Focus On Prunes 3 Key Ingredients
Some things don’t change. Adequate prune orchard mineral nutrition is critical to maintaining high yields and good orchard health. Focus on maintaining the “Key 3” — nitrogen (N), potassium (K), and zinc (Zn). Although in certain orchards other elements may be important based on local conditions, adequate N, K, and Zn are needed across Northern California for good prune orchard yields.
Some things do change. Fertilizer costs are up. Prune prices are steady to dropping. Maintaining good return on your fertilizer investment is key to staying profitable. Don’t pay more for fertilizer than you need. Here’s a quick prune orchard fertility review, in order of importance to prune production.
Potassium (K) is the most important mineral nutrient in prune production. Prune fruit accumulate K steadily from bloom through harvest. In July and early August, as root growth slows and soil moisture is used rapidly, tree K uptake may decrease. However, fruit K needs are unchanged and fruit “pull” K from the rest of the tree, especially leaves. This causes leaf K content to drop, risking deficiency if leaf levels were not high enough going into summer.
K deficiency starts a devastating cascade of trouble: deficiency leads to leaf scorch and drop, which leads to sunburn, which leads to cytospora infection, which leads to limb or scaffold death, which leads to a loss of orchard production that can last for years. Keeping adequate K in a prune orchard is essential to sustainable production. Prune fruit need a lot of K. Prune orchards with a heavy crop need a lot of K.
Traditional University of California annual K fertilizer maintenance programs suggests 400 to 500 pounds of potassium sulfate (0-0-50; sulfate of potash) per acre banded in the fall in orchards that are solid set irrigated or shanked in where orchards are cultivated or flood irrigated. Orchards with well-drained soils that receive significant winter rains can use potassium chloride (0-0-60; muriate of potash) at about the same rate. Those rates cost $200 to $220 per acre in today’s market — a huge amount of money to ante up before you know what your crop will be like the next year.
Wait For Spring
Alternative plans to a big slug of potassium in the fall are 1) injecting K fertilizer through drip or microsprinkler irrigation lines in the spring and summer, 2) a steady foliar program of a minimum of the equivalent of 100 pounds of potassium nitrate (KNO3) per acre or 3) some combination of option 1 & 2. In-season K fertilization allows growers to check the crop size before putting on any fertilizer. A light crop may require very little if any fertilizer. Traditional fertilizers such as ground applied potassium sulfate and foliar applied potassium nitrate are proven effective. Be careful when looking at new products, as those have risks as well.
Potassium thiosulfate (KTS, 0-0-25) is an effective liquid fertilizer that can be injected through microirrigation systems. However, high rates of KTS (more than 10 gallons per acre per application) can damage or kill trees depending on the orchard conditions. Alternatives to KNO3 for foliar K fertilization are available in the market. Many are liquids that are easier to mix than solid fertilizers. Potassium nitrate is a good, efficient foliar fertilizer that won’t burn leaves when applied at reasonable rates (20 to 25 pounds per acre in 100 gallons).
When using liquid K foliar fertilizers to replace KNO3 in a spray-only program (no soil applied K fertilizer) in an orchard carrying a good crop, use the amount of material equivalent to 100 pounds of KNO3 per acre per season. Multiple sprays will be needed, just as in a KNO3 program. Otherwise you run the risk of under-supplying your orchard with K. A solid K fertilizer program is a cornerstone of a good prune orchard management program. Cutting corners with your K fertilizer program can put the health of your orchard at risk.
Apply N Soon
Nitrogen (N) is essential for good prune production and tree health. N-deficient prune trees make fewer flowers and therefore set smaller crops. Prune trees with low N levels are more susceptible to bacterial canker than trees with adequate levels of N. Trees store N over the winter in woody tissue to use in spring growth. An orchard with a good leaf N level in July leaf samples (good storage reserves) shouldn’t need N fertilizer until mid-April the next year once the crop can be checked. Light crops mean less N fertilizer need, much like K.
If July leaf N levels are low, an application in the fall, before trees go dormant, may be warranted. If you plan to do this, remember that trees have limited N storage space and that once leaf drop occurs, prune trees don’t absorb N from the soil. If you are going to apply fertilizer N in the fall, use low rates (less than 50 pounds per acre) and get it on early (not later than September).
Spring For Zinc
Zinc (Zn) is important to healthy growing points in plants. Bloom through spring is the time when the most growing points are found on plants and so is the period of highest Zn demand. To meet this timing need, Zn is usually applied as a foliar fertilizer in the fall or spring. A high rate (20-plus pounds per acre) of zinc sulfate (36%) sprayed in the fall once natural leaf drop begins delivers Zn to prune trees and removes leaves. In my experience, early defoliation following a fall zinc sulfate spray won’t occur if the orchard is dry.
A good alternative to a high rate of Zn in the fall is 4 to 6 pounds per acre 52% zinc (neutral zinc, etc.) in the spring, preferably before leaves reach full size and no later than mid-May. There are many different Zn foliar materials. Many effectively move Zn into trees, but cost and risk of phytotoxicity vary from product to product. Talk with your pest control adviser about the most cost effective options that supply sufficient elemental Zn.