There have been several posts over the past few weeks about fertilizing mature almond trees. This data has been based upon long term studies held primarily in Kern County with some data points from across the state. Not all of this, however, is applicable to determining the rates of fertilization for developing almond orchards.
At best, there are “educated guesses” in regards to identifying the proper rate for developing trees. Whole tree assays have found about 480 lbs of N/acre are stored within the woody biomass of mature trees. In order to reach that amount, trees would have to be accumulating nitrogen within the woody tissue at a rate of 20-50 lbs/acre/year until maturity (10-12th leaf). Once the tree begins to bear (~3rd/4th leaf), nutrients must be added to compensate for what is removed by the crop.
Recently, I began to study the effect of various rates of fertilization on first leaf almond trees. I have heard a broad range of nitrogen applied to these orchards, ranging from 20-80 lbs an acre, and sometimes more. Since the root zone of these trees is limited, there is a chance that nitrogen may be wasted. To address this “hole” in the data, I established a study on a sandy soil testing varying rates of nitrogen applied using both conventional and controlled release fertilizers. Rates chosen for the experiment were based upon the estimates from the whole tree assay/assumptions of N accumulation.
It is important to note that this is the first year of this study, and therefore the results may be heavily influenced by the trial location, methods. I was planning to withhold the data until I am able to have a replicate study, but there have been requests for the information. The results were summarized and presented as a poster at the 2012 ABC conference.
Even though preliminary, there are some practices that we should consider at this early stage:
1. Trees may not respond to nitrogen linearly (i.e. more applied= more growth). This trial suggests that growth was decreased at higher rates of applied nitrogen. This was observed in both the conventional and controlled release fertilizer. It is important to note that one of the flaws of the study is that a N-P-K blend fertilizer was used, so this reduction of growth may be due to one or interaction of the three elements.
2. In this trial, a rate somewhere between 15-30 lbs/acre provided maximal growth. Since the N was applied as a granular around the base of the tree, this was the assumed amount of N delivered to the rootzone of the tree. In a fertigated system, there may be substantial amount of N delivered outside of rooting zone, which may be lost, requiring more N for maximal growth.
3. The use of conventional fertilizer did not perform as well as the controlled release, suggesting a reduction of N efficiency. It is unknown how efficient fertigation systems are in delivering nitrogen to the young tree.
4. A single application of controlled release fertilizer worked as well as, if not better than 6 applications of conventional fertilizer. This implies potential for saving money on labor using these products.
5. Leaf tissue nitrogen concentration did not appear to influence growth rate.
6. These rates do not take into account mineralization, which may contribute N to the system. It is important to note that the soil contained 0.3% organic matter (Atwater sand). Soils with higher organic matter content may achieve maximal growth with less applied N.
With this preliminary study, a major question remains – why are farmers seeing increased growth responses with nitrogen rates well over the tested (45 lbs/acre) rates? Is it that we are that inefficient in delivering nitrogen to the rootzone of a young tree, or is there something else?
I began another study to help address these questions. This year’s study is looking at variable rates of N alone to see if we observe the same reduction in growth at higher rates. If the reduction is not observed, there may have been some negative interaction of P and K on tree growth. I have also initiated studies within second leaf trees.