Effect Of Long-term Application Of Potassium On Soil K, Crop Yield And Quality In Selected Soils From North China

By using the eight fixed sites K fertilization field trials established in 1992/1993 in Northeast(Jilin, Liaoning and Heilongjiang), Northcentral(Shanxi and Hebei) and Northwest(Ningxia,Qinghai and Xinjiang) regions, effect of long-term application of K fertilizer and wheat straw returned to soil on soil K status, on K fixation capacity of soils, and on crop yield and quality were investigated. The fixed site field trials were selected to represent the main cropping systems(single cropping of winter wheat, single cropping of spring maize, winter wheat and summer maize double cropping) and main soil types in the north of China. The treatments in the fixed site field trials include check with N and P fertilizer applied only(NP), KC1 application with different rates(NPK, or NPK1 and NPK2), and with wheat straw returned to soil(NP+St, NPK+St). Soil samples were taken from 0-20 cm and 20-40 cm in soil profile from field plots with different treatments. By using different extraction solutions, the amount of soil K in different forms, their distribution in various forms and their availability to plants were evaluated. Potassium fixation capability of the selected soil samples were determined by soil K adsorption study. The main findings were as following:1. Compared with NP treatment, long-term application of K fertilizer and wheat straw increased content of water-soluble K, non-specifically adsorbed K, non-exchangeable K, mineral K and total K of the soil. But the content of specifically adsorbed K was not significantly affected, indicating that the amount of specifically adsorbed sites for K, the wedge site of the 2:1 clay minerals, was not affected by the different treatments. The effects of K fertilization and straw returned to soil on different forms of soil K and their distribution varied greatly from location to location because of the difference in soil and the environment conditions. Potassium input either with KCl or straw showed greater influence on different form of K in top layer(0-20cm) than that in 20-40 cm layer soil. The effect of KCl application was more significant compared with straw returned to soil treatment. Potassium input from either K fertilization or straw returned to soil significantly increased the proportion of water soluble K and non-specifically adsorbed K, hence increased the proportion of available K in the total soil K. Significant correlation was found between amount of potassium taken up by crops and the available K and slow released K levels in the top layer soil. The effect also differs from site to site because of differences in cropping systems, in climatic conditions, in soil clay mineralogy.2. Potassium fertilizer application and straw returned to soil increased plant K uptake. The amount of plant K uptake in NP, NP+St, NPK1 treatments in all field trials were greater than the corresponding K input, resulting in a negative balance of soil K budget, which was more severe with NP treatment. Positive balance was only found with NPK2 and NPK-St treatments. The highest removal of soil K was found from the field trial of winter wheat-summer maize double cropping system in Hebei province of Northcentral region, with annual removal of 200-400 kg K₂O/ha. Straw provided part of the K uptake by crops, reduced the depletion of soil K. Compared with available K and slowly released K levels in the original soils collected at the beginning of the fixed site experiments, the available K and slowly released K contents in the treatments without K input(NP treatment) or with low level of input(NP+St) were reduced, while that in high K input treatment(NPK2 or NPK+St) increased. Change degree of soil available K was-51.9 mg/kg～40.6 mg/kg and slowly available K was-158.6 mg/kg～71.0 mg/kg in treatment of NP or NP+St, however, change degree of soil available K was 14.1 mg/kg～131.4 mg/kg and slowly available K was-8.6 mg/kg～150.7 mg/kg in treatment of NPK2 and NPK+St. The influence of K input on soil K status varied remarkably due to the differences in K input, K uptake by crops, and the dynamic process of K transformation in the soils.3. Significant different K fixation capacity among the selected soil samples were found, with clear trend of K fixation capacity increasing when moving the location from the west toward the east. When the K concentration in the surrounding solution was 1000 mg/L, the average K fixation by soils from NP treatment were 348.5 mg/kg,442.9 mg/kg,468.5 mg/kg for soils from Northwestern, Northcentral and Northeastern regions, respectively. Continuous application of K fertilizer or straw returned to soil significantly reduced K fixation capacity of the soils. Soil K fixation capacity increased with the increase of K concentration in the surrounding solution, but the percentage of the total K in the solution fixed by soils was reduced with increase of K concentration. The K fixation capacity in subsoil layer(20-40 cm) was greater than that in the top layer soil(0-20 cm). Significantly negative correlation was found between soil K fixation capacity and soil available K, slowly released K, and soil organic matter contents, which was influenced by the different soil conditions at various locations.4. Long-term application of K fertilizer and straw returned to soil also affected content of other nutrients in the soil. Soil active organic matter and total organic matter contents were increased with both K fertilization and straw returned to soil treatment, with straw returned to soil showed more significant influence. Continuous application of K fertilizer and straw returned to soil reduced soil NH₄⁺-N, available Ca and Mg contents, which might be attributed to the possible competition of added K⁺ with NH₄⁺, Ca²⁺ and Mg²⁺ for the same adsorption site in the soil colloid system. The total P content in treatment with K fertilizer application and with straw returned to soil decreased, but the available P increased. The possible reason was that the balanced fertilization in NPK treatment and the addition of organic matter in the straw returned to soil treatment increased total P removal from the soil with increased crop yields, at the same time, mobilized P in the soil-crop system. Significant positive correlation was found between soil available K and soil active organic matter, while negative correlation found with NH₄⁺-N, available Ca, available Mg and available B contents.5. The effect of K fertilizer application and straw returned to soil on crop yield varied with location and cropping system, and influenced by potassium supplying power of soil. In general, application of K fertilizer and straw returned to soil increased crop yield. The average yield increase by K fertilizer application ranged from 5% to 8% for winter wheat and 12% to 19% for summer maize in field trials from Northcentral region, and that for spring maize located in Northeastern region ranged from 10% to 20%. The effect on crop yield in Northwestern region was relatively small, with average yield increase from 4% to 6% for wheat and from 3.0% to 6.5% for maize. No significant yield increase from K application was found in the field trial located in Qinghai province. It appeared that maize was more tentative to K application than wheat, and double cropping system in Northceantral region accelerated soil K depletion and resulted in significant crop yield response to K application soon after the establishment of the field trials. Application of K fertilizer and straw returned to soil treatments significantly increased K content in the plants and the total K removal by crops. More than 80% of K in plants was in the straw or stalk. Application of K fertilizer and straw returned to soil also increased content of N, P and the secondary nutrients and micro-nutrients in kernels, but reduced content of most micro-nutrients in leaves and stalks, indicating that K application might have improved the translocation of nutrients from vegetative parts to kernels.6. Long-term application of K fertilizer and straw returned to soil increased content of crude protein, lipid, soluble sugar and amino acid of wheat and maize, and wet gluten of wheat. But the effect was reduced when K fertilizer was over used(in NPK2 treatment). The effect varied from location to location, and the K fertilizer treatments generally showed greater effect than straw returned to soil treatment. The positive effect of K fertilization and straw returned to soil on crop product quality might attribute partly to the direct influence of improved K nutrition in the plants, and might also affected by improved balance nutrition and soil condition. The associated mechanism needs to be further investigated.