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Coping With Chloride

PNP  (Pacific Nut Producer)  March, 2012
Joe Traynor

Why potassium chloride could be your best fertilizer bet

Terms used herein:
potassium = K chloride = Cl
KCl = potassium chloride = muriate of potash (MOP)
K2S04 = potassium sulfate = sulfate of potash (SOP); K20 = potassium oxide

Rising fertilizer costs, esp. for potassium fertilizers, are taking an increasing bite out of farming budgets. Potassium (K) is necessary to maintain high almond yields – a 3000 lb/acre almond crop removes over 200 lbs of K20 from the orchard each year. When considering potassium fertilizers, the choice is usually between two materials: potassium sulfate (sulfate of potash) or potassium chloride (muriate of potash). The cost of other K fertilizers per unit of K20 is prohibitive.

Potassium sulfate is 52% K20; potassium chloride is 61% K20. Besides having a significantly higher K analysis, potassium chloride is also cheaper per ton and cheaper yet on a cost per unit of K20 basis. When forced to choose between the sulfate and chloride forms of K, many growers opt for sulfate because of concerns that chloride will harm their trees. These concerns are well-founded because almonds are a salt (and chloride) sensitive crop. In many cases, however, potassium chloride should be the material of choice. An explanation follows.

To get a handle on the potential of Cl damage to crops, one must first know how much chloride is being applied. Potassium chloride is 47.5% Cl (note: adding 61% K20 to 47.5% Cl gives 108%, but there is no K20 in KCl, only K and Cl; KCl is 51% K, which makes the numbers come out — 51 + 47.5 (+ 1% impurities) = 100% (see sidebar K20 vs. K for further explanation).

A 500 lb/acre application of muriate of potash supplies 305 lbs of K20 per acre but also 238 lbs of Cl, a scary figure, but will it damage trees? How many lbs of Cl can almond trees tolerate? Fortunately, there are guidelines to answer this critical question. Irrigation water contains Cl, and guidelines have been established (by UC and others) as to how much Cl can be safely applied to almonds in a given year. These guidelines state that irrigation water with a Cl level of 140 ppm or less is safe for almonds (Cl levels can be higher for almonds on the salt-tolerant hybrid rootstocks). To be ultra-safe, let’s figure that irrigation water should contain no more than 100 ppm Cl for almonds. A 100 ppm Cl water supplies 270 lbs of Cl per acre foot of water (an acre foot of water weighs 2.7 million lbs). At a reasonable figure of 3 acre feet applied annually, water with a Cl content of 100 ppm would safely apply 810 lbs of Cl each year, a figure well above the 238 lbs from 500 lbs/ac. of KCl.

In order to know the amount of KCl that can be safely applied to your orchard, you need to know the Cl content of your irrigation water. Cl is often reported in milliequivalents per liter (meq/l); to convert to ppm, multiply by 35.5 (1 meq/l of Cl = 35.5 ppm). Allowable amounts of KCl for different levels of Cl in water are given below:

AMOUNT OF KCl ALLOWED PER YEAR AT DIFFERENT LEVELS OF Cl IN IRRIGATION WATER

ppm Cl lbs Cl applied/year* Amount of KCl allowed per year
10 ppm 81 1536 lbs (936 lbs of K20)
25 ppm 202 1280 lbs (780 lbs of K20)
50 ppm 405 852 lbs. (520 lbs of K20)
75 ppm 607 427 lbs (260 lbs of K20)
100 ppm 810 best to use sulfate of potash or closely monitor leaf Cl
*in water; assuming 3 acre feet of water applied per year

Most orchards don’t need more than 250 lbs/ac, of K20 annually, an amount supplied by 400 lbs of KCl. As long as your irrigation water is less than 80 ppm Cl, a 330 lb/ac. application of KCl (200 lbs of K20) should be plenty safe, esp. for trees on hybrid rootstocks.

Potassium fertilizers are usually applied within a tight time frame while the Cl in irrigation water is spread throughout the year. Soils can tie-up applied K (esp. soils on the east side of the Valley that are derived from granitic parent material) so, for soil applications of K fertilizers it is best to apply the material in a concentrated band on both sides of the tree row. Applying the entire KCl allotment at once minimizes soil tie-up of K, but does give a temporary spike in soil-solution Cl, however with a good follow-up irrigation and on well-drained soils, Cl should not accumulate in the root zone because Cl is readily leached.

Probably the best way to apply KCl is in the irrigation water during the growing season, flooding the root zone with K, starting at petal fall and continuing through May, a period of maximum K use by almonds. KCl is 3 times more soluble than potassium sulfate, making water-run applications easier. Applying K in the irrigation water minimizes soil tie-up of K, and, if an early frost takes the crop, K fertilization can be eliminated for that year. Water-run applications of KCl should be followed by a short irrigation to leach some of the Cl past the root zone; Cl is easily leached, K resists leaching.

Growers using low-salt ditch water (e.g., snowmelt from the Sierras) can use KCl to apply maintenance rates of K20 without concern of Cl toxicity. A possible advantage of sulfate of potash is that it supplies the essential nutrient, sulfur, however sulfur deficiency is rare to non-existent on irrigated crops in California. Most well water contains ample sulfur in the form of sulfate, and when ditch water is used, most growers apply gypsum (calcium sulfate) to improve water penetration, negating the need for additional sulfur.

When KCl is used, monitoring Cl levels in leaves will ease concerns about Cl toxicity. Whether or not KCl is used, Cl levels in leaves will increase during the growing season and peak in September. Cl levels in leaves should be below 0.2% throughout the summer. Most growers that use KCl have been pleased to find that leaf Cl does not rise to potentially harmful levels and symptoms of Cl toxicity – marginal burning of leaves – are never seen. Growers that restrict KCl applications to the amounts in the above table should have no problems with Cl toxicity. For many growers, switching from sulfate of potash to muriate of potash is a sound economic decision.

Note: Although almonds are considered a Cl-sensitive crop, at least 3 crops – strawberries, avocados and kiwis – are considered super-sensitive to Cl. KCl should not be applied to these 3 crops.

K20 vs. K in Fertilizer Analysis

Fertilizer analysis for potassium (K) is given as % K20 (potassium oxide) rather than % K. The original methods used to analyze a material for potassium ashed the sample, which gave K20 as a final product. % K20 (rather than % K) was reported. When the material was ashed in a pot, the resultant ash was called potash, a designation that survives today. An enterprising salesman could market 100% potash!!! – the entire ash at the bottom of the pot – but consumers would be very disappointed with the K (and K20) content of such a dubious product.

Dividing % K20 by 1.2 gives the % K. Sulfate of potash is 52% K20 but only 43% K. Muriate of potash is 61% K20 but only 51% K (both materials have about 1% impurities). Triple-mix (N-P-K) fertilizers also report K as K20, thus a 15-15-15 material is 15% K20 but only 12.5% K. Note: the K in triple-mix fertilizers is almost always from muriate of potash.

There has been talk for years about reporting K in fertilizers as K (rather than K20) but it hasn’t happened and isn’t likely to happen. Those that sell fertilizers would probably not be in favor of switching to a system that would give the impression that their product was a sub-par material, particularly if they had to compete with a material that reported K as K20.