Phosphorus: Does your orchard need it?
Phoebe Gordon, UCCE Madera and Merced Counties
Take home message: Phosphorus fertilization is only needed if your trees are below the July sufficiency ranges, and documented deficiencies in orchard and vineyard crops in California is extremely rare. There may be some benefit to providing phosphorus to young trees, however evidence to date does not show mature orchards need it.
Phosphorus is a macronutrient and one of the nutrients found in complete fertilizers, but we don’t talk about it much in orchard crops. Long story short, most research involving phosphorus fertilizers hasn’t yielded many positive results. Deficiencies are so rare in California that the almond and pistachio production manuals published by UCANR only devote a few sentences to the nutrient. However, results of a recent almond replant trial I did with Greg Browne, detailed in a poster at the 2019 Almond Conference have got me thinking about the nutrient.
Before I continue: you should be diagnosing nutrient deficiencies in your mature orchard with July leaf tissue analyses (Table 1). If your trees are sufficient, you shouldn’t need to worry about phosphorus. Oversupplying any nutrient will not improve your orchard’s performance, as the orchard crops we grow do not accumulate nutrients to save for later. In fact, most plants reduce phosphorus uptake when their supplies are sufficient. Furthermore, applied phosphorus stays put in soils, so a single application can last many years.
| Crop | Critical Value | Sufficiency Range |
|---|---|---|
| Almond | 0.1 | 0.1 – 0.3 |
| Peach, Nectarine | 0.1 | 0.1 – 0.3 |
| Pistachio | 0.14 | 0.14 – 0.17 |
| Plum, Prune | 0.1 | 0.1 – 0.3 |
| Olives | 0.1 | 0.1 – 0.3 |
| Walnut | 0.1 | 0.1 – 0.3 |
Table 1: Leaf critical values and sufficiency ranges for commonly grown California orchard crops.
Phosphorus, like nitrogen, is found in a lot of compounds in plants. It’s a critical component in the building blocks of DNA and RNA. It’s part of the compounds that form cell membranes, it’s involved in metabolic processes as part of a high-energy but unstable compound (ATP), and it is involved in some cell regulatory processes. Deficiency symptoms can be difficult to diagnose, as they’re typically just reduced above-ground growth, and leaves that are a deeper green. In severe cases, leaves can become purplish or even exhibit burn. Flower initiation and seed formation could be reduced, and leaves may fall off prematurely. (Again: applying P in excess of plant needs will not enhance flower and seed formation!). Orchard crop removal rates are for the most part very low (Table 2), which is one reason why deficiencies are so rare.
| Crop | P2O5 removed | Harvest unit |
|---|---|---|
| Almonds | 18-20 lbs | 1000 lbs of kernels |
| Prunes | 5 lbs | Ton fruit removed |
| Pistachios | 7 lbs | 1000 lbs marketable yield |
| Olives | 2-4 lbs | Ton of fruit |
| Stone Fruit | < 1 lb | Ton of fruit |
| Walnuts | 10 lbs | Ton of harvested nuts |
Table 2: Removal rates for commonly grown California orchard crops.
Phosphorus is immobile in in soils. Plants only take it up in the form of orthophosphate (H2PO4- or HPO42-). In low pH soils it will bind to iron and aluminum minerals, and in high pH soils it will bind to calcium, forming low soluble compounds. It can be helpful to think of P as being in one of several ‘pools’ in the soil:
1) P that is completely available for uptake, found in soil water
2) P that is mostly available for uptake
3) P that is somewhat available for uptake
4) P that is mostly unavailable for uptake
Soil tests mostly show the amount of phosphorus available in pools 1 and 2. P found in pools 3 and 4 may eventually become available for uptake. There is VERY little P available in pool 1, and any applied P will rapidly move into pools 2, 3, or 4.
Because of the different ways P can be bound in soils based on the soil pH, the tests that are performed by your soil lab will differ. The Bray I/Weak Bray test is used in neutral to acidic soils and the Olsen Bicarb test is used in basic/alkaline soils. For soils that are around pH 7.0 you may be provided with both values depending on the laboratory; the numbers will differ as the reagents used for extraction are different. It is important to understand this, as what is considered as ‘moderate’ or ‘high’ vary drastically based on the test performed (Table 3). These tests also do not remove all of the available P in a soil, so the values cannot be converted to pounds of P2O5 per acre. It’s better to think of the tests as an indicator as to whether your crop will respond to fertilization. It is thought that orchard crops may be able to remove adequate P in soils with 5 ppm (Olsen Bicarb).
| | Weak Bray (in ppm P2O5) | Olsen Bicarb (in ppm P2O5) |
|---|---|---|
| Low | < 20 ppm | <10 |
| Medium | 20-40 | 20-Oct |
| High | 40-100 | 20-40 |
| Very High | >100 | >40 |
Table 3: Soil P test interpretation values. Note: these do not take into account specific crop soil critical levels.
Because phosphorus is immobile in soils, the main method of uptake is diffusion. An extremely tiny amount of phosphorus is dissolved in soil water; once this phosphorus is taken up by plants, some bound phosphorus will dissolve into the soil solution and move toward roots. The speed at which P can move toward roots is extremely slow, so the amount of P in soil that roots can access is only a few millimeters around a root. Most plants have evolved root hairs that increase the amount of soil that can be exploited for immobile nutrients, though a small minority of plants, like pecans, do not have root hairs.
