Plant nutrient levels are important, especially when heavier almond crops are produced. Nitrogen should not be added in late fall or winter due to leaching and potential loss. Potassium, foliar-applied zinc, and boron however, should be added if needed. Zinc sulfate can play an important role in breaking the disease cycle of rust and shot hole when applied as a foliar spray in November.
These fungal diseases can remain on older leaves through the winter and be an inoculum source that infects new leaves in the spring. Young vigorous orchards often become zinc deficient due to their excessive growth and development.
Zinc deficiency symptoms often appear in late summer and are referred to as ‘little leaf’ or ‘rosette’ and are characterized by a shortening of the internodes toward the tips of the shoots and small narrow leaves. Often leaves are bent upward on either side of the mid-rib. Zinc foliar sprays are often applied in the fall to correct deficiency, either as zinc sulfate (neutral zinc 52 percent) or in a chelated form. I personally don‘t like waiting so late in the season that the sprayer blows most of the leaves off during the application.
I often make my zinc applications in mid to late October in order to make sure the leaves burn from the zinc. There is environmental concern that continued zinc sulfate sprays (10 pounds to 15 pounds of zinc sulfate in 100 gallons of water) will lead to soil contamination, so you might consider more frequent chelated zinc applications during the season. Chelated zinc sprays can be applied safely at anytime during the season when other applications are being made.
Many growers apply too much nitrogen and not enough potassium or boron, especially if you are irrigating with water from an irrigation district. Boron nutrition often goes unnoticed and should be monitored with hull samples at harvest. If hull samples are less than 80 ppm boron, your trees could be deficient and you may be experiencing a yield loss as a result. Boron can be applied to the soil, in herbicide mixes, or in foliar applications. Soil applications can be made (not in a band like potassium) at rates of 2 pounds to 4 pounds of actual boron per acre (10 pounds to 20 pounds of 21 percent product). Granular boron can be broadcast on the soil while soluble boron formulations can be injected into micro-irrigation systems.
I have often observed potassium deficiency in young vigorous second-generation orchards. In many of these cases the first-generation orchard probably had plenty of potassium stored in the soil and the grower didn‘t observe deficiency symptoms and didn‘t apply potassium. But that first-generation orchard probably used up most of the available potassium in the soil, and when the second-generation orchard is planted potassium is limited. Then potassium deficiency symptoms can become visible in young second-generation orchards and growers may end up applying as much potassium as nitrogen.
Young, vigorous growing orchards often display symptoms in late spring to early summer on leaves of new shoots. Leaves can turn pale and develop marginal necrosis and roll into a boat shape or the classic ‘Viking‘s prow’ symptom. The ‘Viking‘s prow’ symptom typically shows first in the tops of trees and later throughout the whole tree. As potassium deficiency progresses, fruit bearing spurs often die and spur renewal is reduced.
The current crop is not affected but future yields are reduced, making the correlation between potassium deficiency symptoms and reduced yields difficult in a single year. Butte is a good indicator variety of potassium deficiency, displaying symptoms while other varieties do not. Recovery from potassium deficiency is a long-term process, once you see leaf symptoms the trees are already deficient and you may experience yield loss before you can correct the problem. Leaf analysis should be performed annually in July to prevent symptom development.
The current UC recommendation is to keep leaf potassium levels at or above 1.4 percent in July-sampled leaves.
An almond tree in production uses as much potassium as nitrogen, and similar to nitrogen, 10 pounds of potassium are needed for every 100 pounds of nutmeats. Thus, a 2,000-pound per acre crop will need approximately 200 pounds of potassium to replace the potassium that was removed in the crop. Potassium is released slowly in the soil and not readily leached.
Because the potassium ion (K+) is positively charged it can easily be bound with negatively charged clay particles and become unavailable to the tree. For this reason, we typically apply potassium sulfate in bands next to the tree rather than broadcasting. Potassium sulfate is preferred over potassium chloride because of salt accumulation in the soil. UC research has demonstrated that annual fall applications of 500 pounds per acre of banded potassium sulfate would maintain potassium levels.
Two thousand pounds per acre of potassium sulfate can correct a deficiency for four or more years — but it‘s expensive. In clay soils double-concentrated bands may be necessary. You may also wish to apply gypsum, calcium sulfate, as a band over the top of previous potassium bands. The calcium ions (Ca++) will displace potassium bound to clay particles, making more potassium available while also improving water penetration.
Liquid potassium fertilizers can be applied effectively in-season through your irrigation system, and drip systems are probably better for potassium applications than micro-sprinkler because the amount of K+ per wetted area is higher, and as a result, potassium will penetrate further into the root zone and be more available. But liquid potassium is generally more expensive and you may not apply as much potassium per acre compared to banding potassium sulfate. Avoid applications that spread potassium applications over a large soil area because it will become bound to the soil and unavailable to the tree. Foliar in-season potassium sprays can alleviate symptoms more quickly, but are relatively expensive and their effect is short lived.
Applying potassium through compost
One of my favorite methods of applying potassium is through compost. While working with organic almond growers I came to appreciate the benefits of compost. Organic almond growers typically apply 10 tons of com-post per year in order to get their desired 200 units of nitrogen per acre (assuming 1.0 percent nitrogen in the compost). But when growers purchase compost they get a lot more than just nitrogen. A typical batch of compost (New Era) that I will use as an example had 1.32 percent nitrogen, 2.75 percent potassium, 1.45 percent phosphorus, 2.6 percent calcium, 0.5 percent sulfur, 1.16 percent magnesium, and 30.2 percent organic matter.
Assuming you are an organic grower applying 10 tons of compost per acre, you are applying 264 pounds nitrogen, 550 pounds potassium, 290 pounds phosphorus, 520 pounds calcium, 100 pounds sulfur, 232 pounds magnesium, and 6,040 pounds organic matter.
Our San Joaquin Valley soils are typically very low in organic matter and we know that it is a critical component of many nutrient cycles, especially the nitrogen cycle, so anytime you can increase your soil‘s organic matter you can increase soil fertility. I know many conventional growers that are applying compost, especially because of the added value and expense of potassium in addition to organic matter and other additional nutrients.
In light of Salmonella concerns, I recommend compost over the use of fresh animal manures. All composts are a little different, but they should come with a certified nutrient analysis and a statement that it has tested negative for Salmonella, E. coli, and Staphylococcus (bacterial pathogens).
From the certified nutrient analysis you can determine the nutrient value of the compost and rate you intend to apply. The compost analysis I examined had reached a high temperature of 152 F and averaged a thermophilic temperature of 140 F from 90 to 120 days. I would love to see more growers using compost and I believe the use of compost will enhance soil quality and ultimately orchard productivity.