Acidification Characteristics Of Red Soils Under Long-Term Fertilization And PH Threshold Of Major Crops

Soil acidification has become one of the most serious issues that Red Soil Regions?main grain producing areas in south China?are facing,which has endangered food security and restricted the sustainable development of agriculture.However,to date it is unclear about the acidification rate and trend of red soils,and the response relationship between crop yield and soil pH.In this study,25 long-term fixed-location monitoring sites,9 long-term fertilization experiments,274 soil samples from Qiyang County,Hunan province,5 parent material pot experiments and more than 100 published articles were used.By meta-analysis the acidification of red soils was characterized,and pH threshold of 4 main crops?wheat,maize,rice and soybean?was determined,and the effect of exchangeable Al:Ca ratio on crop yield was assessed.The study provided a quantitative target for acidic soil pH promotion by liming,and also a scientific basis for soil quality assessment and fertilization in acidic soils.The major results and conclusions were as follows:?1?The acidification of red soils proceeded in two stages:slow and fast acidification occurred in 1980-1990 and after 2000,respectively,where average 0.85 pH units were decreased at about 42%monitoring sites within 15 years?annual decrease of 0.01?0.12 pH units?.Soil acidification should go hand in hand with the application rate of chemical nitrogen fertilizers,in which unobvious acidification was observed under the application of 90 kg N ha^-1 y^r?-1?,whereas significant acidification occurred under the application of more than 100 kg N ha^-1 y^r?-1?,with the decrease in soil pH even more than 2 units.On the contrary,manure fertilization could effectively relieve or inhibit soil acidification.Thus,total application rate of 150 kg N ha^-1 y^r?-1??chemical N fertilizer:90 kg N;organic N fertilizer:60 kg?was recommended in Red Soil Regions.The acidification trend of soils in Qiyang County was predicted using surface pH values of uncultivated land adjacent to croplands as their initial pH,showing that soil pH in 77%of towns would become strong acid?pH<4.5?within 50 years even within 10-20 years.?2?Plotting the crop yield against the soil pH often yields an S-shaped curve,by which soil pH threshold?at 95%maximum yield?of major crops?wheat,maize,rice and soybean?was determined as follows:wheat 5.39>maize 5.29>soybean 5.10>rice 5.07.Soil pH threshold could be considered to use as taget value for the management and control of acidic soils.Those factors and their critical values significantly correlated to crop yield were also determined as follows:exchangeable aluminum?Exc Al?<1.24 cmol kg^-1,exchangeable calcium?Exc Ca?<10.45 cmol kg^-1,exchangeable magnesium?Exc Mg??0.74 cmol kg^-1,soil organic matter?SOM?? 12.58 g kg^-1,and available phosphorus?Ava P?? 12.22 mg kg^-1.Further study showed that SOM,Ava P and Exc Mg were not limiting factors for crop growth but both Exc Al and Exc Ca drove the response of crop yield to soil pH during soil acidification,with the critical value of Exc Al:Ca ratio of 0.108.?3?There were obvious differences in critical values of soil pH,Exc Al,Exc Ca and Exc Al:Ca ratio in red soils developed from different parent materials,where crop grown on red soil developed from red sandstone showed the stronger acid resistance,whereas it from plate shale exhibited the most weakest acid resistance.For the former these values were 4.95?5.45,2.30?4.36 cmol kg^-1,3.79?5.12 cmol kg^-1,and 0.15y0.25,respectively;For the latter they were 5.51?5.70,0.15 cmol kg^-1,7.07-7.28 cmol kg^-1,and 0.006?0.08,respectively.