Iowa State University's new nitrogen fertilizer recommendations for corn (outlined in Pm-1714) encourage using the end-of-season test for cornstalk nitrate rather than other common tissue tests. This deserves an explanation that starts with the basics of tissue testing.

The objective of tissue testing for N is to measure the sufficiency of nitrogen for growth, where the term "sufficiency" describes the supply of N relative to the needs of the plants sampled. The measurement shows N sufficiency during the period before the plants are sampled. N sufficiency is described in terms of being inadequate, optimal, or excessive.

A tissue test involves sampling a specific plant tissue at a specific physiological age, and then measuring some characteristic of that tissue. The characteristic most often measured has been the concentration of a specific form of nitrogen, such as nitrate or total nitrogen, expressed on a dry-weight basis or at a specified water content.

Concentrations of N in plant tissues can provide a numerical index of sufficiency only if the tissue test has been calibrated through research trials. Calibration is learning about the natural relationships between concentrations of N in the tissue and sufficiency of N for plant growth. This is done by conducting trials in which various rates of N are applied, and both yields and concentrations of N in the tissue are measured. Response curves are fit to describe how yields and concentrations change with rates of additional N.

Information from calibration trials are used to categorize concentrations of N into "zones." The "zone of poverty adjustment" shows where, in the response curve, small increases in available-N increase both the concentration of N in the tissue and yield. On the other hand, the "zone of luxury uptake" shows where, on the response curve, small increases in available-N increase the concentration of N in the tissue but not yield.

The concentration of nutrient that falls between the zone of poverty adjustment and the zone of luxury uptake is the "critical concentration." Any concentration lower than the critical concentration indicates deficiencies of N, and the lower the concentration the greater deficiency. Any concentration higher than the critical concentration indicates excesses of N, and the higher the concentration the greater the excess. The term "critical level" often is used when a tissue characteristic other than concentration of nutrient is measured (e.g., amount of light absorbed or reflected).

A tissue test is most useful if calibration trials conducted across a wide range of conditions reveal essentially the same critical concentration. If the critical concentration remains constant across all conditions of interest, then collection and analysis of a single sample provides a reliable assessment of the N sufficiency in the plants sampled.

In practice, however, the measured critical concentration of nutrients differs from site to site. It is difficult to determine how much the variability should be attributed to errors in measurement rather than to actual variability in critical concentrations. Actual variability can be caused by weather, hybrid, planting density, and many other factors. Practical application of tissue testing, however, only requires that the observed variability in critical concentrations is much smaller than the variability in concentrations normally found where the test is to be used.

Critical concentrations may not be the same as optimal concentrations. Critical concentrations are determined by the plant's physiological characteristics. Optimal concentrations, however, depend on factors such as prices for fertilizer and grain, environmental costs, and uncertainty associated with weather and other factors. Critical concentrations often can be defined as a single value, but optimal concentrations need to be defined as a reasonable range of concentrations.

Tissue testing provides site-specific assessments of N sufficiency that cannot be attained easily by testing soils or measuring yields. The current movement toward site-specific management of N, therefore, makes tissue testing more important than it was in the past. Indeed, tissue testing is likely to be an essential tool for evaluating and improving new technologies that enable site-specific management of N.

Producers should expect a substantial increase in the number of tissue tests available in the future. In the past, for example, remote sensing has not been considered tissue testing, but recent advances in technology enable its use to measure plant characteristics that indicate N sufficiency. From a practical point of view, remote sensing could be considered a form of tissue testing that can be used to determine N sufficiency, but only if the basic principles of tissue testing are applied correctly.