Robson, 1991). Roots in dry soil are also capable of nutrient uptake from the root zone (Nambiar, 1976(Nambiar, , 1977. Kirkham (1983) and Xu and Bland (1993) presented models of water movement that adequately describe the movement of water and solutes in similar splitroot systems based on the development of gradients favoring movement to roots in drying soil. These processes are key survival mechanisms for woody plants with or without benefit of irrigation. Under drought conditions, plants use these mechanisms to "mine" water and nutrient resources held in the subsoil and to survive periodic drought (Caldwell and Richards, 1989). These mechanisms are important for nonirrigated plants but for irrigated plants they probably contribute little to the water or nutrient balance.Roots growing in the wetted portion of the root zone function as the primary tissue of nutrient and water uptake and root hormone production. The island of resources created in the wetted root zone is analogous to a shoot growing into the light and responding by developing lateral shoots and leaves. Root development in the wetted zone integrates the effects of the local environment with the physiological relationships between individual roots and other plant tissues. Generally, roots in a given environment develop more rapidly if other roots on the same plant have less access to resources (Gersani and Sachs, 1992). Generalized responses of roots when they encounter a resourcerich patch of soil include increased nutrient uptake and life-span, and changes in carbon allocation and morphology. Increases in the kinetics of nutrient uptake by roots (Caldwell, 1994;Jackson et al., 1990;Robinson, 1994) generally precede root proliferation. It is not clear, however, if root exudation is reduced in nutrient-rich zones (Caldwell, 1994).Roots may or may not live longer in response to additions of water and nutrients (Gross et al., 1993;Pregitzer et al., 1993, and citations therein) as a strategy to minimize carbon cost while maximizing nutrient acquisition (Eissenstat, 1991; Eissenstat and Yanai, 1997). The carbon economy of roots can significantly affect root biomass in relation to the cost of foraging for nutrients. Root biomass is reduced when nutrients occur in patches, rather than being homogeneously distributed (Crabtree and Bazzaz, 1992). Numerous studies demonstrate that as little as 15% of the root system can supply the entire plant's N needs (Burns, 1980;Robinson et al., 1991). Acquisition of diffusion-and interception-controlled nutrients, e.g., phosphorus, probably requires more root growth for nutrient foraging than does acquisition of nutrients that move predominantly by mass flow.Root morphology is affected by the chemistry of nutrient-rich sites. Roots supplied with ammonium have increased total root length and surface area, while those supplied with nitrate have longer laterals (Schortemeyer et al., 1993). These results explain, in part, why nitraterich zones have less root biomass and use less water than those in ammonium-rich patches (F...