Nitrogen Monitoring through Spectral Analysis: David Slaughter

Nitrogen Monitoring in Tree Crops: The Need for Improved Protocols

David Slaughter, Yun (Carolyn) Zhang

The standards for leaf tissue analysis and nitrogen management in fruit and nut trees were established in the 1950's and 1960's and have been only modestly refined over the subsequent years. The standard consists of taking at least 10 leaves from nonfruiting branches, July - Sep. The choice of this standardized protocol has been a compromise necessary to overcome the variability in leaf tissue nutrient contents that occur throughout the year and within the tree.

There has been no study (to our knowledge) to determine the relative sensitivity of this standard leaf with any other leaf type, or time of sampling. Furthermore, the timing of this sample combined with lab processing time provides the grower with no opportunity to adjust fertilization practices in the current season. The inadequacy of current tests is further compromised by the high cost of sampling which in turn results in growers taking a limited number of leaf samples to represent large areas of production.

There is a clear need and a strong industry desire for leaf nitrogen determinations that are quicker, cheaper, and more flexible so that a grower could monitor with more frequency and precision.

The Challenge

Nitrogen concentrations in almond and pistachio leaves decrease significantly from leaf-out in the spring to full expansion and finally, senescence. The primary determinant of this N change is the changing ratio of non-structural to structural composition of the leaf. That is, young leaves have a high proportion of protein N to leaf dry weight. As leaves age, this proporton of N decreases as the proportion of cell wall material increases. Therefore, phenological leaf age can have a profound effect on leaf N analysis, and must be taken into account in the development of any sampling scheme.

Our Objectives

We propose the development of a leaf nitrogen analytical tool that will provide the simultaneous determination of N concentrations and leaf age. Our goals include:

• Testing the feasibility of developing a low-cost near infrared-based optical sensor that can replace the standard leaf sampling and analysis protocol.
• Optimizing this method so that N determinations can be made and interpreted at any time during the growing season.
• Investigate the impact of water status on the performance of Near Infrared-based spectral analysis for individual leaf N prediction

The challenge is to develop an instrument that is low-cost, fast and sensitive.