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for prolonged plant availability. They have relatively high cation exchange <br />capacities, are generally slightly alkaline and have low EC and ESP values. The <br />data in fact indicate that onlv nitrogen and phosphorus may be potentially limit- <br />ing to plant growth. These characteristics are typical of what might be expected <br />in northwest Colorado (Berg, 1975; Moore et at, 1975: Berg, 1976; and Bauer et <br />al, 1978). <br />In the absence of water stress, one of the principal factors determining the rate <br />at which the root system will grow in a soil is the amount of nutrients available <br />to the root from the aerial parts of the plant and from the soil. If the soil is <br />too low in nutrients, then suitable fertilizers should be added to promote a <br />better developed root system, and increase above ground productivity. Adequate <br />nutrients early in the growing season will encourage improved plant root penetra- <br />tion into the soil. <br />Nutrient deficiencies however, cannot be interpreted in isolation of soil mois- <br />ture availability. Tisdale and Nelson (1975) found that at given levels of <br />• applied nitrogen, an increase in the amount of available soil moisture raised <br />straw yield, which for their experiment was greatest at the highest rate of <br />applied nitrogen. Conversely, an increase in the rate of nitrogen at any given <br />level of applied moisture increased wheat yield, which was greatest at the high- <br />est level of applied water. This illustrates that there is very little improve- <br />ment in plant yield following nitrogen applications until the soil moisture con- <br />tent reaches some threshold value. <br />Nutrient deficiencies in soils are common and widespread in semiarid and arid <br />regions. However, responses to fertilizers applied to correct these deficiencies <br />are not always forthcoming because of the influence of limited water supply. In <br />areal extent, the most widespread soil deficiencies occur with the macro-elements <br />nitrogen (N) and phosphorus (P). <br />Studies over the past 10 years have shown that coal-mine spoils in the western <br />United States which are predominately derived from calcareous Cretaceous and Ter- <br />tiary age shales (overburden) are usually deficient to extremely deficient in <br />rJ <br />3-54 <br />REVISED FEB 13 '87