Fertilizer K placement guidelines

The major objective of all approaches to fertilizer application regardless whether applied as single nutrient or as a compound fertilizer is to ensure the most efficient uptake of fertilizer K by plant roots.

K fertilizers are most commonly applied to the soil by hand or using mechanical applicators. K fertilizers may be broadcast (usually before planting), top-dressed (on the growing crop) or side dressed in bands and strips along the crop row. Broadcast and side dressed fertilizer is usually incorporated in the uppermost layer of the soil. K fertilizers can also be placed in the soil in a particular location in relation to the crop’s roots. Row placement or ‘side banding’ involves the mechanical placement of fertilizer below and to one side of the seed at planting. For tree crops, single K fertilizers or compound fertilizers containing K can be placed in the planting hole before planting. For mature bushes and trees, K fertilizer may be placed together with organic mulch in pockets and furrows around or between plants, broadcast over the weeded circle or over the entire soil surface.

Plant uptake can be increased by applying soluble K fertilizer nutrients to the crop as a foliar application, with irrigation water (fertigation) or (where economically and technically feasible) in solution culture.

Timing is particularly important where large application rates of K fertilizer are required. Avoid application before or during periods of rain but apply fertilizer to coincide with periods of rapid crop growth when the crop’s K requirements are greatest. In the end, a range of different fertilizer placement methods may result in the greatest uptake efficiency.

When applied to or placed in moist soil, KCl fertilizer dissolves rapidly and its constituents (i.e., K and Cl) enter the soil solution in the ionic forms K+ and Cl-. K+ ions move by random thermal motion (diffusion) from places of higher concentration towards locations where the concentration has been reduced due to plant uptake (e.g., around the tips of roots). K ions are also attracted to negatively charged clay and organic matter particles where they may be exchanged for other cations (e.g. Ca2+, Mg2+, H+, Al3+).

The movement of K+ by diffusion towards the roots is more rapid when the concentration of K+ in the soil solution has been increased and this explains why K fertilizer placement in close proximity to the crop roots often results in increased K uptake efficiency. K placement may help to ‘protect’ fertilizer K from being adsorbed or ‘fixed’ and rendered unavailable to crop plants by clay minerals.

The efficient placement, splitting and timing of K fertilizers in coarse textured, sandy or highly weathered tropical soils with a small cation exchange capacity can help to reduce leaching losses and increase fertilizer recovery efficiency.