Studies On Effect Of Low-molecular-weight Organic Acids On Phosphorus Availability In Calcareous Chao Soil

Simulating the types and the concentrations of organic acids that plant roots excreted under phosphorus stress, the effect of low-molecular-weight organic acids on phosphorus availability in calcareous Chao soil been studied with equilibrium adsorption-desorption method, chemical extraction, soil inorganic phosphorus fractionation, respectively. The main results were as follows:1. When phosphorus isothermal adsorption curve described with Langumir equation, the adsorption reaction constant K, the max adsorption quantity Xm, and the max buffer capacity all reduced because of the presence of organic acids. In the neutral condition, all these organic acids could reduce phosphorus adsorption by calcareous Chao soil, and the extent of reduction is closely correlated with the kinds and concentrations of organic acids. The ability of organic acids to reduce phosphorus adsorption by soil was in the order of oxalic acid, citric acid > malic acid, tartaric acid > acetic acid; the extent of reduction increased with increment of organic acid concentration in adsorbing solution; the effect of organic acid on decreasing phosphate adsorption varied with the addition order of them. The relation between capacity of phosphorus adsorption and the addition order of them are: when the organic acid was introduced before phosphate addition< when phosphate was added after organic acid and discarded through centrifuging citric acid >tartaric acid.4. The available phosphorus content quickly increased by application of different synthetic phosphate, and then quickly decreased by time, and were steady after 30-45 day. The organic acids inhibited the trend of available phosphorus decreasing by time, so available phosphorus kept a high level in a long time in soil. The inhibiting ability of organic acids followed the order: oxalic acid, humic acid > citric acid, malic acid > tartaric acid.5. In the tested calcareous Chao soil, the amounts of organic phosphorus (OP) components was in the sequence: middle active OP >active OP>middle stable OP>high stable OP. Organic acids increased the content of higher-mineralization-rate organic phosphorus (middle active OP and active OP), so improved organic phosphorus mineralized to inorganic phosphorus or directly absorbed by plant root system.6.Under phosphorus stress, organic acids increased the availability of phosphorus in calcareous Chao soil, and prompted the growth of crops.