Phosphorus fertilization is beneficial for newly planted almond trees

Phoebe Gordon, Orchard Systems Advisor, Madera and Merced counties

Greg Browne, Plant pathologist, USDA-ARS

Jamie Ott, Orchard Systems Advisor, Tehema, Shasta, Butte, and Glenn Counties

Brent Holtz, Pomology Advisor, San Joaquin County

Take home message: Phosphorus fertilizer applied at a rate of 5 or 6 oz of P2O5 benefited newly planted almond trees in three different locations under a variety of replant scenarios.

Phosphorus is a macronutrient and one of the nutrients found in complete fertilizers, but we don’t talk about it much in orchard crops.  Or, if you have heard someone from UC Cooperative Extension talk about it (like me!), you were probably told that fruit trees don’t need it. Well, I’m happy to tell you that I was wrong – at least when it comes to first leaf almond trees.

The reason why conventional wisdom suggests that fruit trees do not need phosphorus fertilizer is because previous research didn’t always show a yield or growth response, and deficiencies are very rarely seen in the Central Valley. This is despite phosphorus being a macronutrient – or a nutrient that is required by plants in large amounts.  One reason that documented deficiencies are rare could be because trees have permanent root systems that stay in place season after season, giving them a head start over annual crops on obtaining this soil immobile nutrient. An additional reason is that trees (as well as most crop plants) will become infected with mycorrhizal fungi, which are known for helping trees obtain soil immobile nutrients, particularly phosphorus. The last is that tree crops export relatively little phosphorus in comparison to nitrogen and potassium (Table 1).

Table 1: Removal rates for commonly grown California orchard crops. Source: https://www.cdfa.ca.gov/is/ffldrs/frep/FertilizationGuidelines/

Our research on phosphorus fertilization emerged out of work on soil anaerobic disinfestation (ASD) in almonds, led by Greg Browne. ASD is a fumigant-free way of disinfesting soils utilizing soil microbes that have been stimulated with an easily metabolizable carbon source.  Greg has consistently found that the most effective substrate is rice bran, which also adds a heavy load of nitrogen, phosphorus, and potassium to soils. We teamed up to test ASD on a large scale in Chowchilla and threw in a fertilizer trial to try to tease out any nutrient effect from the ASD substrates. The orchard in this trial was a Butte/Padre on Nemaguard, planted in February 2019. The grower had recycled the previous orchard at this site, and it wasn’t fumigated prior to planting. The soil phosphorus levels ranged from 7 to 11 ppm using the Olsen bicarb test, which would be rated as moderate phosphorus availability (Table 2).

A note to help the reader understand our growth measurement: we used trunk cross-sectional area (TCSA), which is a way of measuring tree growth and vigor. It is a non-destructive way to estimate the surface area of the trunk if you were to decapitate a tree about 12 inches from the ground. TCSA is positively correlated with yield in the early stages of a tree’s life (before canopy closure).

Table 2: Soil P test interpretation values.  Note: these do not take into account specific crop soil critical levels. 

In this trial, we tried out several different fertilizer formulations (Table 3). All fertilizer was applied to the base of the tree, buried in holes dug into the ground that were about one foot away and six inches deep, and we used this method for our later experiments as well. Because nitrogen will leach out of the soils with heavy irrigation, we split the applications for the +N and +NPK to reduce leaching risks. We also split the +P application to remove application frequency as a factor. The slow release and micronutrient blend were applied once. We applied the first fertilizer application after leafout and every month afterward. We accidentally skipped one application of the +N/+NPK/+P treatments, but we found that this did not affect the results – the trees that received the lower rates of the +N, +NPK, and +P treatments were no different in size than the trees that received the slow-release formulation (Fig. 1). We only applied fertilizer in the first year after planting, so any effect seen in the second year is a continuation from the initial fertilizations.

Table 3: Fertilizer treatments and application rates in the West Chowchilla trial. Fertilizer treatments were only applied in the first year, in addition to the grower’s application of 10.5 lbs N/acre through the irrigation system and 1 lb N, 2.1 lb P, and 4.3 lbs of K/acre split into two foliar sprays.

Figure 1: Changes in tree trunk cross sectional area over two years of growth in Padre trees planted on Nemaguard. Error bars indicate the 95% confidence interval.

We were shocked by what we found: phosphorus fertilizer resulted in as much growth as the +N treatment and the two complete formulations. The Butte trees were not affected by any fertilization treatment (Fig. 1).  

Because these results were so unexpected, we decided to test phosphorus-only fertilization treatments in another location. The next trial was in a grower’s orchard east of Chowchilla, which was planted in the 2020/2021 winter in fumigated ground. This grower also recycled the previous almond orchard, and planted Monterey and Nonpareil on Viking. One critical thing to note: the grower applied 3 tons of composted chicken manure, which was concentrated in the area that was fumigated. Because of this, the starting phosphorus levels were fairly high, ranging from 18-31 ppm using the Olsen bicarb test. I was skeptical that we would see results here, based on the manure addition and the high soil phosphorus levels, but once again we were surprised. Monterey (Fig. 2), but not Nonpareil, grew more when fertilized with phosphorus-containing fertilizer.

