Author : Tattao, Dante A.

Summary : Studies were made on Packchong clay, reddish brown lateritic soil (Paleustult) at the National Corn and Sorghum Research Center, Packchong, Nackon Ratchasima, Thailand to evaluate the effects of long-term nitrogen-phosphorus (N-P) fertilizer, cropping systems, and rainfall on corn yields and soil properties. One study compared application four rates of N-P (subscript 2)O (subscript 5) (O-O, 60-60, 120-120, and 180-180 kg/ha) over 10-year period. The other study compared seven treaments of sole corn and its rotation or intercropping with mungbean, soybean, or peanut with or without fertilization for seven years. When the soil fertility was low, grain and stover yields did not correlate with rainfall. At high levels of N-P (subscript 2) O (subscript 5), however, yields responded to rainfall. The highest agronomic yield occurred with the addition of 120-120 fertilizer while the best economic yield was obtained with fertilization of 90-90 kg N-P (subscript 2) O (subscript 5). Soil organic matter content tended to increase with rate of N-P fertilizer. Soil mineral N and available P likewise accumulated with continuous application of N-P (subscript 2) O (subscript 5). All forms of hydrolyzable N fractions and inorganic P also tended to increase with rates of fertilizer application. Soil pH and surface soil extractable calcium (Ca) and magnesium (Mg) decreased at higher levels of fertilization. The level of deficiency, however, was not yet critical enough to warrant addition of these nutrients. The concomitant increase in the level of manganese (Mn) with fertilization may be a cause for worry in that toxicity may occur in the future. Aggregate stability, bulk density, and carbon-nitrogen ratio tended to decrease with fertilization over the period of ten years. The populations of mycorrhiza and asotobacter, were not affected by fertilization, although this may soon change when the soil becomes acidic enough to effect these microbes.