Estimating Nitrogen Needs = Estimating Your Crop

Research by Dr. Patrick Brown’s group at UC Davis has been reviewing nitrogen use efficiency of almonds. In their studies they have shown that almonds, when properly fertilized (i.e. not over/under-fertilized),  export between 60-65 pounds of N from the orchard for every 1000 kernel pounds harvested. This number includes all green weight removed to achieve the 1000 kernel pound yield – roughly 4000 pounds of hulls, shells, leaves, debris, and kernels. Interestingly enough, they also have found that orchards that are under-fertilized will export less N (65 lbs).

Knowing this, is it possible to determine the nitrogen needs of the trees for the season? Simply stated, “Yes,” but only if an accurate estimate of crop can be made. The ability to accurately estimate a crop is gained through experience and the taking of careful notes from the early season to allow comparison with the sheets provided at harvest. Once a crop estimate is determined, the estimated kernel yield/acre can be multiplied by 60 pounds to determine the total amount of N required/acre for that year. Since almond goes through a period of several drops and weather events (i.e. frost, hail, etc), this adjustment can be modified in season to account for the change.
Never-the-less, applying only 60 pounds of N for every 1000 kernel pounds will not meet the trees need because the application efficiency of nitrogen is not 100%. Further work within the study demonstrated that almonds are much more efficient than previously thought, with roughly 70-75% of the nitrogen applied through a micro-irrigation system making its way into the tree. Taking this use efficiency into account, we need to multiply the pounds required by crop demand by 1.4, giving a number of 84 pounds of N for every 1000 kernel pounds.
So how does all of this relate to nitrogen leaf sampling? Leaf sampling provides the ability to determine if the nitrogen content within the leaves is at an adequate level to allow growth. Previous work by the UC has found that a value of 2.2-2.5% N within leaves sampled from non-fruiting spurs in mid-July is the range to achieve optimal growth. Below 2.2% is deficient, which will hinder growth, above 2.5% will yield more vegetative, weaker growth. Since sampling in July is often too late to address tree N deficiency, Dr. Brown’s group has also been working to determine earlier timings for leaf sampling. In their work, they have found that a non-fruiting spur leaf nitrogen value in April above 3.5% is sufficient for crop and tree growth within a mature orchard (8 years and older). In other words, if the leaf N analysis comes back greater than 3.5%, applications of N need only to match the demands of the crop load, as there is ample N within the tree to provide for the vegetative growth. Younger trees that are developing may need more N.
On a personal note, I wanted to make you all aware that the this large, multi-year study is just starting to conclude, and the regional variations/adjustments will hopefully be determined. In my mind, a big “Thank You” to Dr. Patrick Brown’s group, which include Sebastian Saa Silva, Saiful Muhammad, Emilio Laca, as well as Blake Sandon and Paramount Farms for all of their effort, as this project was a huge endeavor.
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