The method for predicting the nitrogen (N) requirements of irrigated wheat that is recommended by the University of Arizona requires preplant soil, plus mid-season stem nitrate analysis. Additional information on the relationships between N rates, stem NO₃-N levels and grain yields are needed for the wide range of agronomic conditions typical of Arizona's wheat growing areas. Three N fertility trials were conducted at the Maricopa Agricultural Center to, 1) measure the accuracy of the current University of Arizona procedure on soils of contrasting texture; 2) to evaluate the use of the current stem testing procedure on two durum varieties, "Aldura" and "Westbred-881"; and 3) to evaluate the effect of various N forms on the levels of NO₃-N in stem tissue for wheat grown in a clay loam soil. The University of Arizona procedure was found to over predict slightly the amount of N required for optimum economic return on sandy soils where the maximum yields obtained did not exceed 5100 lbs. grain/a which is considerably below the expected yield possibility for these sites. The procedure accurately predicted the amount of N required for optimum production on a clay loam soil (175 lbs. N/a)at a maximum yield of 6000 lbs. grain /acre. "Aldura" and "Westbred-881" were remarkably similar in their response to a wide range of N applications. There was no significant difference in the yields of these two varieties, but "Westbred-881" did contain somewhat higher protein levels. Little statistical or practical differences were observed in the quantities of N contained in the stem tissue of these two varieties; this should help simplify the interpretation of stem NO₃-N values for various wheat cultivars. The chemical form of N applied to wheat grown in a clay loam soil had no significant effect on the quantity of NO₃-N measured in stem tissue at any time during the growing season. The currently recommended procedure for predicting optimum N fertilization rates in wheat productions shows considerable promise but needs further evaluation, particularly under high - yielding conditions.

The high yielding spring wheats grown in Arizona usually require applications of fertilizer nitrogen (N) to achieve optimum grain yields and acceptable quality. The University of Arizona's currently recommended procedure (preplant soil plus periodic stem tissue analysis for NO₃-N to predict the N needs of wheat) is not widely used by Graham County growers for various reasons. A nitrogen fertility trial was conducted at the Safford Agricultural Center during the 1986-87 crop year to: 1) examine the relationships between basal stem nitrate-N levels, grain yields of durum wheat, and N fertilizer rates; and 2) to test the accuracy of the recommended procedure for predicting the N needs of durum wheat. Five rates of N from O to 419 lbs N /acre were applied in three split applications. One additional N treatment was made as indicated by the current University of Arizona procedure. Maximum grain yields of 5500 to 6200 lbs /a and protein levels in excess of 14.5% were attained with the application of at least 186 lbs NIA. An untimely early season irrigation induced a temporary N deficiency condition for all plots, which may have kept grain yields below the maximum yield possibility for this site. In spite of this, the amount of N predicted by the University of Arizona procedure (197 lbsN/acre) did attain an adjusted economic return which was not significantly different from the maximum numerical yield that was achieved for any of the other N treatments.

Current University of Arizona recommendations require periodic stein NO₃-N tests to determine nitrogen (N) fertility status of wheat crops. Lack of data on the importance of sample handling techniques, plant part selection, grinding criteria and extraction conditions have resulted in a reluctance by some growers and laboratory operators to utilize this test procedure. A laboratory study was carried out to examine factors important in wheat stem tissue analysis for NO₃-N. Sample handling, fineness of tissue grinding, and different extraction ratios were examined to determine their effects on NO₃-N recovery. Detailed partitioning of wheat plants at the 3-4 leaf, joint and boot growth stages was conducted to document which plant part is the best indicator of the N status of wheat. Optimal recovery of tissue NO₃-N existed for stem tissue separated immediately in the field and dried within 8 hours; stem tissue ground to 30 mesh or less and extracted for at least 30 minutes; and when using a sample size of 0.1000 g, in conjunction with 25 ml of extractant (i.e. 1:250 plant tissue to extractant ratio). Partitioning data confirmed current University of Arizona wheat tissue sampling guidelines which suggest sampling of the basal portion of the stem tissue.

Obtaining optimal yields of spring wheat in Arizona normally requires applications of fertilizer nitrogen (N), and occasionally phosphorus (P). The University of Arizona currently recommends preplant soil tests for NO₃-N and P, plus periodic stem tissue NO₃-N analyses to predict the N and P needs of wheat. Preplant application of P within the root zone of growing plants is suggested due to the immobility of P in soils. Split applications of N broadcast to dry soil preceding irrigations are generally recommended. Collecting additional data to calibrate and refine current guidelines for interpreting soil and plant test values is an ongoing need in Arizona. An experiment was conducted at the Safford Agricultural Center during the 1986-87 crop year to evaluate the response of "Aldura" durum wheat to banded and broadcast N and P, and split applications of N on a clay loam soil testing low in NO₃-N and available P. Maximum grain yields of over 4,500 lbs./A were obtained by banding of 40 lbs. P₂O₅ /A and 32 lbs. N/A as 16-20-0 at planting and broadcasting 118 lbs. urea-N/A prior to seeding. Stem tissue NO₃-N analyses revealed that N deficient conditions prevailed throughout the growing season in all fertilizer treatments. Treatments in which the preassigned rate of N was split into three applications produced the lowest yields due to serious N deficiency early in the season. The stem NO₃-N tissue test proved accurate in predicting N status and a stem. PO₄-P tissue test seemed reliable in monitoring P nutrition of durum wheat.