In determining N use, start with the amount of nitrogen left after the crop is harvested and work back. Nitrogen applied to the field, but not used by crop or removed in harvested product is viewed as a risk for eventual loss to the environment, says Hartz. This is what growers should strive to reduce or eliminate.

One of the first steps in budgeting nitrogen is to start with how much nitrate nitrogen is in the soil before the first seasonal N is applied.  This varied widely in 29 fields Hartz monitored in developing his research.

Hartz says each part per million nitrate nitrogen in the soils represents 4 pounds of nitrogen per acre in the top foot of soil. Therefore, N credit could be as much as 4 pounds of N per acre for each ppm of nitrate nitrogen above 5 ppm, he says.

In order to determine how much N is in the soil, Hartz says to sample in the drip wetted zone of the field below the first 2 to 3 inches of the soil. Do not sample bed shoulders.

Pre-plant fall and spring organic amendments also add to the soil nitrate nitrogen levels, and these must be factored into nitrogen budgeting.

To estimate soil nitrate nitrogen that could wind up in leachate, he suggests multiplying nitrate nitrogen by three or four. For example, he says if root zone soil nitrate nitrogen is 15 ppm, leachate is likely to be 40 to 60 ppm.

In processing tomatoes, nitrate nitrogen loss is likely to be in the range of five to 15 pounds of nitrogen per acre inch of water. Tissue analysis can improve N efficiency during the growing season. However, Hartz says, tissue analysis, as currently used, “more often leads to increasing a grower’s normal N fertilization program than decreasing it.”

Here are some guidelines Hartz offered for both evaluating leaf N and petiole N. Leaf N evaluation is more reliable now than petiole sampling.

Leaf N from 20-fields indicated:

• Three to seven weeks post-transplant leaf N may indicate limited soil N availability, but plant N update is not rapid enough to put a strain on soil N supply; therefore, high values do not necessary indicate high soil N.

• Seven to 11 weeks post-transplant is a period of peak N uptake and leaf N can fall rapidly; by the first red fruit stage leaf N comfortably above the sufficiency level indicates that N fertigation can stop.

• Eleven weeks until post-transplant: After first red fruit fields with very low leaf N are candidates for continued fertigation; high leaf N pre-harvest indicates excessive nitrogen applications.

Petiole nitrate nitrogen data from 20 fields indicated:

• Three to seven weeks into the season very low petiole nitrate nitrogen indicates limited soil N availability. However, in the sufficient range, higher values do not necessarily indicate more soil N.

• Seven to 11 weeks is when petiole values fall rapidly and it is difficult to set a sufficiency level. However, Hartz said maintaining 4,000 ppm nitrate nitrogen through first red fruit is not required.

• Eleven weeks through post harvest. After first red fruit, the only thing that petiole analysis can tell a grower is that excessive N has been applied.