Webster soil association area and found that 60% of the samples from calcareous soils contained less than 20 ppm Zn and 18% had less than 15 ppm Zn. The critical level of Zn in corn leaf tissue has been suggested as below 20 ppm (Jones and Eck, 1973), although others (Jones, 1967; Viets et al., 1953) have proposed critical levels as low as ppm. It seems reasonable to assume that the critical level might vary with different soils, environments, corn cultivars, and other factors. A number of investigators (Massey and Loeffel, 1966, 1967; Shukla and Raj, 1976; Shuman et al., 1976) have observed that corn inbreds and single crosses vary in the amount of Zn they accumulate under constant conditions. Halim et al. (1968) considered this to be an important reason that tissue Zn content was frequently not closely related to Zn deficiency symptoms. The Zn soil test in use at the Iowa State University Soil Testing Laboratory (ISUSTL) involves a chelate (DTPA) extraction developed in Colorado by Lindsay and Norvell (1978). In 1977, an unpublished summary of the results from 3,786 Zn analyses performed at the ISUSTL showed that 62% of the soils tested adequate (about 1.0 ppm Zn), 25% were defined as mar ginal (0.6-1.0 ppm), and 13% were classed as deficient (0.5 ppm or less). An updated summary by Eik (1980) involved 12,869 samples and showed 57%, 29%, and 14% testing in the 7 adequate, marginal, and deficient ranges, respectively. Gogan (1975) achieved a predictive success of 75% in separating Iowa soils into nonresponsive and responsive classes for corn grown under greenhouse conditions, when a value of 1.30 ppm DTPA-extractable Zn was chosen as the critical soil test level. This value exceeds that of 0.8 ppm Zn established for corn under greenhouse conditions in Colorado by Lindsay and Norvell (1978) and of 0.5 ppm Zn in California by Brown et al. (1971). Apparent differences in these defined critical levels were probably due to variation in the experimental preparation, corn cultivars, cropping techniques, or to possible differences in physical-chemical properties among the Iowa, California, and Colorado soils. The existence of a critical DTPA-extractable Zn level for corn grown under field conditions in the riorthcentral United States has escaped the attention of this investigator. Field studies were initiated in 1977 and 1978 to determine whether corn grown on selected Iowa soils would respond to Zn fertilization. Two inorganic Zn fertilizers were included to assure that any measured responses were not dependent on the applied Zn source. It was hoped that any response could be related to soil, environmental, and source characteri sties. In 1979, studies involving soil injection of a chelated Zn solution (ZnEDTA) were started. These were instigated 8 primarily because of concern as to whether failure to obtain response to Zn fertilization in the earlier experiments was due to Zn placement or large variations in plant popula tion within experimental areas. 9 MATERIALS AND METHODS Experimental Procedures Long-term studies ...
