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Bank on It

Healthy orchards and vineyards withdraw potassium from your soils, so be sure to make a deposit.

By Joe Traynor

There has been increasing interest in potassium (K) fertilization of orchards and vineyards in recent years, and with good reason. A high-yielding orchard or vineyard can remove 100 to 300 pounds of K from the soil every year. Multiply this by 10 or 20 years, and it becomes apparent that soils, especially sandy soils, will eventually incur a debt that must be paid. Providing ample K can increase fruit size, even out maturity, and reduce stress.

Two decades ago, the accepted method of applying K to permanent crops was to shank K down the tree or vine row during the winter. Two drawbacks to this method are the possibility of salt injury from high amounts of the K materials normally used, and the fact that most soils will tie up K within weeks of application, rendering it unavailable to the roots.

Orchards and vineyards require virtually all of their K during the period of rapid fruit growth – from post-bloom through May. Winter application of K is simply not efficient.

The proliferation of low-volume sprinkler and drip irrigation over the past 20 years provides growers with the ideal K delivery system. K can be applied when roots are active and at the time of greatest need, during rapid fruit development. Soil tie-up of K is minimized and, should an early frost take the crop, the expense of K fertilization can be eliminated.

Here is a look at the four main K’s.

The Four Fertilizers

A COMPARISON of the four main potassium fertilizers is indicated in the following table. Maintenance application rates should be 100 pounds of K2O (potassium oxide) annually, starting shortly after bloom.
Material %K2O Solubility #/100 gallons Cost Per Ton (estimated)
Muriate of potash (KCI) 61 300 $150
Sulfate of potash 52 100 $240
Potassium carbonate 68 940 $880
Potassium thiosulfate 25 819 $420

Source: Scientific Ag Co.

Muriate Of Potash

Muriate of potash is the K fertilizer of choice in almost all cases because of its significantly lower cost. Muriate’s relatively good solubility makes for easier water-run applications. Some growers hesitate to use muriate of potash because of its chloride content, but at the relatively low rates used for K maintenance this should not be a concern unless irrigation water is high in chloride – over 180 ppm. Growers concerned about chloride should monitor the levels through leaf testing. The level will increase during the season, but should be kept below 0.2%.

Sulfate Of Potash

The only discernible advantage of sulfate of potash for most crops is “free” sulfur nutrition, as the material is 18% sulfur. However, the possibility of sulfur deficiency in orchards and vineyards is remote, as most well water supplies ample sulfur. For other water sources, gypsum (calcium sulfate) is usually applied to maximize water infiltration, and gypsum supplies plenty of sulfur.

Potassium Carbonate

Potassium carbonate (PC) is an intriguing material for orchards and vineyards under low-volume sprinkler or drip irrigation because of its anti-acid properties. PC is amazingly soluble – three times as soluble as muriate of potash and nine times as soluble as sulfate of potash. PC is the K of choice when soil pH is low, but it is more hazardous than other materials because of its alkilinity.

Potassium Thiosulfate

Potassium thiosulfate (KTS) has an acidifying effect on soils and is therefore the K fertilizer of choice on high pH or high-lime soils. Most varieties are tolerant of high-lime soils, but there are exceptions. If trees on a high-lime soil show chlorosis, KTS should be used if K is needed.


Joe Traynor is the manager of Scientific Ag Co. in Bakersfield, CA. E-mail questions or comments about this article to afg_edit@meisternet.com.