| Paddy-upland rotation is one of the dominant cropping systems in the southern regions of China, which plays an important role in increasing crops yield, maintaining soil fertility and ameliorating soil. Soil potassium deficiency in paddy fields is becoming the key limiting factor for sustainable agricultural production because of the consumption of enormous amounts of K for crop cultivation. Therefore, it has became the main reasonable measure of applying potash fertilizer to promote our agricultural development with deep significance of increasing crops yields, improving fertilizer efficiency and decreasing pollution under the condition of being short of potash resources in China. This paper studied the effect of K application on yield, economic profit, nutrient accumulation, K fertilizer efficiency, nutrient balance and soil fertility in rice-rapeseed rotation system, furthermore, the nutrient accumulation regularity of rice and rapeseed and the dynamic change of soil K was studied basing on them. The special purpose was to provide evidence for how to carry out reasonable application of K fertilizer in paddy-upland rotation system. The main results were as follows:1.In the experiment, the rice dry matter accumulated more and more with the plant growing. In the whole growth period, the amounts of dry matter accumulation in fertilization treatments were more than the control. The application of K increased yield and profit benefit significantly. The rice and rapeseed yields reached the highest increase up to 603kg/hm2 and 350 kg/hm2 brought by 150kg K2O/hm2 application. Besides,the application of the most K fertilizer in Qichun brought about the yield of rice and rapeseed up to 775 kg/hm2 and 393kg/hm2 respectively, which led to the best net income of 292yuan/hm2 and 895 yuan/hm2.2.The result showed that the nutrient content decreased with the crop growing, but the accumulation amount increased. The promotion of potassium fertilizer on nutrient contents of N and K were significant, but slight effect on phosphorus content. For the nutrient accumulation, fertilizing K not only enhanced the absorption of K, but also having significant influence on N and P accumulation.3.In rice-rapeseed rotation, K apparent efficiency grew up after dropped with K fertilizer continuously rising in rice season but kept steady in rapeseed season. The K use efficiencies of rice in two experiments were 51.7% and 61.3% and the ones of rapeseed were above 80%.The partial factor productivity and physiological efficiency fell down when K fertilizer rising during the two seasons.4.Two methods of calculating nutrient balance were compared by this experiment. The result showed that the normal calculating system just simply by the fertilizer input and crops output coud not truly reveal the nutrient condition of farmland ecosystem. This paper recommended a more accurate method which made inputs of the rice and rapeseed seedling, rice stubble and rapeseed stem as the total input and crops output as the total output. The nutrient banlance in rice-rapeseed rotation showed that N and P were in surplus, but the K in short in Wuxue and in surplus in Qichun when more K fertilization.5.Analyzing data of the soil K content, after rice harvest, the contents of available K and slow-available K in topsoil were reduced, so did of available K content in subsoil but not for slow-available K. K content in each section enhanced after rapeseed season.6.During the entire rice-rapeseed rotation, the K content of soil kept changing, and in the whole process K content in fertilizing treatment was higher than that in control with the transferring among the soil available K, slow available K and mineral K all the time.
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