While soil samples are helpful for checking soil pH and determining any potential problems with competitive uptake between nutrients in the soil, pecan leaf samples tell you the fertility status of the actual trees.
Using soil and leaf samples together allows pecan growers to match their fertilizer applications with the actual needs of the pecan tree rather than just shooting in the dark. This provides an excellent opportunity to save a lot of money on fertilizer cost.
The general recommended period for leaf sampling is July 7-August 7. I would shift these dates by about one week this year since crop development is running 7-10 days behind normal. Why does it matter when you sample?
Concentrations of N, P, and Zn on a leaf dry weight basis start off relatively high early in the season, decline rapidly, reach a fairly steady state after mid-June, and then decline near leaf fall. K tends to start high, then decreases and plateaus at about the same time as N, P, and Zn. Most of the P and Zn that accumulates in the leaves have done so by the time the leaves reach full size. Calcium (Ca) accumulates in the leaves as the season progresses, peaking in August-September. Magnesium (Mg) Manganese (Mn), and Boron (B) also tend to increase as the season progresses, but to a lesser extent than Ca.
Some growers have started taking leaf samples throughout the growing season to try and determine the fertility needs of the tree. This is really unnecessary and only an added cost. Leaf analysis does not necessarily reflect the actual use pattern of mineral nutrients. They merely indicate concentrations of those elements in the leaf at the time of sampling. The tree knows what it needs and when it needs it.
Concentrations of mineral nutrients in the leaves change as leaves emerge, expand, and finally senesce in the fall. For many elements, the least change in concentration occurs from early July-early August. Leaf samples should be taken at this time because critical levels established through experimentation and observations are based on sampling done during this period. Comparison of problem trees with healthy trees can be done by leaf sampling at any time, providing one is aware of how element concentrations change during the season.
So, how should you take leaf samples?