It’s Time to Give Potassium Some Consideration

Frequently considered the third wheel in the macronutrient clique, potassium generally doesn’t receive the respect it is entitled to. Potassium deserves more appreciation. Its demand in plant nutrition typically runs 80-140% of a crop’s nitrogen demand. But, how often do you apply potassium in an amount similar to your nitrogen application? Many of us try to skimp by and rely on the residual soil bank to supply our crop’s potassium needs.

Potassium has many key roles in plant nutrition:

1. Regulates plant cell osmotic potential, affecting cell expansion, cell contraction, gas exchange, and movement of leaves in response to light.
2. Required for various metabolic activities and physiological functions such as photosynthesis, enzyme activation, and production of sugars and carbohydrates.
3. Adjusts pH within plant cells.
4. Crucial to translocation of assimilates within the plant.
5. Required for nitrogen uptake and protein synthesis.
6. Increases winter hardiness.
7. Increases drought tolerance and water use efficiency.
8. Known to improve resistance to disease and lodging.
9. Improves quality of seeds and fruit.
10. Increases oil content.

Potassium in the soil comes naturally from the physical and chemical weathering of K-containing primary materials such as micas and feldspars. Potassium is also released from the interlayers of the layer silicate minerals such as illite, vermiculite, and smectite. Irrigation water in some areas carries potassium in solution. Decomposition of crop residues and manures also release potassium into the soil. However, in high yield production agriculture, these sources of potassium can be rapidly depleted, and residual soil test levels will decline, and productivity may suffer.

Crops with high K demand in high yield environments will benefit from additional K fertilizer application. Potassium applied as fertilizer replenishes and augments the readily exchangeable and solution K pools in the soil. Replacement of this K is necessary to avoid long-term depletion of the soil K nutrient reserves. Readily exchangeable K resides on the cation exchange sites of the soil, and it is this readily exchangeable K, and some K in solution, that are primarily available for plant uptake.

During the most recent 10 years, potassium fertilizer prices have been elevated relative to historical levels. Potash prices have settled some, and if you were one of those who tried to skimp by the last few years, now is the time to consider replenishing your potassium bank.

Soil sampling and analysis are effective in measuring soil K status, and determining whether additional K application is warranted. Helena’s HyGround® Soil Management Services can further define variability of residual soil K levels within a field. When sufficient variability is present, a variable rate application of fertilizer K may be effective in meeting crop needs more precisely.

Above is an example of a 297 acre winter wheat field in the Northern Plains. The map on the left shows residual soil K status as determined by our HyGround Soil Management Services. The map on the right shows the laboratory’s variable rate K recommendation. Average fertilizer K needed for the variable rate HyGround approach is 78 lbs MOP/A compared to a flat rate K recommendation of 58 lbs MOP/A based on the average soil test K value for this field. In this example, the grower would need only 0.63 bu/A increase in yield to pay for the additional 14 lbs MOP/A recommended by the HyGround approach (calculations based on $360/ton MOP, and $4.05/bu wheat price). It is the small adjustments like this that can lead to greater yield potential and higher nutrient use efficiency. Potassium is an important element in crop nutrition, so give it the consideration it deserves.

- Michael Larkin, Precision Information Manager, Fresno