For a number of years, growers have been successful in meeting their orchard potassium (K) requirements by applying low rates of K fertilizer as either a broadcasted dry material or fertigation injections of low rates of liquids. This technique defies what much of the old literature regarding low rates of K has stated.
Potassium has always been considered a material that had to be applied in concentrated bands above the root zone, the rationale being that if the application was not concentrated, the K would be rapidly fixed by the negative sites in the soil clay and organic fractions. Once fixed, they would be unavailable to the roots.
While this is still a generally true statement, cultural factors that have evolved in the tree crop industry have changed this.
Decades ago when cross disking, spring-toothing, weed-knifing, leveling, and rolling were common practices in orchards, a lack of shallow feeder roots due to this root damage and repeated soil compaction passes were the norm.
A Change In The Root Structure
Today, in non-cultivated orchards the use of planting berms no cross working, and the use of light-weight weed control applicators have changed the landscape.
Another major change has been the trend towards less flood and furrow irrigated orchards in favor of low-volume sprinkler and drip irrigation systems. Along with increasing efficiency, these systems, as a result of increased frequency of applications, produce trees with a very dense, shallow root system (see photos).
This tremendous volume of fine feeder roots just millimeters below the soil surface provides a large surface area of potential absorption directly below the placement of the K materials.
A five-year study in a mature bearing almond orchard in California demonstrated that low rates (400 pounds/acre) of sulfate of potash broadcasted in the undisturbed centers of the tree rows produced yields and leaf K levels comparable to the conventional concentrated bands out in the drive rows.
This provides growers with a lower-cost method of spreading out their expense of the pricey K materials over several years instead of taking a large financial hit in a given year.
The distribution and proximity of the root mass directly below the drip emitters or micro-sprinklers also helps to explain why growers who have been applying low rates of fertigated potassium thiosulfate (KTS), potassium carbonate, or other liquid K formulations have been successful in sustaining leaf K values. Traditionally they had been told these low rates would be tied up in the soils and never make it to the roots.
One must keep in mind that the local soil texture will have a lot to do with this response. Low-cation exchange capacity (CEC) soils may still require larger quantities of topical broadcast applications to meet the trees’ total K needs, but this does not mean that a substantial amount of this requirement cannot be met through maintenance or supplemental doses of low rates of dry-broadcasted or fertigated injections of K.