Leaching of excess salts from the root zone of apedal soils

[摘要] English: In South Africa a huge amount of energy was spend on irrigation research over the past twodecades, mainly to optimise water application in order to prevent crop water stress. In thequest to conserve water for transpiration, researchers tended to neglect the importance ofdrainage or percolation, which eventually results in the accumulation of salts in the root zone.Salts also accumulate in the root zone where shallow water tables are present. Farmers alongthe Lower Vaal River expressed their concern about yield losses induced by build-up of saltsin the root zone. The detrimental affect of salinity on field crops are extensively reported inthe literature and the only way to address the problem is through leaching. Sustainableutilization of these saline or potential saline soils depends on adequate natural drainage orartificial drainage systems, which ensures a net downward flux of water and salts below theroot zone for optimum development and functioning of roots. This dissertation focusesmainly on the management of salts in the root zone of apedal soils.The research was conducted on two soil types (Clovelly and Bainsvlei) reconstructed in 5000litre lysimeters on the experimental farm, near Bloemfontein, of the Department of Soil, Cropand Climate Sciences (University of the Free State). A total of 30 lysimeters, 15 per soil typearranged in two parallel rows under a moveable rain shelter were used. It was assumed thatthe artificially prepared soil profiles are stable because more than 10 cropping cycles werecompleted before the commencement of this experiment.The first aim of Chapter 3 was to address the effect of irrigation water salinity on theaccumulation of salt in the root zone under shallow water table conditions. A total of 612 mmwas irrigated with irrigation water salinity treatments that varied between 15 and 600 mS m-1.Results showed that in the absence of drainage, salts will accumulate in the root zone at analarming rate. In fact, salinity of the soil water almost doubled with respect to that of theirrigation water during only one growing season. These various saline profiles were used tocharacterise the impact of soil water salinity on the hydraulic characteristics of the two soilsunder investigation. After saturation of the profiles, drainage curves were in situ determinedby allowing water to drain freely from the profiles for approximately a month. Thesedrainage curves revealed that the initial soil water salinity did not significantly influence thehydraulic characteristics of both soils. It was possible to quantify the amount of salt removed during a drainage cycle. Although both soils are apedal, the two soils differed markedly intheir discharge rates and amounts.Chapter 4 had focused on quantifying the pore volume of water required to leach excess saltsfrom the profiles. It was found that piston flow can describe the leaching process, becauseone pore volume of drainage was sufficient to remove 100% of the excess salts, irrespectiveirrigation water salinity or soil water salinity. The results also showed that it is more efficientto remove 80% of excess salts in stead of 100%. On freely drained soils it is thereforepossible to effectively and efficiently manage the salinity level of the root zone throughcontrolled irrigation in excess of crop water demand, when necessary.Complex dynamic models are helpful in understanding the nature and complexity of solutemovement in soils, but unfortunately they are not widely used by irrigators and managers.The final objective (Chapter 5) was to derive a simple model capable of estimating the depthof water required to remove excess salts from the root zone. The non-linear exponentialassociation (y = a {1- exp –b x}) of the in situ determined leaching curves provided the bestmathematical description of the fraction of excess salts removed in relation to the depth ofleaching water required per unit depth of soil. Verification of the proposed model showedthat it is possible to accurately estimate the leaching requirement for effective and efficientmanagement of root zone salinity in apedal soils. It was recommended that the proposedmodel should be expanded to include more soil types.

[发布机构] University of the Free State