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<br />4. NUTRIENT AVAILABILITY <br /> <br />Besides climate and nutrition, the principle determinent of forage <br />production was nutrient availability. Therefore, the supplemental watE~r effect <br />on quantities of nutrients available for plant growth was studied. 1. Light <br />rain showers might increase rates of nitrogen fixation by algae or bacteria <br />present in surface soils of Bouteloua grasslands (Johnson et al., 1982). 2. <br />Failure of Mycorrhizae to increase or decrease with watering (Stahl et al., <br />1981) suggested but did not prove, that facilitation of nutrient accumulation <br />by fungi would be unchanged. 3. The apparent unaffected decomposition rates <br />(Weaver, 1981b) would not support Perry's (1976) prediction of producti.on <br />benefits due to more rapid cycling of soil nutrients. With Forbes McClellan <br />data were gathered on numbers of bacterial and protozoa to help explain the <br />surprising result; the numbers have not been analyzed. 4. Dry regimes differ <br />from wet ones in the degree to which nutrients were lost by leaching <br />(Woodmansee, 1978). Since nutrients in soils of the high plains were rarely <br />leached out of the root zone, it was doubtful that small increases in summer <br />rainfall would incre~lse leaching. Additions la,rge enough to wet soils deeply <br />(as in the fall wet, spring wet, or wet treatme!nts) would probably increase <br />leaching. Soil samples may still be analyzed for mobile ions such as sodium, <br />chloride, potassium and to test these propositions. <br /> <br />5. CONSUMERS <br /> <br />Several studies ~1ere conducted to determine whether past consumer <br />populations would grow to harvest water-induced yield increases. Attack rates <br />of leaf spot fungi and chewing insects were not changed (Yount and Weaver, <br />1981), but small supplements would have possibly increased attack rates of <br />nematodes (Weaver and Smolik, 1981), and large supplements would have probably <br />increased attack rates of sap-feeding insects (Haglund, 1981b). Increased <br />forage production in wet treatments would have implied an increase in predator <br />numbers or consumption rates; increased densities were, in fact, observed <br />(Haglund, 198Ib). Production benefits of supplemental water would have <br />exceeded pest losses; determination of whether pest control would be economical <br />would require estimates of program costs (probably available) and the degree to <br />which consumption would be reduced, i.e. the benefits. Losses to chewing <br />insects could be estimated, but not to other predatory groups such as bacteria, <br />fungi, protozoa, nematodes or sap-feeding insects. <br /> <br />6. SOIL EROSION <br /> <br />Increases in vegetation density due to supplemental water were expected to <br />more than compensate for any increased erosive force due to movement of the <br />water unless cloud-seeding would produce rare and very heavy showers (Haglund, <br />1982). <br /> <br />7. OTHER STUDIES <br /> <br />The soil water-range production model of Wight and Hanks (1981) was being <br />adapted to and calibrated with HIPLEX data to allow estimation of the effects <br />of a wider variety of treatments (Johnson and Weaver, unreported). The <br />interaction of water with nitrogen fertilization (unreported data) and grazing <br />(Haglund, 1981a, b, aend c, Weaver and McClelland, 1981, 1982) was considered. <br /> <br />49 <br /> <br />