Phosphorus nutrition in plants can be aided by mycorrhizae, but only in plants that form mycorrhizal associations. Mycorrhizae are symbiotic fungi that invade plant roots. Plants provide energy to the mycorrhizae, and mycorrhizae in turn provide some nutrients, notably phosphorus. These infections happen when plants are extremely young, and research that has looked at infecting trees with mycorrhizae at planting has not been effective for various reasons. Additionally, many of these fungi are already found in soils.
Photo of P deficient walnuts. Photographer is likely Serr.
There has been very little research that I could find that examine phosphorus in mature California orchards; Serr published a paper in 1960 on overcoming P deficiency in walnut orchards growing in volcanically derived soils: trenching or applying phosphorus in circles around the tree was effective in fixing the deficiency symptoms. A much more recent publication on potassium fertigation in almonds, done at Nickels Soil Laboratory in Arbuckle, CA, included monopotassium phosphate among several other potassium fertilization treatments. While leaf tissue analyses for phosphorus were not reported, the authors noted that the trees were not deficient. Soil P levels ranged from 6 to 15 ppm (Olsen bicarb). The potassium treatments were applied annually for five years. In the first four years there were no differences between fertilizer formulation or application method, but in the last year the treatment that included phosphorus (the applied rate would have supplied 1 lb K2O and 1.5 lbs P2O5 per tree) had the highest yield: approximately 200 lbs/acre above the next highest yielding treatment, banded potassium sulfate at a rate of 2 lbs of K2O per tree. It also outperformed other fertigated potassium treatments that supplied 1 or 2 lbs of K2O per tree. The orchard was in its 10th leaf at the conclusion of the trial. Since this trial was not set up to examine whether the orchard was deficient in phosphorus (which would have required a treatment with only phosphorus), the results should be taken with a grain of salt. However, given the significant increase in yield above other potassium-only treatments, it is probably worth examining phosphorus fertilization in other mature almond orchards.
Senescing leaves on P deficient nectarine trees. Photographer not recorded; copyright belongs to the University of California Division of Agriculture and Natural Resources.
More recently, I worked on an Anaerobic Soil Disinfestation trial in Chowchilla, California with Greg Browne (USDA ARS), and Jamie Ott (USDA ARS) and Abdur Khan. The full results were presented during the poster session of the 2019 Almond Conference. We found that applying 6 oz of P2O5 per tree below the soil surface in the first growing season resulted in approximately a 1 inch trunk circumference increase over the control, which received very low rates of nitrogen, phosphorus, and potassium (the control was applied to the entire field, so the P only treatment would have received the same low rates of N and K). The increase in growth was no different than fertilizing with two different types of complete fertilizers (15-15-15, applied at a rate of 5 oz of N per tree; and 15-9-12 applied at a rate of 6 oz of N per tree) or 5 oz of nitrogen applied as urea. The full results will be detailed in a future article, and we will be continuing to monitor this orchard in its second year.
Research on application methods has shown that broadcasting phosphorus fertilizer on the soil surface may be ineffective, as it binds to soils very quickly. One study examining the movement of surface-applied monoammonium phosphate found that with 3” of irrigation applied after the fertilizer, the phosphorus only moved 1-2 inches below the soil surface, regardless of soil type. 6” of irrigation only moved the fertilizer one inch deeper. However, modern microirrigation systems result in a proliferation of roots very close to the soil surface, so this depth may be sufficient for plant uptake. If you need to fertilize your orchard trees with phosphorus, the best way to apply it is one that gets the phosphorus into the root zone. You can trench it in or drill it into the soil, just be sure the fertilizer is in the wetted zone of the irrigation system. Fertigation has been found to be effective at moving phosphorus deeper into the root zone, however it can precipitate with calcium found in irrigation water, so check your water quality first and perform a jar test before fertigation. If you must surface apply it, band it under the emitters and follow with a heavy irrigation. Surface banding will result in a high concentration of phosphorus, which will increase the likelihood that binding sites will be saturated and the nutrient will move deeper into the soil. Most forms of inorganic fertilizer will be adequate for fertilization; the only exception is rock phosphate, which is only appropriate for acidic soils. Since applied phosphorus is rapidly bound to soil particles, applying it in such a way that it gets to the roots is more important than the specific formulation. Phosphonates (such as phosphorous acid), which are compounds that have fungicidal activity, do not provide phosphorus to plants.
Composts and composted manures can be a source of phosphorus, but to my knowledge the P credits from the application methods used in orchard crops (surface application with no incorporation, with likely loss of the organic material during harvest activities) has not been examined. However there is an analogue in annual cropping systems: no-till agronomic crops. In these systems, soil immobile nutrients become concentrated in the top six inches of soil. These are systems where manure (not always composted!) would be applied every year, and if the application rate of manure is made based on the nitrogen requirements of these crops, this would result in overapplication of P every year and a buildup of P in the soil. Based on P’s imbobility in soils and the fairly low rates of compost to orchards, I would speculate that the P contribution from surface applied composts and composted manure is low, but given the extremely low P needs of orchard crops, I wouldn’t completely discount it.
Changing soil pH to change P availability has had mixed results and is not recommended, especially in crops with low P needs like orchard trees. An exception to this is in plants growing in extremely acidic soils (pH less than 5.5), where the acidity and high levels of aluminum can impede root growth and nutrient uptake. We do not naturally have these extremely acidic soils in the Central Valley.