Residual effects of 20 annual applications of ammonium sulfate and triple superphosphate for corn on properties and productivity of oxic paleustults

Experiment consisted of 4 application rates of N and P fertilizer, i.e., 0-0, 60-60, 120-120 and 180-180 kg N-P2O5/ha/year in the forms of ammonium sulfate and triple superphosphate. The pH of the surface soil and sub-soil was decreased with increased rates of the fertilizers with more pronounced effects in the sub-soil. The EC of the surface soil was not affected by the fertilization while that of the sub-soil was increased with increased rates of the fertilizer. The CEC of the surface soil was not affected by the fertilizers while that of the sub-soil decreased with increased rates of the fertilizers. The OM and total-N contents of the surface soil tended to increase with increased rates of the fertilizer while those of the sub-soil were not affected by the fertilization. The available P of the surface soil was dramatically increased with increased rates of the fertilizers while that of the sub-soil was less affected. The exchangeable K, Ca and Mg either tended to decrease or significantly decreased with increased rates of the fertilizer, with more pronounced effect in the sub-soil. The DTPA-extractable Fe of the surface soil was increased with increased rates of the fertilizers while that of the sub-soil was not affected by the fertilization. The DTPA-extractable Mn and An of the top-layer soil was slightly increased with increased rates of the fertilizer whereas those of the middle-layer soil were similarly decreased by the three application rates o fertilizer and those of the bottom-layer soil were not affected. The DTPA-extractable Cu of soils of the top and middle layers was slightly increased with increased rates of the fertilizer whereas that of the bottom-layer soil was slightly decreased with increased rates of the fertilizer. The bulk density of the surface soils showed slight trends to decrease with increased rates of the fertilizers while that of the sub-soil showed no effect of the fertilizers. The aggregation of the soil was decreased with increased rates of the fertilizers. The hydraulic conductivity of the saturated soils was not affected by the fertilization. The infection on corn roots of arbuscular mycorrhizal fungi (AMF) showed consistent trends to decrease with increased rates of the fertilizer whereas AMF spore intensity in the soil was not affected by low rates of the fertilizer but decreased with increased rates of the fertilizer at high rates of fertilizer. The population of the free-living N2-fixing bacteria in the soil showed consistent trends to increase with increased rates of the fertilizer. The productivity of the soil was increased with increased rates of the fertilizers.