Sacramento Valley prunes are most typically deficient in nitrogen, potassium and zinc. Tissue samples taken in 2003 from 23 orchards in the Integrated Prune Farming Practices (IPFP) project showed 67 percent of the sites were nitrogen deficient (less than 2.2 percent), 72 percent were zinc deficient (less than 18ppm) and none were deficient in potassium (less than 1.3 percent) or boron (less than 30ppm).
Current production economics may be responsible for less applied nitrogen and more orchards in the deficient range. Fall application of nitrogen is not suggested since uptake efficiency is very poor as leaves fall and trees enter dormancy. Fall is an excellent time to address zinc and/or potassium nutrition.
Many farmers are following a good potassium program, which along with an extremely light crop for 2004 may explain why none of the IPFP surveyed sites were potassium deficient. Potassium deficient trees show symptoms in early to mid summer, maybe earlier for severe deficiencies. Leaves become pale and leaf size, shoot growth and fruit size are reduced. By mid summer, leaves turn a buckskin color and develop marginal scorch. Scorch can involve the entire leaf especially after hot weather or with a heavy crop. Fruit sunburn and shoot dieback may occur. Tree vigor, water stress, diseases and/or sunburn can mimic potassium deficiency. A July leaf sample is useful to confirm if potassium is low. When potassium deficiency is confirmed, a grower needs to select solution that works best for their orchard. Not every technique will fit every orchard.
For fall applications, potassium sulfate (54 percent K20) or potassium chloride (63 percent K20) are the most common choices and are soil applied about leaf drop. Potassium chloride has been used safely but can cause chloride toxicity if chloride remains in the root zone. One way to improve the safety of potassium chloride is to apply it slightly later to avoid any chloride uptake if leaves are still on the tree and active.
Potassium chloride should not be used on weak trees, young trees, orchards with water tables, hardpan, stratified soils or any restriction which would prevent chloride from moving out of the root zone. Chloride should be applied early enough to provide for adequate leaching (approximately 10 inches of rainfall). If rain fall is insufficient then winter irrigation would be recommended. If in doubt, use potassium sulfate.
One strategy is a massive application using 1,500 to 2,500 pounds per acre. Massive applications have the best chance for quickly correcting deficiency, last for 3 to 5 years depending on application and yield, but are expensive in the year of application. Retired Butte County Farm Advisor Bill Olson developed a maintenance program using about 500 pounds of potassium sulfate (270 lbs K20) per acre per year. Potassium is applied by banding or shanking down each side of the tree row about 4-5 feet away from the tree trunk. Remember where the band is and apply each year to the same location. Shanking is preferred in disked orchards and broadcast applications have not worked well. The strategy is to concentrate potassium and get it as deep into the root zone as possible.
Zinc deficiency symptoms appear early in the season. Symptoms are delayed bud break of both vegetative and flower buds. Developing leaves are small, chlorotic and appear in tufts (little leaf). Severe zinc deficiency usually results in terminal shoot dieback and smaller fruit. Leaves mildly efficient are slightly smaller with chlorotic areas between lateral veins. Often by mid summer, shoot growth tends to hide zinc deficiency symptoms making diagnosis more difficult. July leaf samples are useful for confirming zinc deficiency. Look for values below 18 ppm for prunes.
Many techniques are available to correct zinc deficiency, each has advantages and disadvantages. Probably the easiest and most effective correction for prune, are fall foliar applications of zinc sulfate (36 percent metallic zinc) applied at 10-15 pounds per acre. Rates can be adjusted upward to achieve leaf removal but watch for phytotoxicity.