Plant tissue tests can be useful for diagnostic analysis of potential nutrient deficiencies. Hall (78) was the first to propose plant analysis as a method for determining normal plant tissue nutrient concentrations. Macy (79) proposed the basic theory, which stated that there is a crop specific critical level/threshold concentration of a nutrient in plant tissues. Concentrations exceeding this threshold signify luxury consumption. Those equaling the threshold indicate sufficiency, and concentrations below the critical level/threshold imply deficiency. Over the years, estimates of levels of deficiency, sufficiency, and excess were obtained based on laboratory analysis of plant tissues sampled from apparently healthy plants. These initial compilations have been modified from data where growth response was correlated to tissue concentration as data from solution culture, greenhouse, and field experiments became available.

This page presents plant tissue nutrient concentration data compiled for the major vegetable crops produced in Arizona. Vegetables produced in the area show consistent response only to nitrogen (N) and phosphorus (P). Thus, the data presented here do not contain locally field-calibrated data for all the crop-nutrient combinations. The plant tissue nutrient test diagnostic criteria assembled here were largely based on data from other publications (46, 48, 73, 75, 76, 81, 82, 83), modified by results from more recent, familiar, or regional research (52, 65, 66, 70, 71, 84, 86, 87), and the author’s own experience and unpublished data (14). These criteria could be helpful in determining growth-limiting nutrients and validating current fertilizer management practices. 

As noted in companion pages of this website (Sections 2.3 and 2.5), potassium (K) and micronutrient fertilizers are not routinely applied to vegetable crops grown in the Lower Colorado River Basin Region, primarily because crop responses to application have not historically been observed to these nutrients. However, periodic tissue testing for K and micronutrients could help forecast potential deficiencies if natural soil sources become depleted. The values compiled here represent best estimates of critical level/threshold plant tissue nutrient concentration levels for the major vegetable crops grown in the Lower Colorado River Region (Tables 4-3-1 and 4-3-2). 

Because plant tissue tests are collected in-season, with the exception of N, they are generally post-mortem for the crop sampled. However, they provide indications of soil nutrient status for subsequent crop management decisions. Furthermore, plant tissue test results can be sensitive to small sampling errors associated with the plant part sampled or growth stage of the crop and environmental conditions at the time of sampling and crop variety. Thus, test results showing in-tissue nutrient concentrations that are slightly less than the nutrient thresholds should be verified through follow up soil and plant tissue testing prior to the introduction of significant modifications to existing fertilizer management programs.

For many of the important vegetable crops grown in the Lower Colorado River Region, threshold in-tissue nutrient concentration data do not exist for sulfur (S) and molybdenum (Mo). However, as noted in Section 2.4, S is abundant in soils of the region, and hence, the major vegetable crops of the region do not exhibit S deficiency. Vegetable crop Mo deficiencies are also not found in the soils of Arizona.