Effects of soil properties on precipitation use efficiency

Abstract

As other non-agricultural users need and are able to pay higher prices for irrigation waters it will be increasingly essential to make more efficient use of precipitation in crop production. Major problems constraining the efficiency with which rainfall is used are: large portions of this water run off or are evaporated from soil surfaces; root zone water holding capacity is not sufficient to hold water needed to sustain good crop production; and the random nature of the precipitation and associated probabilities of crop failure deter investments needed to achieve improved efficiency of water use on high value crops in rain fed lands. Gravel and organic mulches have shown potential for reducing annual evaporation to as low as 10 cm of water and for facilitating entry of expected intensities of rainfall. Under such mulches, soil water is almost as dependable as “money in the bank”, and can be retained for use in seasons when high value crops are most efficiently grown. Measurements of such stored water plus past weather records can provide bases for calculating probabilities of adequate water being available for crops whose water requirements are known. Most of these water requirements have been determined when these crops were planted close together and root zones, and in some cases the crop canopy, overlapped. Research is needed to determine the degree to which wider spacing of high value vegetables and fruit crops under mulched conditions can be used to increase probabilities of economic production when water supplies are marginal. For tree crops, where planting density is not temporally flexible, but small amounts of irrigation water may be available at high cost, management systems are needed to optimize use of rain and ascertain when the economic benefits are sufficiently great to justify use of the irrigation water. Such sporadic use requires that the supply system can provide water on demand. Increases in rooting depths by modifying soils with restricting layers can serve as a cost effective alternative to irrigation under some climatic regimes. Such regimes should be identified and costs of increasing rooting depths should be evaluated and reduced where possible. Availability of municipal and industrial waste products, whose producers may be willing to pay costs of transport and incorporation, may provide economically feasible means of removing such restrictions. Production levels and environmental consequences of incorporating such wastes-cum-amendments in soils need evaluation to provide bases for developing optimized and acceptable management systems.