Figure 2: Changes in trunk cross sectional area for Monterey trees on Viking rootstock. Error bars indicate 95% confidence intervals.

After seeing the results confirmed in a different location, we were interested in breaking down different practices that growers may do when replanting orchards, and seeing how these components may be affected by phosphorus fertilization. For instance, one of the grower locations was fumigated prior to planting, the other wasn’t.  We didn’t think that wood chips would affect phosphorus availability, but it’s a fairly new practice: perhaps increased growth due to phosphorus is due to wood chip incorporation?

We decided to test these questions out at was at the University of California Kearney Agricultural Research and Extension Center (Kearney REC) in Parlier, California. We performed two different research experiments here to examine how phosphorus interacted with fumigation and whole orchard recycling (WOR). At this site, Nonpareil and Monterey on Nemaguard were planted in May 2021. The soil phosphorus levels ranged from 9 to 16 ppm, and it was planted in a former peach orchard, where the trees were burned prior to planting.  The wood chips were imported from a different site and treated as an experimental unit.

The first Kearney REC trial examined phosphorus and nitrogen in fumigated or unfumigated soil, and the second added wood chips into the mix (Table 4).  Because we were conducting research with wood chips, we also threw in two nitrogen treatments: the recommended rate for first leaf almond trees (between 3 and 4 oz of nitrogen per tree) and a higher rate to account for possible nitrogen immobilization (5.5 oz N per tree). Like in the previous two trials, we buried the phosphorus fertilizer in the soil soon after planting, but we surface applied urea on a weekly basis and immediately irrigated it in to reduce the chance of volatilization.

Table 4: A breakdown of the Kearney REC experiments

In Kearney REC Experiment 1, which was only examining fertilization and fumigation, only the Nonpareil responded to phosphorus fertilization (Fig. 3). Trees that were fertilized with phosphorus fertilizer grew significantly more than trees that received 5.5 oz of nitrogen per tree. Fumigation did not affect the response to fertilization in this trial. We didn’t find any additional growth response if we added an extra 2 oz of nitrogen per tree, which supports other work done by Mae Culumber and Brent Holtz: in a WOR site, no additional nitrogen fertilizer is needed if applications begin close to planting/leafout.

Figure 3: Nonpareil, planted on Nemaguard in 2021, response to fertilizer treatments in Kearney REC experiment 1. Fumigation did not change the response to fertilization treatments, so results are combined across fumigation treatments. Error bars indicate 95% confidence intervals.

In Kearney REC Experiment 2, which examined fertilization, fumigation and WOR, Nonpareil trees in fumigated or nonfumigated soil responded to added phosphorus fertilizer in the first year (Fig. 4). Monterey also responded to fertilizer applications, but the response was influenced by fumigation (Fig. 5): the growth response to applied phosphorus was greater in fumigated rather than unfumigated soil.

Figure 4: Nonpareil, planted on Nemaguard in May 2021, response to fertilizer treatments in Kearney REC Experiment 2. Fumigation did not change the response to fertilization treatments, so results are combined across fumigation treatments. Error bars indicate 95% confidence intervals.

Figure 5: Monterey, planted on Nemaguard in May 2021, response to fumigation and fertilizer in Kearney REC Experiment 2. The addition of wood chips did not affect results, so the analyzed results were combined over wood chip treatments. Error bars indicate 95% confidence intervals.

We found no effect from the addition of wood chips – there was no stunting of growth with 3.5 oz of nitrogen per tree, and there was no boost in growth when applying more nitrogen. Likewise, phosphorus fertilizer did not influence trees planted in soil containing wood chips.

To conclude, in all four fertilization experiments, at least one cultivar responded to applied phosphorus. Given the relatively low cost of this fertilizer and the consistent results across planting scenarios (fumigated or unfumigated ground, or ground with or without the previous orchard recycled in it), we suggest that all newly planted almond trees would benefit from phosphorus fertilizer soon after planting.

One thing to note: we found that soil phosphorus levels did not accurately predict almond response to phosphorus fertilization.  When we looked into other research trials that were conducted in other trees species, we found the same thing – soil phosphorus levels did not accurately predict whether fertilization would result in a growth response.  It is possible that a shot of phosphorus soon after planting serves as something like a starter fertilizer – close to the trees and ready for uptake before they have the chance to explore the surrounding soil.

We are unsure as to why only one cultivar responded to added phosphorus fertilizer at three of the four trials. In the case of the East of Chowchilla trial, the Monterey was planted several months before the Nonpareil (Nov 2020 versus Feb 2021), however this was not the case in the other three experiments. It is possible that small differences in seedling blocks in the nursery could impact later growth responses. It is also possible that cultivars have different nutrient requirements that have not been fully quantified.  

Thank you to our grower cooperators and the Kearney REC crew for supporting these trials. This research was partially supported by the Almond Board of California.

Figure 6: Kearney REC Experiment 1