When formulating a nutrient plan for an orchard crop, the terms “always” and “never” are frequently referenced. Though they may be used in statements that are generally true, one must consider all the variables that may influence choices such as micro-climates, crop physiology, and water availability and quality.
A couple cases in point: “Always apply potassium in concentrated bands or it will be tied up by the soil and unavailable to the plant.” It is well understood that potassium can be readily bound to the negatively charged soil clay particles and have its availability in the exchange solution reduced. The rationale behind the mass dose application is to saturate the negatively charged sites in the zone, thus allowing the remainder of the material to be present in the exchangeable phase. While this is not a false assumption, there are many cultural nuances in orchard crops that have challenged this principle.
Nuances In Crops
Not all soils have such high clay content that this universally applies. Many coarse textured sands have such a low cation exchange capacity (CEC) or organic matter (OM) content that there is essentially very little capacity to fix potassium. There are many areas where orchard crops are being grown on soils with CEC’s of 2 or less or OM’s of less than 0.25%. In some of these soil types, care must be taken to not apply concentrated doses of fertility products or crop injury may occur, especially if applied during an active part of the growing season.
Shallow Root Masses
With the advent of drip and micro-sprinkler irrigation systems, the high percentage of extremely shallow, dense masses of root networks makes low dose applications of strip broadcasted sulfate of potash (SOP), or low rate fertigation injections of liquid solubles like potassium thiosulfate (KTS) or other in-season potassium blends highly effective. Detection of these applications are immediate and can be seen in the current season’s leaf analysis. It must be pointed out however, that usually these low dose applications are used to supplement total potassium needs and cannot be relied on to provide the total demand for the crop at an economically competitive level.
Nitrogen Application To Ensure Uptake
Second case: “Never apply soil nitrogen when the tree is not actively growing and transpiring.” Again, this statement is generally true, but there are situations where it may not apply. The rationale here is that if N is applied when there is no uptake by the tree, it is subject to leaching and volatilization losses.
Root activity begins in tree crops well ahead of leaf-out. Almonds have significant new root growth starting in mid-January, even though a significant leaf canopy is not present until early March.
Though uptake of soil nitrogen may not be great prior to March, some applications are made at this time to have the nitrogen in place so when activity increases, the product is already present.
Not all growers have the availability of irrigation water to apply N on demand. In some districts, first water is not available until April, well into the current growing season. This is why in some cases growers apply N earlier so rainfall can help move the material into the rootzone where it is immediately available when the current growth flush starts. Keep in mind that these applications are not in lieu of a good management plan where stored N is applied the previous year to address the N demand at bloom.
This just illustrates that there are occasions when application timings due to water availability or the acquisition of a new property that had been neglected nutritionally may be in need of a quicker response time from applied nutrients.