ObjectivesUnderstanding the effect of soil initial acidity on the potassium availability will provide basis to remediate acidified soil.

MethodsA typical quaternary red clay soil in Qiyang County, Hunan Province, was acidified into soil pH gradient of 4.0, 4.5, 4.8 and 5.2 (refer to acidified). For each pH gradient, half of the soil was added with lime to regulate the pH to a final value of 6 (refer to limed). A winter wheat pot experiment was then conducted using the treated soils.A nylon root bag was placed inside each pot, and 1.65 kg of the treated red soil was add into the pot (150 g in nylon bag). Wheat biomass and potassium uptake were investigated after 80 days' growth of winter wheat, and the changes of potassium content in different forms of rhizospheric and bulk soils of winter wheat were measured and compared.

Results1) The biomass of winter wheat (roots and shoots) was significantly higher in all the limed treatments than in the corresponding acidified treatments; The biomass was positively and significantly (P < 0.05) correlated to the soil initial pH value for both acidified and limed soils, but it was only positively and significantly (P < 0.05) correlated to the potassium uptake for acidified treatments. 2) The concentration of soil available potassium was markedly higher in bulk soils than those in corresponding rhizospheric soils for both sets of soils except for acidified soil of pH 4.0 where the same value was observed; soil available potassium in rhizospheric soils of acidified treatment and in bulk soils of limed declined with increasing initial soil pH value. 3) Soil potassium ion saturation was significantly higher in bulk soils than in corresponding rhizospheric soils for both sets of the soils; Soil potassium ion saturation in rhizospheric soils of acidified treatment declined with increasing soil pH value; whereas it showed an increasing trend in bulk soils of both acidified and limed treatments. 4) The winter wheat biomass was positively and significantly (P < 0.01) correlated to the deficit of soil available potassium in rhizospheric soil for acidified soils, and the deficit rate of soil available potassium in rhizospheric soil was positively and significantly (P < 0.01) correlated to either the potassium uptake by winter wheat, or the deficit rate of potassium ion saturation of rhizospheric soil for acidified soils. whereas, a significant positive (P < 0.05) correlation was observed between the deficit rate of soil available potassium and its initial soil pH value in rhizospheric soil for limed soil.

ConclusionsWithin the pH range of the experiment, under the acidifying condition, available potassium in rhizospheric soil increased with the decreasing soil pH value, but the amount of potassium uptake by winter wheat seedlings and their biomass were increased with increasing soil pH. This discrepancy suggested that it is the soil acidity that is governing potassium absorption of winter wheat on acidified red soil, rather than soil available potassium. Lime application could reduce the acidity of soil, therefore efficiently enhance the growth of winter wheat and the amount of the potassium absorption. Liming should be carried out best before the pH value dropped down to 5.0, and after liming, appropriate amount of potassium containing fertilizer is needed to ensure the desired soil available potassium level.