Nutrient Accumulation And Ion Exchanging Properties In Soil Under The Sunlight Greenhouse Cultivation

The cultivating areas of sunlight greenhouse in China during the past 20 years haveincreased rapidly,and it has become one of main types of protected cultivation in China.Mostfarmers have paid a great attention to the application of fertilizer in sunlight greenhouses.Excessive application of fertilizers is very common in most regions.It resulted in a series ofproblems,including nutrient imbalances,over-accumulation of salts and harmful substancesin soils,low use efficiency of fertilizers,degradation of soil quality.These problems havelimited the sustainable development of sunlight greenhouse in China.Although someresearchers had dealt with these problems,most of them have concentrated on saltaccumulation in soil and the optimum application of fertilizers in greenhouses.Ion exchangein soil and solution interface is one of major exchange way of substances between soilparticles and growing plant roots in terrestrial ecosystem.Ion exchange in soil has a closerelationship not only with the supply of nutrient in soil,but nutrient movement andtransformations in soils.Obviously,the excessive application of fertilizers in sunlightgreenhouse will affect the ion exchange balances in the soils.The problems taken place insunlight greenhouse is the phenomena of ion imbalances in soil.However,there are limitedstudies about the effect of sunlight greenhouse cultivation on nutrient accumulation and ionexchanging properties in soils.Therefore,we carried out this research in the sunlight greenhouse bases located inGuanzhong plain,Shaanxi Province with the different methods(chemical analysis,incubationmethod,and physi-chemical methods)to study the effects of sunlight greenhouse cultivationon the nutrient accumulation and ion balances(K⁺,Ca²⁺,Mg²⁺)in the soils,and effects ofaddition of different forms of nitrogen fertilizers(urea,ammonium carbonate,and ammoniumsulfate)and phosphorus and potassium fertilizers on the electric conductivity(EC)and ioncompositions and ratios of soil solution,and the effects of different concentrations and ratiosof K⁺ and Mg²⁺ions on the adsorptions of K⁺,Ca²⁺and Mg²⁺ions by the soils.The majorconclusions were as follows:(1)The EC and concentrations of K⁺,Na⁺,Ca²⁺,Mg²⁺,NO₃^-ions and ratios of K⁺/Ca²⁺, K⁺/ Mg²⁺in soil solution of greenhouse obtained with the centrifugation method withoutadditioin of fertilizer were 2.5,95.0,16.6,1.9,3.2,4.0,31 and 39 times of that in arable soil,respectively,indicating the serious accumulation and imbalances of ions in the sunlightgreenhouse soil in research regions.(2)Compared to the openland soils,the sunlight greenhouse cultivation significantlyincreased the contents of exchangable K⁺ and Mg²⁺ions in the soils,and had not significanton the content of exchangable Ca²⁺ions in the soils.On the contrary,the sunlight greenhousecultivation significantly decreased the saturation of Ca²⁺ions in soil colloids,and the ratioesof Ca/K and Mg/K in the soils.This is related to the excessive application of K fertilizers inthe sunlight greenhouses.Tomato deficient in magnesium usually occurs in some greenhousesin the research region.This could not be explained by the contents and saturation ofexchangable Mg²⁺ions in the soils,because of higher levels in greenhouse soil than that inopenland soil.The ratio of Mg/K in the greenhouse soils was lower than the openland soil.Imbalance of Mg/K in the greenhouse soils is considered as the major reasons for magnesiumdeficiency in tomato due to the antagonistic interaction between K⁺ and Mg²⁺ions and highneed of Mg²⁺ion by tomato.(3)Addition of different forms of nitrogen fertilizers only had some effects on the EC ofsoil solution in the first week of incubation,after that the differences among the differentforms of N fertilizers were not significant.The EC of soil solution was significantly increasedas the increasing of N application rate,indicating the high contribution of nitrogen rate to theaccumulation of salts in the soil.The nitrate concentration in soil solution was increased withthe rates of N fertilizer added and the incubation time.The application of N fertilizer alsoincreased the concentrations of K⁺,Na⁺,Ca²⁺and Mg²⁺in soil solution,especially theconcentration of Ca²⁺and Mg²⁺ions.More attention is need to pay to the effects of Napplication rates on the accumulation of salts in soil,and the leaching of K⁺,Na⁺,Ca²⁺andMg²⁺from soil solution.(4)The application of P and K fertilizers had different effects on ion composition of soilsolution.Addition of P fertilizer significantly decreased the EC and concentrations of Ca²⁺,Mg²⁺ions both in arable and greenhouse soils,its effect on concentrations of K⁺,NO₃^--N andNH₄⁺-N ions in the soils was not significant.As the increasing addition of K fertilizer,the ECand concentrations of K⁺,Na⁺,Ca²⁺,Mg²⁺ions and ratios of K⁺ / Ca²⁺,K⁺ / Mg²⁺in solutionsof arable and greenhouse soils increased.It is concluded that adquate application of Kfertilizer is important in preventing the excessive accumulations of salts,cation imbalancesand poor structure of greenhouse soils.(5)The activity ratios between K⁺,Ca²⁺,and Mg²⁺ions of two greenhouse soils used(i.e., Loess soil and fluvo-aquic soil)at the point where no change of exchangeable K⁺ takes place(AR₀)were about 14.8 and 6.9 higher than the corresponding open land soils,respectively.The buffer capacities of greenhouse soils to add K+ were lower than that of open land soil.The changes of Gibbs free energy of K⁺,Ca²⁺,and Mg²⁺exchange(â–³G)of greenhouse soilsindicated these soils had the potential deficiencies of Ca²⁺,and Mg²⁺induced by the excessiveapplication of potassium fertilizer.(6)Ion adsorption isotherm curves of K-Mg and K-Ca both in greenhouse and openlandsoils followd the S-shape curve.As the increasing of K⁺ ion saturation in soil particles,theselectivity coefficients of the K-Mg and K-Ca equilibria decreased.The selectivitycoefficients of the K-Mg and K-Ca of the preference and non-preference isotherms indicatedthat the soils used had a turn-point for selectively adsorbing K⁺ ions.For K-Mg equilibrium,the exchangeable K⁺ ions in soils corresponded for the turn-points for selectively adsorbingK⁺ ions were 1385.5,1355.5,1007.1 and 1106.3 mg/kg for the Loess soils in openland andgreenhouse,and the fluvo-aquic soil in openland and greenhouse,respectively;For K-Caequilibrium,the exchangeable K+ ions in soils corresponded for turn-points for selectivelyadsorbing K⁺ ions were 580.3,575.6,415.4 and 355.1 mg/kg for the Loess soils in openlandand greenhouse,and the fluvo-aquic soil in openland and greenhouse,respectively.When theconcentrations of exchangeable K+ ions in soils were lower than the correspondedexchangeable K⁺ ions at the turn-points,K⁺ ions could exchange the exchangeable Ca²⁺,andMg²⁺ions adsorbed by soil colloids.