Effect Of Soil Acidification On Magnesium Nutrition Of Pomelo And Mitigation By Lime And Mg Supplementation

In recent years,a series of agricultural problems caused by soil acidification has attracted widespread attention.China ranks first in the world in pomelo production,and Pinghe County（Zhangzhou City,Fujian Province）is the largest production pomelo base in China.Driven by economic benefits,excessive fertilizer input in pomelo orchards is more common,which results in soil acidification and seriously affects the sustainable development of pomelo production.Meanwhile,soil acidification leads to soil magnesium（Mg）deficiency,which causes the leaf chlorosis as a serious problem in pomelo production,thus limits fruit yield and quality of pomelo.In current research,took Pinghe County as the research area,firstly,soil and leaves samples from 101 pomelo orchards in 10 villages and towns were collected,and the soil samples from 30 unplanted crops（without fertilization）were taken as control,the characteristics of soil acidification in pomelo orchards and Mg nutrition in soil and leaves were monitored.Secondly,a field experiment using lime combined with Mg fertilizer was conducted,so as to provide theoretical and practical basis for promoting sustainable development of pomelo production.The main results obtained were as follows:（1）Soil acidification of pomelo orchards in Pinghe County is severe,and long-term over-fertilization as the main factor intensified the trend of oil acidification.The average soil pH values of 0-20 cm and 20-40 cm in pomelo orchards were 4.38 and 3.97,respectively,which were 0.61 and0.88 units lower than the control.The specific characteristics were as follows:the closer to the circle of fertilization site,the lower soil pH and base saturation（BS）,while the higher concentrations of exchangeable acid（H⁺）and aluminum(Al³⁺);and the worse trend of soil acidification in subsurface layer（20-40 cm）than that in surface layer（0-20 cm）.Soil exchangeable Mg concentration in 50.8%of 0-20 cm soil samples and 78.1%of 20-40 cm samples were below the critical level.36.6%of leaf samples were lack of enough Mg nutrition.As soil pH decreased,it enhanced the antagonistic effects among Al³⁺,Mn²⁺,H⁺and Mg²⁺,thus affected Mg uptake in the leaves.（2）The results of adsorption-desorption experiment showed that proper application of lime could increase adsorption and desorption of soil Mg and improved Mg bio-availability.However,over-use lime（more than3.6 g lime per kilogram of soil）could decrease the desorption amount and desorption rate of soil Mg within the same addition of Mg application rate,and also decrease the capacity of Mg available to crop.It is thus necessary to determine an appropriate dosage of lime in acid soils.According to the results from Mg adsorption-desorption experiment and field trial,the dosage of 1.8 g lime per kilogram of soil（that is equivalent to 1 kg of lime（calcium carbonate））is the ideal dosage to improve soil acidification and ensure soil Mg availability in acid soils.（3）Results from field experiment including two-growing seasons（2018 and 2019）showed that,liming significantly improved soil pH,soil exchangeable acid content.The order of positive effects on different soil layers in the orchard was 0-10 cm>10-20 cm>20-40 cm.Lime and Mg supplementation improved availability of soil N,P,K,Ca and Mg,while reduced the availability of soil Fe,Mn,Cu and Zn.Concentrations of N,K,Ca and Mg in the leaf,peel and pulp increased,however,those of Fe,Mn and Al in leaves,peel and pulp decreased.The pomelo yield ignificantly increased 30.6%by lime application and Mg supplementation,mainly due to increased numbers of fruits per tree,compared with control.In addition,lime application and Mg supplementation significantly increased edible rate,soluble solids and soluble sugar content in fruit,decreased titratable acid content in fruit.