FIELD SAMPLING CONSIDERATIONS FOR THE STEM NITRATE TEST IN PEPPERMINT

The stem nitrate test for peppermint (Mentha piperita var.) is a promi sing nitrogen (N) management tool. When used properly, this test may a id in obtaining significant savings of fertilizer costs and in the pro tection of groundwater quality. There are several factors related to e nvironmental conditions, N management, and sampling procedures that ha ve not been evaluated and may confound interpretation of test results. The objective of this study was to measure the response of stem nitra te concentrations to factors that would be expected to influence the t est and develop guidelines for the collection of stem tissue. The fact ors considered here were i) solar radiation effects on both hourly and daily scales; ii) spatial variability; iii) differences between alter native plant materials; and iv) the temporal response of tissue nitrat e concentrations to soluble N application. The most influential of the se variables were the type of stem material (a 441% effect at p=3.55E- 6) and the number of stems collected to estimate the field mean concen tration. It was found that the variance of the sample population and t he number of stems required for a given sampling error could be greatl y reduced by only collecting stems from within the plant canopy. Colle cting only these stems, 30 stems were found to be adequate to estimate the field mean concentration within 10 to 15% of the true population mean (p<0.05). Statistically significant differences in stem nitrate c oncentrations were produced by variations in solar radiation on both h ourly (p<0.05) and day length (p<0.01) scales. When measuring the diur nal response, a 17% reduction in stem nitrate concentration was observ ed over a nine-hour period from 12:00 hours to 21:00 hours. On the day length scale, an 80% reduction in incoming solar radiation produced a 29% increase in stem nitrate concentrations after three days of shadi ng. In the analysis of stem nitrate spatial variability, no discernabl e range of autocorrelation was detected indicating a purely random dis tribution of stem nitrate concentrations on the 1-150 m scale. Given t his finding and under the conditions of the analyses (late season with stem nitrate in excess of critical levels), it is not important that samples collected for this test fully cover the field being assessed, despite the intuitive appeal of full-field sampling as a standard proc edure. The response of stem nitrate concentrations to soluble N applic ation was minimal, probably due to plant N status in the test plots be ing well above the critical deficiency content prior to application. W ith the data produced from these investigations, users of the peppermi nt stem nitrate test are presented with a method to collect data in th e field whereby N management interpretations of the test can be more c onsistent and reliable. In addition, these results indicate the need f or researchers to fully report the method of sampling employed when pr esenting finding for stem tissue tests.