Growth response of rice (Oryza sativa L.) and corn (Zea mays L.) to phosphorus as influenced by soil agregation

Results of the study revealed that the soil aggregate fractions differed in both native and high P soil. The larger aggregates contained greater amount of clay, exchangeable Al and oxalate-extractable Fe than the smaller aggregates. Clay content, exchangeable Al and Fe in 4.8 mm aggregate fractions were about 22%, 49% and 8% higher than those of 0.106 mm aggregate fraction, respectively. The smaller aggregates on the other hand contained more organic carbon (OC) and total P than the larger aggregates. Total P content and OC of 0.106 mm aggregate fraction were about 11% and 19% higher than those of the 4.8 mm aggregate fractions, respectively. The Melich 1 extractable P was always greatest for the smallest aggregate regardless of whether extracted as is or extracted after short-term incubation with newly applied P. This trend was unafected by grinding of soil samples, indicating that factors other than exposed surface area contributed to the differences in extracted P among aggregates. The larger aggregates with higher clay, exchangeable Al, Fe probably contributed to stronger P holding resulting in less extractable P in those aggregates. In contrast, higher organic carbon with higher total P in small aggregates might have contributed to higher extractable P in these aggregates. The increase in extractable P by soil grinding and thorough shaking of soil in the extractant was very small and only significant in high P soil. Short-term phosphorus buffering coefficient also was not affected by soil aggregation. In greenhouse experiment, using flat clay pots minimized soil aeration related problems especially in small aggregates. The level of soil in the clay pots was kept at only 4 cm and the soil was kept moistened from the bottom by capillarity. Plant height, leaf area, shoot dry weight, root length, and dry weight and total P in shoot of both corn and rice were significantly increased with decreasing aggregate diameter from 4.8 mm to 0.106 mm. The greater plant height growth and total P in small aggregates were attributed to adequate soil aeration, longer and finer roots, more soil-root contact for water and nutrient including P and less impedance for root growth. The effect of soil aggregation on short-term plant P availability of newly added P was small because plant growth response was the same over all P rates. The results of this study point out that in Siniloan soil, an Ultisol, soil aggregation had little effect on short-term PBC [phosphorus buffering coefficient] and short-term plant P availability of newly added P in the soil. However, in high P soil, the current soil tests which requires grinding and shaking soil sample might over estimate available P status of the soil since some of this P located inside aggregates may not be available for the plant. These results are based in short-term soil incubation and about five weeks of plant growth in the greenhouse. Longer-term experiments may be needed to confirm these results