To decide how much spring nitrogen to apply, wheat specialists advise submitting two nitrogen tissue testing samples of wheat leaves to the Agronomic Services Division of the N.C. Department of Agriculture and Consumer Services. One sample will be used to measure plant nutrient content, the other to measure biomass. Both tests are necessary to obtain the most precise recommendation. The fee is $5 per sample for N.C. residents and $25 per sample for residents of other states.
"Tissue sampling should be done when wheat reaches Zadoks growth stage 30 (GS-30), said Michelle McGinnis, the division's field services chief. "To determine growth stage, wait until wheat begins to stand up tall and straight. Then pull several plants, split the stems from the top to the base and look for the growing point. Before GS-30, it will be just above the roots; at GS-30, it will have moved about one-half inch up the stem.
"Once GS-30 is reached, growers should immediately collect tissue samples and matching above-ground biomass samples, McGinnis said. "This is especially true if wheat is lush due to warmer weather or early planting dates. If the crop's need for nitrogen is not met at this time, then tillers will abort and yield will be reduced.
To properly run a nitrogen tissue testing sample, cut wheat plants about one-half inch above the ground in 20 to 30 representative areas throughout a field. Generally, two large fistfuls of leaves will make a good sample. Remove dead leaves and weeds before placing the sample in a paper bag.
A biomass sample, on the other hand, should contain all the above-ground wheat-plant tissue from one representative, 36-inch section of row. In broadcast fields, collect all the plants from one square yard. Place the sample in a paper bag, and write the sample ID from the corresponding tissue sample and the word "biomass on the bag.
Collecting biomass samples has only recently become part of the nitrogen determination process. Dr. Randy Weisz of N.C. State University developed a method of using biomass weight along with tissue test results to calculate more site-specific nitrogen recommendations. His approach takes into account crop-growth differences due to planting date, row spacing and moisture levels. For wheat grown on large acreages of poorly drained soils, however, growers should consult with an agricultural adviser about whether this method is likely to be useful.
Upon receiving the NCDA&;CS plant analysis report, growers should first look for the biomass and nitrogen percentage values. These values and certain crop planting details help determine the appropriate nitrogen rate, based on Weisz's interactive tool.
North Carolina growers wanting more information about this method should contact their regional agronomist, county Cooperative Extension agent or other agricultural adviser. Regional agronomists, in particular, can offer advice on how to collect and submit tissue and biomass samples, and how to interpret and use plant analysis report data.