The Critical Value Of Soil P Leave For Corp And Environmental Safety In Different Soil Types

Sufficient soil phosphorus(P) is important for achieving optimal crop production and good quality, but excessive soil P levels may create a risk of P losses and associated eutrophication of surface water environment. It is of great significance to determine a reasonable level according to the critical soil P levels for crop yield and environmental safety of different soil to ensure food and environmental safety. This research based on a 24-year long-term fertilization experiment on ‘The national fertility and fertilizer efficiency monitoring base of loess soil’ in Yangling, Shaanxi province, long-term history data and the related analysis of soil were used to determine the critical value of soil Olsen-P for crop yield and soil P leaching threshold. Laboratory test were used to study the P leaching critical value, and designed to assess the main factors that affect the risk of P leaching by 18 kinds of farmland soils widespread all over of China, of which the corresponding physicochemical properties such as soil pH, organic matter, CEC, active Fe and Al, exchangeable Ca and Mg, and clay content were also analyzed to research the relationship between soil properties and the critical value. Pot experiments were carried out to determine the critical soil P levels for winter wheat and summer corn yield of 8 different soils. Conclusions were as follows:(1) The critical value of soil Olsen-P was 26.2 mg·kg-1 for winter wheat, and 13.9 mg·kg-1for summer maize on Lou soil. There existed an evident change-point on soil Olsen-P along with the increasing of soil total P, which was total P at 1.15 g·kg-1 and Olsen-P at 34.8 mg·kg-1, and the Olsen-P increased greatly with the increasing of soil total P above the change-point. Similar change-point was found also between soil Olsen-P and CaCl2-P, the risk of P leaching increased greatly when Olsen-P was above 39.3 mg·kg-1. Under the condition of no rotation of winter wheat-summer maize, the application rates of phosphorus fertilizer(P2O5) were 80~95, 60~80 and 10~15 kg·hm-2 respectively, when soil Olsen-P were <25 mg·kg-1, 25~40 mg·kg-1 and >40 mg·kg-1 for winter wheat. The application rates were70~85, 55~70 and 10~15 kg·hm-2 respectively, when soil Olsen-P were<15 mg·kg-1, 15~40 mg·kg-1 and >40 mg·kg-1 for summer maize. Under the condition of winter wheat – summer maize, the application rates of phosphorus fertilizer(P2O5) were 100~120, 90~100, 80~90, and 15~20 kg·hm-2 for winter wheat, and the application rates of phosphorus fertilizer(P2O5) were 50~60, 45~50, 25~40, and 5~10 kg·hm-2 for summer maize, when soil Olsen-P were <15, 15~25, 25~40 and >40 mg·kg-1, respectively.(2)Soil P leaching critical point varied considerably between different soils, Olsen-P changed between 23.64~144. 69 mg·kg-1, CaCl2-P changed between 0.12~2.01 mg·kg-1. The critical value decreased with soil pH, exchangeable Ca content, and increased with the active Fe and Al, clay content and initial Olsen-P level. The mathematical model based on soil properties(soil pH, organic carbon, activated alumina, exchange of calcium, carbonate, clay, initial Olsen-P and initial CaCl2-P)provide a reference for comprehensive evaluation of the influence of soil properties to leaching risk.(3) The critical levels of soil Olsen-P was 13.1~26.2 mg·kg-1 for winter wheat and 9.78~16.0 mg·kg-1 for summer maize. Combine with the previous research results of the field test, the agronomic threshold of soil Olsen-P was 12 mg·kg-1 for tide soil,and Olsen-P was 20 mg·kg-1 for most other soils.(4)The Leaching threshold of tested soil is about 1~4 times that of the agronomic threshold. The reasonable level of soil Olsen-P between Leaching threshold and agronomic threshold vary greatly from soil to soil, so the management of soil phosphorus should make different.For soils such as tide soil and grey desert soil who have a high leaching risk,the soil Olsen-P should be controlled in the range of 12~20 mg·kg-1. For acid soil such as red soil, the soil Olsen-P should be controlled in the range of 20~40 mg·kg-1.For calcareous soils such as Lou soil the Olsen-P should be controlled in the range of 20~40 mg·kg-1, but the phosphorus application rate should be less.