Nutrient Accumulations And The Relationship Between Potassium And Magnesium In The Soil Under Sunlight Greenhouse

In recent years, cultivating the vegetables in protected facilities such as sunlight greenhouses has increased rapidly, and this cultivating method has become an important part of modern agriculture in China. Development of protected cultivation of vegetables has tremendous impetus not only in developing of rural economy but also in increasing farmer's income. The crops cultivated in sunlight greenhouses have a short growth period; and the greenhouses are frequently planted, and heavy fertilized. Over-fertilization is a very common in the production. It results in the low yield and poor quality of the crops, and also the low soil quality, especially the imbalance of nutrients in soil, and the salinity and acidity of the soils. The deficiency of magnesium, which usually occurs in acid soil, is reported in the calcareous soil under sunlight greenhouse. Therefore, it is necessary to study the effects of sunlight greenhouse cultivation on the nutrient accumulation in the soils; and the effects of different application of K and Mg fertilizers on the contents and ratios of different ions in soils, and on the growth and nutrient uptake of tomato.In this research, we had studied the accumulation of nutrients in the soils under the sunlight greenhouses in Shaanxi Province, and evaluated the effects of the different ratio of potassium and magnesium nutrient addition on the content and ratios of different base cations in soil, and on magnesium uptake by tomato. The main results showed as follows:1. Compared to the arable fields, the contents of nutrients and the electric conductivity (EC) in the greenhouse soils were increased significantly with the cultivation of the crops in greenhouse; on the contrary, the soil pH was decreased. As the cultivation of the crops, the salt accumulation in the greenhouse soils was also increased. The salt content in the greenhouse soils with long-term cultivation period was significantly higher than that under short-term greenhouse, and also higher than that in the arable fileds. The changes of the exchangeable ions in greenhouse soils were different from that of arable soil. The content of exchangeable and water-soluble magnesium in greenhouse soils was as high as that in arable field soils. However, the ratio of exchangeable potassium and magnesium in the greenhouse soils of Chang'an District and Yangling District were greater than 0.5. It is concluded that the magnesium deficiency of tomato in the greenhouse may relate to the imbalance of the two ions in soils.2. The incubation experiment showed that the contents of nitrate, water-soluble salt and the exchangeable ions, EC in greenhouse soils under the incubation period were higher than that in the arable field; and the soil pH was lower than that in the arable filed. The content of NO3--N was increased significantly under the treatment added with nitrogen fertilizer after the 63-day of incubation. It indicated the ammonium nitrogen was changed into nitrate after 63-day of nitrogen mineralization; the soil pH was declined in all treatments, the EC was increased significantly. There was no fixed trend of the changes of content of water-soluble salt in all treatments during incubation. The content of exchangeable potassium increased in greenhouse soil, and the content of exchangeable magnesium was decreased in the treatment of adding NPK fertilizer but no magnesium fertilizer, but it was increased in the other treatments. The contents of the exchangeable potassium, calcium, and magnesium in the arable field were increased.3. After the 63-day of incubation, the ratio of potassium to magnesium was increased in the treatments added with the increasing application rates of potash fertilizer, whether with or without magnesium fertilizer, and lower than the treatment without fertlization. It showed that the application of magnesium fertilizer can adjust the ratio of potassium to magnesium in soil. It is considered that magnesium deficiency of tomato was induced by the excessive content of potassium in soil.4. Compared to treatment without any fertilizer, the dry yield of tomato in greenhouse soils did not increase significantly, indicating the contents of nutrients was rich in greenhouse soils. The dry weight of stem, leaf, root and total weight of tomato plant were increased when the potassium fertilizer was added and no magnesium fertilizer was added. Higher rates of potassium fertilizer rate decreased the dry weight of stem, leaf, root and total weight of tomato plant. Compared to the treatment without magnesium fertilizer, the height of plant, Chlorophyll content and rate of photosynthesis were decreased when magnesium fertilizer was added. This showed that the application of magnesium fertilizer inhibited the tomato to absorb magnesium and limited the growth of tomato. In a certain range, the increasing amount of potassium addition inhibited the magnesium uptake by the crops. The soil pH, the content of available phosphorus and potassium, water-soluble salts and the exchangeable ions in the soils of the different treatments were decreased after harvesting the tomato.