Nitrogen is the most important mineral nutrient in almond production. It is absorbed as nitrate or ammonium by active roots. Mature almond trees use 80% of their total annual N requirement between March (full leaf out) and mid-May – June (kernel fill). If the N isn’t available in this key application window, yield can be lost.
To maximize yield (income) while minimizing N losses from the root zone ($ losses) that hurt growers wallets and may harm the environment attention to the four R’s – Right Rate, Right Time, Right Place, and Right Material — is needed in all orchard fertility programs. Different nutrients are needed at different times and amounts by almond trees. Spring is a key time for almond orchard nutrient management and the key time for nitrogen inputs.
Regardless of the form of fertilizer N applied – urea, ammonium, or nitrate – and unless nitrification inhibitors are used, most soluble fertilizer N is rapidly transformed to nitrate within two weeks of application during the growing season in almond orchards in California. Nitrate is highly mobile in soils. Nitrate moves readily to roots with water, a good thing, but also moves readily down out of the root zone with excess water from rain or irrigation. Nitrate leaching is a loss to the grower and a potential groundwater pollutant.
The key to sustainable almond N fertility is matching the amount and timing of N delivery to tree N use, as excess soluble N in the soil is easily lost from the root zone. Nitrogen absorption into almond trees is regulated by processes within the tree, not by the amount of available N outside the roots in the soil. This was proved in field studies in the late 1990’s by Drs. Steve Weinbaum and Patrick Brown at UC Davis. Their research showed that almond trees have a feedback mechanism, which reduces further tree N uptake in trees previously exposed to high soil N levels. Soil N in excess of tree N uptake capacity is not absorbed by the tree and vulnerable to loss by leaching from the root zone — towards ground water — with excess irrigation or rain water. That is why matching N delivery to tree N use is crucial to maximizing yield in an almond orchard and avoiding groundwater contamination and possible regulatory limits in N use in agriculture.
The Four R’s – Right Rate, Right Place, Right Time, and Right Material are keys to efficient N management in almonds. Here is a review of the four R’s for N in almonds.
Right Rate: Almond trees provided optimum N nutrition use 65-70 lbs N per 1000 lbs of kernels. This N is in the hull, shell and kernel contained in the crop producing 1000 lbs of kernels. Research results indicate that 20-40 lbs of N per acre per year within the tree is needed to provide N for vegetative growth in vigorous, bearing trees intercepting >70% of light (>70% of the orchard floor is shaded at noon). With efficient, adequate irrigation, growers should shoot for between 80 and 100 lbs of N delivered to the orchard floor per 1000 lbs of kernel produced. That’s should deliver70-88% Nitrogen Use Efficiency (NUE: amount N removed in the crop per amount N applied to the crop). This is a solid, sustainable target. In a perfect world, where fertilizer N went directly from the fertilizer truck into the tree, the target NUE would be 100%. However, since most fertilizer N is applied to the soil and can be used by many organisms (soil fungi and bacteria, weeds, etc.) or lost via leaching or volatilization in addition to absorption by tree roots, it is impossible to maintain high yields over time assuming a 1:1 ratio of N applied to N removed in the crop. [Note: Foliar N fertilizers are, essentially, delivered from fertilizer truck to tree, but are expensive, can only apply, safely, a relatively small amount of N per application (< 10 lbs. N per acre per spray) between bloom and harvest, and are subject to the 60-75% deposition efficiency of current airblast spraying. Foliar feeding may have a niche in overall almond nutrient management, but, based on our current knowledge, it is only a niche.]
Before determining the amount of fertilizer needed, make sure to account for other sources of plant available N in the orchard. This includes nitrogen fixing cover crops, composts, nitrate in irrigation water, etc. Test the nitrate-N concentration of well water used for irrigation. Ground water can provide fairly large amounts of “free” nitrogen – as much as 50-70 lbs N per acre per year in certain areas with high groundwater nitrate levels. The N delivered to the orchard in ground water can come right off the top of your N budget – a natural “deduction” from the total amount of N input required per acre for sustainable production.
Right Time: Current UC recommendations for optimum timings of fertilizer N application are the following. Depending on the location, soil type and risk of heavy rains early in the growing season, apply 20% of total annual N budget between end of bloom and full leaf out. Apply another 30% of total N budget between full leaf out and shell hardening. Apply another 30% of the total N budget between shell hardening and kernel fill or early hull split, dependent upon hull rot history (last application in mid-May if managing hull-rot). A final application amounting to 20% of total N budget can be applied between the end of hull split – when nuts development and N accumulation is complete – and early post-harvest because of possible leaching risk to remaining N. This final application should be considered carefully after obtaining July leaf samples, considering current season yield, and the history of early rains in the region.
Right Place: Nitrogen, as nitrate or ammonium dissolved in the soil solution – the soil water, is absorbed by active roots. The majority of active almond roots are in the top 3-4’ feet of the soil surface in the tree row, away from the compaction of loaded sprayers, etc. in the tractor aisles. Nitrogen fertilizer application should target the roots in two directions. Horizontally across the orchard floor, target the tree row. Vertically, in the soils, target the active the root zone – the top 3-4’ of soil. Nitrogen applied in the tractor aisle or deep in the root zone is less likely to get into the tree than N applied in the tree row and the mid to upper portions of the root zone. What is the best way to target active roots? Apply fertilizer N with micro-irrigation water (fertigate). If using flood or solid set sprinkler systems, concentrate your fertilizer application into the tree row and then irrigate. When fertigating, inject the fertilizer during the middle third of the irrigation set. For example, inject fertilizer N during hours 8-16 of a 24 hour irrigation set. Both nitrate and urea will move readily with irrigation water, so injection of, say, UN-32, early in the irrigation set can move at least some of the N in the application deep in the rootzone, sometimes too deep for efficient uptake by the tree. [Note: Urea is an uncharged molecule and moves readily with water through the soil. Each urea molecule breaks down into two ammonium molecules plus some carbon dioxide, but this process can take longer than the duration of an irrigation set. Thus, in the time frame of an irrigation set — usually a day or less — urea and the N contained in it is very mobile in the soil.]
Right Material: Dr. Patrick Brown, Professor in the Plant Sciences Department at UC Davis, recommends that growers use “compatible, soluble, and balanced” fertilizer materials when adding N to almond orchards. Over a three year, detailed field study, Dr. Brown and group found no differences in almond yield when equal amounts of N was applied as CAN17 or UN32 – two very common fertilizer N sources.