Re-evaluate your potash application- getting enough?

Nutrient removal without replacement

In many Western soils, the importance of maintaining adequate levels of potassium is frequently overlooked.  The rocks and minerals that formed many of the soils in this region were naturally high in potassium.  When these soils were first farmed, there was little response to added potassium in many cases.   

Continuous cultivation requires nutrient replacement

Many years of high-yielding crop production has resulted in a mining of this resource.  For example, an alfalfa hay yield of 8 tons/A will remove about 500 lbs K₂O each year.  A 400 cwt/A yield of potatoes accumulates over 400 lb K₂O/A in the plant!  When high-yielding crops are continually harvested and removed from the field, the native potassium resource finally becomes depleted and exhausted.  Even where some potassium-rich minerals remain in the soil, they frequently cannot release their nutrients at a rate to meet the peak demand periods of a rapidly growing plant.

All plants require potassium

A recent study examined the extent of potassium depletion in the Western U.S. (the balance between soil K removal by crops and replacement with potash fertilizer).

• Arizona  4.4 times more removal than replacement

• California 2.0 more removal

• Idaho 4.3 times more removal   •  Montana 6.8 times more removal

• Oregon 2.2 times more removal   • Utah 5.2 more removal

•  Washington  2.6 times more removal   • Wyoming 9.2 times more removal

It is clear that on average in every Western state, we are rapidly depleting soil potassium reserves.  While these averages do not represent every specific field, the overall trend simply cannot continue indefinitely if we want to maintain our current yields.  

Potassium-deficient lettuce

Inside the plant, potassium is vitally important for many enzymes involved in photosynthesis, organic compound synthesis, translocation of important plant materials, and maintaining proper water balance.  Since potassium is mobile in the plant, deficiency symptoms appear first on the oldest leaves as yellowing around the leaf margin or specks between the leaf veins.  However, once deficiency symptoms are visible, plant growth has already declined and the crop continues to lose yield each day. 

High-yielding Idaho potatoes

Regular soil testing is the best way to predict the amount of potassium available for next year’s crop and decide on appropriate nutrient replacement rates.  If this is not possible, keep in mind the amount of potassium removed in past crops and in the coming year.  Don’t wait until deficiencies occur before replenishing the supply of this essential plant nutrient with potassium fertilizer.