Comparison of different methods by means of which water holding capacity of soil is determined and the prediction of water holding capacity from soil texture in coarse-textured soil

[摘要] ENGLISH ABSTRACT:Irrigation scheduling is one of the most important cultural practices in irrigated vineyards. Water holdingcapacity of soil is arguably therefore one of the most important characteristics of a soil as it determineshow much water can be made available to the plant. The measurement of water holding capacity ofsoils is time consuming and costly. In situ determinations are often impractical to determine.For routine determinations, water holding capacity is therefore determined on disturbed samples. Sucha method for example is the rubber ring method. A great deal of criticism surrounds this rubber ringmethod and results are often questioned.The objectives of this study were therefore to determine what the relationship was between undisturbedand disturbed samples and to determine whether compacted samples could give a more accuraterepresentation of the water holding capacity of soil. Soil textural factors influencing the volumetric watercontent of undisturbed, rubber ring and compacted samples at 5, 10 and 100 kPa were investigated. Inaddition, soil textural properties influencing water holding capacity of the respective samples between 5and 100 kPa and 10 and 100 kPa were investigated. The final objective of the study was to developsimple models to predict the volumetric water content and water holding capacity of soil.Undisturbed and disturbed soil samples were taken at various localities to ensure a wide range oftextures. Water holding capacity of undisturbed and disturbed samples was determined at ARCInfruitec-Nietvoorbij using the standard air pressure and ceramic plate technique and the routine rubberring method respectively. Soil samples were also compacted to a bulk density of approximately 1.5g.cm-3 as a further treatment for determination of water holding capacity using the air pressure andceramic plate technique.To investigate aspects of soil texture that could possibly influence volumetric water content of the soil,correlations were done between different texture components and volumetric water content ofundisturbed, rubber ring and compacted samples at 5, 10 and 100 kPa. In order to determine the effectof texture on water holding capacity of the soil, correlations were drawn between texture componentsand water holding capacity of undisturbed, rubber ring and compacted samples between matric potentialranges 5 and 100 kPa and 10 and 100 kPa. The results from this study were used to develop models topredict volumetric soil water content and water holding capacity of soils for a range of soils.Volumetric water content of rubber ring samples at 5 kPa was more than the volumetric water content ofundisturbed samples at 5 kPa. The volumetric water content of rubber ring samples at 5 kPa and thevolumetric water content of undisturbed samples at 5 kPa was correlated by 87%. Volumetric water content of compacted samples at 5 kPa had a 85% degree of correlation with the volumetric watercontent of undisturbed samples. At 10 kPa, the correlation between volumetric water contentdetermined using rubber ring samples and undisturbed samples was 77%. This was identical to thecorrelation between volumetric water content of compacted samples at 10 kPa and undisturbedsamples. At 100 kPa, most of the rubber ring samples' volumetric water content fell below the 1:1 line ofvolumetric water content of undisturbed samples. The volumetric water content of all the compactedsamples was higher than that of the undisturbed samples.Water holding capacity of all the rubber ring samples between 5 and 100 kPa was greater than thewater holding capacity of the undisturbed samples between 5 and 100 kPa. Rubber ring samplestherefore generally overestimated the water holding capacity of the soil. The water holding capacity ofmost of the rubber ring samples between 10 and 100 kPa was greater than the water holding capacity ofthe undisturbed samples. In contrast, the water holding capacity of compacted samples between 5 and100 kPa was less than the water holding capacity of undisturbed samples between 5 and 100 kPa.Water holding capacity of compacted samples was therefore underestimated.The results from this study confirmed that the influence of clay and silt content on volumetric watercontent of undisturbed, rubber ring and compacted samples increased as the suction on the respectivesamples is increased. The influence of fine sand content on volumetric water content of undisturbed,rubber ring and compacted samples decreased with an increase in matric potential to 100 kPa. Mediumsand content of undisturbed, rubber ring and compacted samples had the greatest influence of all thetextural components on the volumetric water content of the respective samples at 5 kPa and 10 kPa.Water holding capacity of undisturbed, rubber ring and compacted samples between 5 and 100 kPa wasgreatly influenced by the fine sand content of the samples. Medium sand content of the samples alsohad an influence on the water holding capacity thereof.To predict the volumetric water content of undisturbed samples at 5, 10 and 100 kPa, the independentvariables were fine sand content, square root of medium sand content and In of medium sand content.In the case of models to predict the volumetric water content of rubber ring samples at 5, 10 and 100kPa, the same variables were used as independent variables. Additional variables such as silt content,the In of silt content, square root of clay plus silt content and the medium sand content. To predict thevolumetric water content of compacted samples at 5, 10 and 100 kPa the terms used were silt content,clay plus silt content, the e-clayplus silt content. medium sand content and the square root of medium sandcontent. The models to predict volumetric water content of rubber ring samples gave the bestcorrelation with the actual volumetric water content of rubber ring samples.The final models to predict the water holding capacity of all the samples between 5 and 100 kPa and 10and 100 kPa used only fine and medium sand parameters as independent variables.Soil textural components do play an important role in determining the volumetric water content ofundisturbed, rubber ring and compacted samples at 5, 10 and 100 kPa. The magnitude of the waterholding capacity between 5 and 100 kPa and 10 and 100 kPa is also influenced by soil texture. Themodels developed to predict the volumetric water content of samples at 5, 10 and 100 kPa and themagnitude of the water holding capacity between 5 and 100 kPa and 10 and 100 kPa could be veryuseful. Both time and money can potentially be saved. Models that can be highly recommended arethe models generated for the undisturbed samples.These are:At 5 kPa, VWCu = 0.47259 - 0.04712 medium sando.sAt 10 kPa, VWCu = 0.41292 - 0.04221 medium sandosAt 100 kPa, VWCu = 0.48080 - 0.00254 fine sand - 0.0865 In medium sandBetween 5 and 100 kPa, WHCu = -29.523 + 3.394 fine sandBetween 10 and 100 kPa, WHCu = -891.794 + 232.326 In fine sand + 38.006 In medium sand

[发布机构] Stellenbosch University