Organic Matter Solubility and Soil Reaction in an Ammonium and Phosphorous Application Zone

Organic matter plays a vital role in soil chemistry and ecology. We studied the effect of N and P fertilizers on soil organic‐matter solubility and pH in a Kennebec silt loam. Soil columns, 7 cm in diameter, were treated with 20 combinations of NH 4 OH‐N and either monoammonium phosphate (MAP) or triammonium pyrophosphate (TPP), then incubated from 0.5 to 16 d. As NH 4 OH‐N rates increased from 0 to 600 to 1200 to 2400 mg kg −1 , dissolved organic matter (DOM) in leachate from the application zone (2.5 cm above and below the application point) increased from 62 to 199 to 389 to 564 mg kg −1 , respectively, averaged over all incubation periods and P treatments. As MAP rates increased from 0 to 300 to 600 mg P kg −1 , DOM in leachate increased from 167 to 308 to 344 mg kg −1 , respectively, averaged over all periods and N treatments. Corresponding rates of TPP resulted in 167, 289, and 410 mg kg −1 of DOM. When NH 4 OH‐N and either P source were applied in combination, interactions between the fertilizers usually synergistically increased organic‐matter solubility, often by a factor of 1.5 to 2.0. Leachates were colored shades of amber, brown, or black depending upon the amount and nature of the DOM. As NH 4 OH‐N rates increased from 0 to 600 to 1200 to 2400 mg kg −1 , pH of leachate from the application zone increased from 6.2 to 8.1 to 9.0 to 9.4, respectively, when averaged over all incubation periods and P treatments. Both P sources decreased pH. Increases in DOM were highly correlated to increases in pH. The R 2 values for the regression of DOM on pH ranged from 0.83 to 0.99.