Magnesium Supplying Ability Of Calcareous Soil Under Solar Greenhouse

Vegetable crop cultivated in solar greenhouses is rapidly expanding in northern China in recent decades and it plays an important role in supplying vegetables during the off-season.However,in order to achieve higher crop production,excessive fertilization is a common practice,and it results in over-accumulatioin of nutrients in soil of solar greenhouses,which has seriously restricted the sustainable agricultural development in solar greenhouse.Magnesium(Mg)deficiency of tomato in solar greenhouse under calcareous soil in northern China has become a serious problem in recent years,and is shaping up as a serious problem as the time goes on.Some studies suggested that appearance of Mg deficiency symptoms in calcareous soil may strongly be associated with over accumulation of nutrients in soil,particularly potassium(K).Therefore,this study was aimed to evalue the contents and ratios of K,Ca,and Mg in soil of greenhouse with different cultivation years.Moreover,the effects of Mg application on tomato yield and contents of K,Ca and Mg in soil were evaluated.The aim of the research was to reveal the relationship between contents of K,Ca,and Mg and their ratios impact on tomato growth in greenhouse soil.It can provide the scientific knowledge for balanced fertilization in Mg deficient greenhouse soil.The main conclusions are as follows:(1)We investigated the application of fertilizer in 70 greenhouses and determined nutrients contents in soil from 30 greenhouses.The results showed that the application rates of fertilizers differed greatly between different greenhouses.The average annual inputs of N,P2O5 and K2 O from inorganic fertilizers were 651,485 and 855 kg/ha,respectively.Nutrient input in greenhouse was significantly high.The contents of available nutrients in topsoil showed significant linearly positive correlations with cultivation years,and water soluble nutrients increased with increase in the cultivation years.The ratio of water soluble K/Mg increased with cultivation years;on the contrary,the ratio of Ca/K decreased,indicating the serious imbalance between cations.(2)The Mg application rate experiments were carried in seven greenhouses in Yangling facilities planting base.The experiment showed that potassium in soil remained at a high level and the contents of exchangeable Mg and water soluble Mg were both increased with increase of Mg dose.At the later stage of tomato growth,soil Mg content and saturation of exchangeable Mg increased with increase in Mg application rates.The tomato yield and soil Mg contents were fitted well with quadratic function.And there was negative correlation between yield and soil potassium content.In addition,the ratio of K/Mg was greater than 0.4 which could result in a potential risk of reducing Mg uptake by K.It appeared that Mg application had no significant effect on correcting the ratio between K and Mg due to the high soil potassium.(3)Pot experiment was carried out to study the effects of different application rates of Mg and K fertilizers on tomato growth,nutrients uptake of tomato and soil nutrients concentration.The treatments were analyzed using two factor(K,Mg)factorial design,with each factor including three levels.The results showed that application of potassium and magnesium increased dry matter of tomato.High potassium rate could low the plant height and stem diameter of tomato.Along with the increase of potassium rate,potassium concentration in plant organs increased;on the contrary,Mg concentration decreased at the same time,especially in plant leaves and fruits.In addition,tomato yield decreased significantly under higher level of K as compared to the lower lever K treatment.Soil exchangeable K and Mg increased as application of K and Mg increased.Compared with treatment without K application(K0),the ratio of K/Mg with high K application(K2)was increased by 20.7%.