Choose Prune Nutrients Wisely
Costs are up, but the need for fertilizer and other inputs to produce a large, high-quality crop remain the same. Using more efficient materials and practices can help control costs relative to income. However, be careful not to trim muscle when you are looking to cut out fat. One example of this is in fertilizer materials and rates.
Dried prunes contain roughly 1% potassium (K) and 0.6% nitrogen (N) on a per-weight basis. At harvest, a prune crop can contain 70% of all the potassium in a tree and half the nitrogen. That amounts to the equivalent of 3-plus tons of potassium sulfate and 4-plus tons of ammonium sulfate trucked out of a 50-acre orchard in a three dry ton per acre crop in one year.
To replace this kind of orchard output, substantial amounts of N and K must be available to heavily cropping prune trees. Potassium is particularly important because of the risk of sunburn and scaffold death following defoliation due to potassium deficiency. University of California (UC) recommendations for maintenance rates of K fertilizer include 250 to 400 pounds of soil-applied potassium sulfate per acre per year — depending on the irrigation system — or 100 pounds of potassium nitrate per acre per year as a foliar fertilizer. The soil provides significant K, but not enough or at fast enough rates at certain times of the year to satisfy the needs of a rapidly growing crop while maintaining leaf health and avoiding deficiency.
Don’t Stop On A Dime
The use of reduced potassium application rates from those listed above should be approached very cautiously. Whether you are considering reducing standard fertilizer rates or using new materials marketed as more efficient than standard products, be careful not to under-fertilize your trees.
Don’t step over a dollar to pick up a dime. For example, a four-year UC research study documented that 100 pounds of potassium nitrate per acre per year, divided into four to five sprays, is as effective in maintaining leaf K levels, fruit size, and total crop yield as a large maintenance rate of soil applied potassium fertilizer (600 pounds per acre per year potassium chloride). Replacing a single spray of 20 pounds per acre of potassium nitrate (9 pounds K2O) with one using 1 gallon per acre of 0-0-26 potassium fertilizer (2.9 pounds K2O) reduces the amount of K2O applied in a single spray by 70%.
You would have to repeat the application twice at the same 1 gallon per acre rate — for a total of three applications — to equal the amount of K2O delivered in a single spray using 20 pounds per acre of potassium nitrate. It would take 15 applications at a rate of 1 gallon per acre 0-0-26 liquid material to match the K delivered in 100 pounds per acre potassium nitrate. You could do it, but I’ll bet it would cost more than the potassium nitrate program.
If the 0-0-26 material — or any other product — is reported to be more efficient than potassium nitrate, check it out in a small block. Even if the material is more efficient than potassium nitrate, if insufficient K per acre is applied in a season using the new material, the orchard may become K deficient. You can’t build a 2,000-square-foot house with the materials for a 1,000-square-foot house, no matter how good the quality of those materials.
A prune orchard carrying a good crop has high K demands. If enough K doesn’t get into the tree to meet crop K demands, there is a strong chance of potassium deficiency, leaf drop, small fruit, sunburned limbs, and loss of fruiting wood. New ideas should be considered, but do the math and/or a small test block before committing to a new nutrient program.