient content, and yield of potatoes (Solanum tuberosum L.). Two sites on a coarse-textured Inceptisol with markedly different cropping histories were selected, one with 5 ppm P, the other with > 200 ppm P extractable with 0.05N HCl + 0.025N H₂SO₄. Fertilizers were banded at P rates of 0, 56, 112, and 168 kg/ha at the low P site, and 0, 24, and 48 kg/ha at the high P site. At the low P site tuber yields and tissue P concentrations were lower with APP than with DAP, while tissue concentrations of Ca, Mg, Mn, and Zn were higher with APP. At the high P site, tuber yield and top dry weight were not affected by P source, but tissue Ca and Mg were higher and tissue P lower with APP. In 1979 and 1980, DAP and APP were applied at a rate of 56 kg P/ha at the low P site. Tissue levels of P again were lower while those of Ca, Mg, Mn, and Zn were higher with APP both years. Tuber yields were lower with APP in 1980. In 1980, DAP and APP were applied at the high P site at the same rates used in 1978. Yields were unaffected by P source but tissue P was lower and tissue Zn was higher with APP. Extractable P levels in soil samples taken after harvest were higher in 1978 with APP. With time, extractable P declined for both sources, and by the following crop season, there were no differences in extractable P due to P source. Extractable P again was higher in 1980 where APP had been applied. The results showed that availability of fertilizer P for winter-planted potatoes was less with APP than with DAP. Less P was found in tops of potatoes with APP regardless of the level of available soil P. Tuber yields, however, were less affected by P source, and the data suggest that most years' tuber yields will not be different with these two sources except on soil with very low levels of available P. Application of APP also resulted in increased uptake of Ca, Mg, Mn, and Zn by potatoes.