Post-harvest application of soil and foliar nutrients are important for optimising almond production for the coming season.
Soil Nutrients - Nitrogen and potassium are the biggest nutrient requirements for almond crops and generally it is recommended that about 1/4 of the total N & K requirements are applied post-harvest, however this should be assessed based on the current season's crop load. Almonds use these nutrients to supply energy and sugars for flower and early shoot development for the next season. It is critical to ensure soil moisture levels are optimal when N & K are applied (or any soil fertilizer for that matter) otherwise fertilizer efficiency/uptake will be low and there is also potential for root damage in dry soils due to the high salt index.
Phosphorous is essential for reproduction, energy production (eg. ATP) and cell division. While almond trees do not have a large P requirement (30 kg P/ha for maintenance in bearing trees) it is critical during the bud-burst to fruit set period so it needs to be available as soon as roots become active in late winter. If soils are low in sulphur then applying Single Super will be beneficial, otherwise MAP/DAP/MKP/APP should all be fine. Some growers may also be applying phosphonate (eg. PHOSIC 600) post-harvest to address any phytophthora concerns. While some companies may claim that the phosphorous (eg. phosphite) in their phos acid products is available to the trees as a NUTRIENT source, this is simply not true at least in the short to medium term. Phosphite that remains in the soil will over time (eg. 6-12 months) be converted to the plant available phosphate form, however it should not be relied upon as a P nutrient source for the coming season.
Post-harvest is also a good time to apply calcium if exchangeable levels are low in the soil or pH needs adjusting. While calcium may be considered a less important nutrient by many, my experience in the field indicates that there is a direct relationship between tissue calcium levels and kernel yield. Calcium is also an important nutrient for reproduction and pollen tube development, so it is essential that tree calcium levels are optimal before they go into dormancy.
Zinc and boron are both essential trace elements for almond production and are required for reproduction, shoot development and fruit filling. The combination of average to good yields with good shoot development this season puts a high demand on boron and zinc so levels in the trees may be depleted and therefore post-harvest application is even more critical in this situation. Boron is quite mobile in lighter soils & so has good availability but can also be lost through leaching if over applied. If soil boron levels are <0.5 mg/kg then look at applying 1-2 kg B/ha. If boron hull levels are also <80 mg/kg consider the higher rate. Unlike boron, zinc is basically immobile in the soil and therefore soil application is less efficient, particularly if soil pH is alkaline. Best zinc availablility will be seen on sandy acid soil types and if exchangeable zinc is <2 mg/kg look at applying 1-2 kg Zn/ha.
Foliar Nutrients - Post-harvest foliar sprays are essentially aimed at 'bud building' for the coming season and nitrogen, zinc and boron are the main nutrients of interest in this regard. Lo bi urea is used in many crops to aid bud development/health and the Almond Board recommends applying a 1% solution (eg. 1 kg urea/100L) in late April and early May. Numerous studies in almonds indicate a consistent benefit from post-harvest foliar boron sprays in terms of improved fruit set and nut retention in the following season (250-500 g B/1000L). Given the high boron requirement for almond production, I would still recommend both soil and foliar application, particularly on low boron soils. Like boron, foliar zinc has also shown to be beneficial for bud development and fruit set, mainly due to its role in auxin production (an essential plant growth hormone).
While there are various factors that determine potential fruit set, nutrition is one that (unlike the weather) can be controlled & manipulated by growers to ensure their trees have the best chance of setting an optimal crop.
Defoliation - There has been some discussion on the best option for defoliation of trees prior to dormancy. Traditionally, high or phytotoxic rates of zinc sulphate have been used and it was thought this would provide some zinc to enhance bud development for the coming season. The other alternative is to use high buiret urea (eg. 70 kg/1000L) to defoliate. In terms of foliar absorption, nitrogen is absorbed much faster (hours) than zinc (2-5 days) which only has limited plant mobility. Also the condition or health of leaves has a major effect on foliar uptake and at this time of the season the condition of leaves is usually not that suitable for foliar uptake anyhow. So the reality is that zinc sulphate will probably not supply any available zinc for the coming season, whereas urea is more likely to supply some nitrogen for bud development.
SJB Soil Applied Nutrients
SJB Foliar Applied Nutrients for Bud-building
Systemic Fungicide for Phytophthora Control
PHOSIC 600 (can be applied to the soil or as a foliar spray)