Study On Soil-plant Nutrition Relationship And Soil Remediation Techniques In Citrus Orchard

Citrus as an important cash crop is widely planted in south China.Soil is an important part of citrus orchard ecosystem,which is the natural basis for all citrus trees to survive.The quality of soil in orchard will directly affect the normal growth of citrus fruit trees.However,due to the long-term application of chemical fertilizers,the soil structure of citrus orchards deteriorates,the degree of acidification intensifies,the ventilation is poor,and the organic-inorganic complex is destroyed.Moreover,most citrus orchards are located in hilly areas with high topographic position,and the orchard site conditions are poor,and the drought resistance and soil moisture conservation ability are low,which leads to the transformation of soil properties to adversity.Therefore,the imbalance of mineral nutrient supply leads to the decline of citrus yield and quality.The research was aimed at the problems existing in citrus orchard production,based on the production practice,the methods of field investigation,field experiment and indoor test were used to systematically study the important problems related to the improvement of citrus industry,such as soil and tree nutrient status,soil acidification treatment,soil structure improvement,soil nutrient regulation.The main results were obtained as follows:1.From 2017 to 2020,335 typical citrus orchards were surveyed in Hunan Province,which revealed the abundance and deficiency of soil-tree nutrients in the main citrus producing areas and identified the main problems in nutrient management of orchards.The soil acidification of citrus orchards in the whole province was serious,and the soil acidity was mainly affected by the exchangeable Al³⁺content in the soil.Moreover,soil p H of86.90%orchards was lower than 5.5.The fertility of soil nutrients was low and the spatial distribution unbalanced,which 65.52%,75.86%and 82.76%of the orchard soils were deficient in available Ca,Mg,and B.2.Soil organic carbon and soil aggregate stability associated with aggregate fractions in a chronosequence of citrus orchards plantations were discussed.Macroaggregates（>0.25mm）were predominant in all orchards and were significantly affected by plantation age.Soil aggregates composition,aggregate-associated C distribution and Contribution of soil aggregates to SOC decreased gradually with decrease in particle size.With the progression of plantation age of citrus orchards,soil MWD and GMD decreased but soil D and K increased.Multivariate statistical analysis showed that SOC in soil aggregates was positively correlated with soil MWD and GMD but negatively correlated with soil D and K,among which>2 mm fraction was the key to the stability of orchard soil aggregates.3.The interaction between soil,leaf and fruit nutrients and quality in citrus orchards was clarified by multivariate statistical analysis.The optimization proposal of soil and leaf nutrients was ultimately constructed when fruit was high-quality.Compared with the optimized value,adjusting soil p H,supplying organic fertilizer and medium trace element fertilizer,improving soil physical and chemical properties and fruit tree nutrition level are key to fruit quality management in the future.4.Objective of the experiment was to develop the preparation materials of high-quality soil conditioner,six kinds of adsorption materials,such as agricultural waste rice husk ash,were selected to compare the adsorption and desorption characteristics of phosphorus.SEM showed that the surface structure of rice hull ash was melt shrinkage,the interlayer was hollow and porous,and the cavities were irregular.According to BET specific surface area analysis,the specific surface area and micropore volume of rice hull ash were 254.648 m²/g and 160.465 m²/g,respectively.The measured phosphorus adsorption curves of adsorption materials were most consistent with Langmuir isotherm adsorption equation,which indicated that the adsorption of PO₄^3-was realized by chemical adsorption of monolayer.The adsorption kinetics characteristics of phosphorus were discussed by changing the adsorption time of phosphorus.The results indicated that the stable adsorption mechanism of single-molecule chemical adsorption dominated the adsorption of PO₄^3-,and there were heterogeneous multi-layer physical adsorption.5.Acid-base titration curves characterized the effects of different control measures on the acid buffer performance of citrus orchard soil.The results showed that the acid-base buffer capacity of yellow soil was significantly higher than that of yellow sand soil,and the tested soil belongs to acid-base sensitive soil.The acid-base buffer curves of soil were obviously inverted S-shaped,but the buffer performance of acid-base sedimentation was quite different among different control measures.With the increase of acid-base titration,the change of p H value showed a weakening trend,which was concentrated in p H>9 and p H<3,that is,the soil in this p H range had strong buffering capacity to acid and alkali.6.The indoor soil culture experiment was used to study and compare the effects of different control measures on acidification and structural improvement of citrus orchard soil.Seven kinds of single and mixed modifiers with different kinds and characteristics,such as organic fertilizer,biochar,rice husk ash,lime,dolomite powder,calcium magnesium phosphate fertilizer and soil conditioner,were selected to improve the dry land soil of citrus orchards with different parent materials.The application of alkaline improver could neutralize soil H⁺,increase soil p H and reduce exchangeable acid content,while the application of bio-organic fertilizer could increase soil organic matter content.Different modified materials had different effects on increasing the content of soil water-stable large aggregates,and rice husk ash was superior to organic fertilizer and biochar in accumulating water-stable large aggregates in soil.7.The effects of organic and inorganic fertilizers combined with alkaline materials on soil-tree improvement in citrus orchards were studied through a two-year field optimization fertilization experiment.Compared with farmers’customary fertilization,the recommended fertilization package treatment could significantly reduce the hardness of 0-30 cm soil layer,increase soil porosity,soil agglomeration and soil structure stabilize,improve the current situation of soil acidification and enhance soil fertility.Organic and inorganic fertilizer combined with alkaline material fertilization package treatment could increase SFW,VC,TSS and TSS/TA ratio of navel orange,and reduce TA content.Comprehensive evaluation showed that treatment 5 had the best effect.