The goal of this study is to develop and test better tools for precision nitrogen measurement in the field, to determine seasonal as well as total nitrogen demands, and to prepare a user-friendly computer program for growers so they can enter local variables and receive best management recommendations for N-fertilization.

Efforts to refine tissue testing procedures failed to develop a method that was superior to the widely used total nitrogen determination. Analysis of leaf nitrate levels, as used in field crops, does not appear to be effective for tree crops due largely to the low and highly variable amounts of nitrate found in mature almond leaves. A three-year comparison [1996-98] of total leaf N, though superior to leaf nitrate determination, was still only poorly correlated with yield. Together these results suggest that the use of leaf analysis alone is inadequate for determining the N-fertilization requirements of almond.

To determine precisely how much N is utilized by a mature almond tree and to define when during the year that the roots are effectively using soil applied N, we conducted the following experiment. Every two to three months from April 1997 through January 1998five mature bearing almond trees were excavated from the soil and the N contained in each tree part was determined (leaves, fruits, young wood, mature wood, roots). Using this sequential harvesting approach it was possible to determine the total N required to satisfy the N demand of fruits, leaves and tree N storage, and it was also possible to determine when during the year N is acquired from the soil. The results will be used to develop a computer program to be used by growers to refine their N-fertilization program.

Results of tree excavation at different growth stages suggested that total removal of N from almond harvest and leaf drop from a mature tree with a per acre nut meat yield of4200 lb was 3.8 lb per tree. Nitrogen stored in wood parts of the plant contributed 1.7 lb per tree to supply the N demand of early season growth. This storage pool was subsequently replenished in late summer. During spring flush, plant growth N needs are satisfied by storage transfer from the stem, roots and uptake from soil (33 percent each). To balance the full year N demands of the almond tree, total N uptake from soil was 4.3 lb per tree. Of this amount, 13 percent was taken up during spring flush (Jan-mid-March), 42 percent between spring flush and nut fill (mid-May), and 45 percent during summer. This information on N uptake throughout the growth season, will provide guidance for determining the timing of split N applications, to optimize the nitrogen use efficiency.

This information is now being used to develop a fertilization recommendation model in which tree N requirements will be determined and N applications will be recommended to coincide with the time of optimum soil N usage. The combination of a precise knowledge of tree demand and the optimization of application method and timing provides an effective and environmentally sustainable approach to fertilization in almonds that has not been previously available. A computer program has now been developed and is currently being tested; this program will provide growers with a powerful tool for the optimization of almond fertilization.