Quantification of the compaction problem of selected vineyard soils and a critical assessment of methods to predict soil bulk density from soil texture

[摘要] ENGLISH ABSTRACT: Besides this overall abstract, each chapter has a separate.abstract under its own heading.Soil compaction is a problem common to many South African vineyard soils, and it has substantialadverse effects on root growth. Although literature reveals that much is known about the inherent soilproperties affecting compactibility, extrapolation beyond the sample population remains a problem.The permanence of expensive soil loosening actions is uncertain on some soils, because a measure bfthe bulk density to which the soil will recompact is not available. This study was conducted tq_docu-me-nt- th-e compaction problem in vineyard s_oils in a total perspective. The final obj~_ftive was to ~ ~ ·-··-----pr~9ict both maximum compactibility (MBD) a_~9 equilibrium field bul_~ Q~£l~i!Y_ (FBD) fr9_r:ri _soil_ textu_r~Idata, and to use other physical soil properties, e.g. soil structure, modulus of rupture, and air-to-water·---- - --------~·~ ----------permeability ratio as ~ackground information to explain _observed and predi60%, or tillage history. Someof the anomalies are outside the scope of the packing model.Some of the general conslusions of the study are listed below:(i) The 2 to 6 mm particle size class should be included for compaction studies and soils should beseparated into at least 1 o size fractions.(ii) Increasing coarse sand contents led to increasing BD's, while increasing clay contents and highercoefficient of kurtosis values led to lower BD's. _(iii) None of the relationships between predicted BD and Observed BD or any of the independentvariables was nonlinear.(iv) None of the measured structural characteristics could statistically be used in regression models todescribe variations in BD, and thus it was concluded that soil structural properties are notnecessarily related to compactibility.(v) An air-to-water permeability ratio of 40 is suggested as a threshold value for soil structural stability.(vi) Modulus of rupture, determined after 12 hours soaking, successfully identified soils that get veryhard upon drying.(vii) The claims that have been made in the literature for the suitability of soil texture as a characteristicto predict BD were justified for selected South African vineyard soils.(viii) Using the simple relationships, developed in this thesis, one can routinely predict MBD and FBDfrom textural data.(ix) The predicted BD's should, however, be interpretated in light of the current knowledge about therelationship between BD and root growth.(x) For screening purppses, e.g. to map a field in compactibility classes, only relative figures for BD aresuffice. (xi) Although several areas for future research in soil compaction are suggested, this thesis has made apromising start t6 the understanding and alleviation of this problem.

[发布机构] Stellenbosch University