Fall is the season to make potassium chloride (muriate of potassium or MOP) or potassium sulfate (sulfate of potassium or SOP) fertilizer applications in walnut orchards. Many walnut orchards, especially heavy producing varieties such as Tulare, Chandler, and Vina, having had two back-to-back large crops, have likely had amounts of potassium removed in the crop. This potassium needs to be replaced, especially in orchards on heavy-textured soils where soil potassium is bound tightly by soil clay particles and not readily available to trees.
Mid-summer leaf samples are the most reliable way to assess the potassium status in walnuts and other orchard crops. Potassium is considered deficient at leaf levels below 1 percent and adequate above 1.2 percent in samples collected in early July.
To move tree potassium levels from deficient to adequate levels, a single large dose of MOP or SOP is required. The amount of fertilizer required depends on the degree of deficiency and the clay content of the soil. Please contact me if you want to discuss this, as there is no simple or single rule of thumb that works in all situations. Once adequate levels of potassium are achieved, lower annual doses can be used in a maintenance program. Past experience in walnuts suggests that yearly doses of around 400 pounds of MOP or SOP will keep tree potassium levels at acceptable levels. This approach – as compared to larger less frequent applications – also allows for annual rate adjustments based on the results of leaf sampling and helps avoid feast or famine cycles of available potassium that can result from less frequent applications.
Fall applied MOP and SOP fertilizers should be applied in bands positioned so as to deliver potassium where the majority of roots are located. If broadcast, potassium is quickly absorbed by clay particles and rendered unavailable for tree uptake. Bands in a young orchard should be closer to the trunks than in older blocks in mature or-chards, banding in the same location in subsequent applications probably helps increase potassium availability under the band, though this has not been rigorously tested in experiments. A convenient way to make sure the fertilizer is placed in the same location is to apply a band on each side of the trees at a location just inside the edge herbicide-treated strips.
There are two considerations in deciding whether to use MOP or SOP: Cost per pound of delivered potassium and the risks of salt damage from the chloride ions in MOP. SOP is roughly twice the cost of MOP, and both deliver roughly the same amount of potassium per ton of fertilizer (SOP is slightly higher). There is no difference in the efficiency or availability of potassium in these materials, once they are in the soil. Potential salt effects from the chloride in MOP could be a problem where soil salinity is already high from other sources – such as some Delta and West Side San Joaquin County locations. East Side orchards are generally free enough of soil salts that adding chlorides as MOP in the amounts normally used to treat deficiencies should not cause problems. Under low-salt and well drained conditions, research and experience has shown that chloride ions are effectively leached from the root zone when fall applications are followed by 8 inches or more of winter rainfall. To be on the safe side, however, it is probably best to avoid using MOP 1) when application rates exceed 1,000 pounds per acre for deficiency correction, 2) in young orchards, and 3) in poorly drained situations where leaching of chlorides from the root zone may be impaired.
In recent years, two new potassium fertilizer formulations have become available: High solubility SOP formulations for drip system injection and liquid potassium thiosulfate (KTS). These formulations offer the prospect of greater convenience and – when injected into localized irrigation systems like drip and microsprinklers – greater efficiency of fertilizer use (i.e. a greater percentage of what is applied actually ends up in the tree). Further testing is needed to confirm these benefits in walnuts over a wide range of the soil and orchard conditions.