Regional Evaluation of Winter Rapeseed Response to K Fertilization, K Use Efficiency, and Critical Level of Soil K in the Yangtze River Valley

The investigation was carried out to study the response of winter rapeseed to potassium （K） feritlization and the critical soil available K level for current winter rapeseed production in the Yangtze River Valley （YRV） of China. A total of 132 field experiments were conducted in fields of farmers in the major winter rapeseed growing areas in YRV in 2000/2001 and 2004/2005 to 2006/2007 during growing season. Results of these field experiments showed that the average rapeseed yield increment resulting from 100 kg K ha-1 application was 358 kg ha-1, an increase over the control CK （no K） of 18.0% in 2005/2006 and 2006/2007. The average internal use efficiency （IE） of K was higher in the CK treatment （21.9 kg grain, kg-1 K uptake） than in the ＋K （100 kg K ha-1） treatment （17.7 kg grain, kg-1 K uptake）. Winter rapeseed required 68.1 kg of K to produce 1 000 kg seed. The recovery efficiency of K fertilizer in rapeseed production averaged 39.3%. The K balance was negative, with an average net removal of 117.6 kg K ha-1 in the CK treatment annually, and 56.8 kg K ha-1 in the ＋K treatment. The results indicated that there was a significant negative relationship between yield increments by K application and soil available K content. Based on the relative yield of CK/＋K at 90% level, the critical level of soil available K （NH4OAc-extractable K） was 135 mg kg-1.

Biotransformation of Earthworm Activity on Potassium-Bearing Mineral Powder

This study analyzes the biotransformation of earthworms on K in potassium-bearing mineral powder （PBMP） under different PBMP recruitments. A mixture of PBMP （10% to 60% mass fraction） and decaying cow dung was used as feed for breeding the earthworms to study the potassium-releasing ability of earthworms on PBMP in soil. The mixture containing 20% and 30% PBMP resulted in good growth and propagation of the earthworms as well as higher conversion rates of potassium. Therefore, the optimum recruitments of mineral powder are 20% and 30%. The mixture of cow dung and PBMP was compared with the mixture of cow dung and corresponding proportions of quartz powder to analyze the conversion rate of earthworms on PBMP in different combinations. After the earthworms were raised with the mixture of cow dung and PBMP （8 ： 2 and 7 ： 3） for 30 d, the contents of rapidly available K and effective K were 10 824.3±35.9 and 11 688.4±16.1 mg.k-1 as well as 10 079.6±62.2 and 10 247.5±172.7 mg.kg-1, respectively. After the earthworms were raised with the mixture of cow dung and quartz powder （8 ： 2 and 7 ： 3） for 30 d, the contents of rapidly available K and effective K were 10 623.3±41.1 and 11 385.5±13.5 mg.kg-1 as well as 9 834.2±51.8 and 9 907.6±11.4 mg.kg-1, respectively. Thus, the contents of rapidly available K and effective K in the mixture of cow dung and PBMP were significantly higher compared with those in the mixture of cow dung and quartz powder （P〈0.05）. The increment contents of rapidly available K and effective K were 201.0 and 302.9 mg.kg1 as well as 245.4 and 339.9 mg-kg1, respectively. Therefore, earthworms can activate and trans-form K into effective K through feeding, digestion, absorption, and excretion. The results pro- vided a new idea of using earthworms to release potassium in low-grade potassium-bearing rocks and obtain the rapidly available K and effective K needed by plants.