The element chlorine is common in nature and exists as an anion (Cl- ). It carries a negative particle charge and competes with other anions such as sulphates and magnesium if it is out of proportion to the other elements. It leaches from soils at about the same rate as nitrates. Excess levels of chlorides reduce soil microbe populations and reduce nitrogen conversion rates. Inputs to soils of chlorine are from natural rainfall, irrigation water, and fertilizers (potassium chloride). Swine and poultry manures have relatively high levels of chlorides compared to cattle manures. Chlorine is commonly found at ambient levels of around 50 mg/l in many soils (California excluded). Many formulations of chemical based NPK fertilizers have very high levels of chlorides. (Potassium chloride is a common source.)
Chlorine is considered a micronutrient and helps regulate the osmotic absorption of other nutrients that plants require. It often accumulates in foliar tissues at levels of 2-20 mg/l-1 of dry matter; even though plants require 10 to 100 times less to grow properly. Thus, deficiency symptoms are rare. A level of 180 mg/l in lighter-type soils is considered the upper range for hops production.
Feeder roots and new shoots appear scorched. Symptoms of toxicity increase during drought conditions; especially if a fertilizer containing high amounts potassium chloride is applied when there is little rainfall to leach away the excess chloride salts. Onset of symptoms can be very sudden – a stressor caused by heat or a missed irrigation.
A concern is when hopyards experience these stress symptoms (which are often diagnosed as a possible potassium deficiency) that the correct action is taken. Muriate of potash (potassium chloride) is often applied to correct the symptoms without doing a soil test first to see what the existing chloride salts levels are. This additional chloride can make the problem worse and ruin the alpha levels of the entire crop.
It is commonly recommended to do a pre-emptive soil test to determine nutrient / salt levels when the hop bines are climbing the twines. If chloride levels are high; then an alternate form of potassium fertilizer, such as sulphate of potash (potassium sulfate) or potassium nitrate should be considered.
If overall potassium levels are low, and muriate of potash is used; it should be applied in the fall season to allow the chlorides to leach out over the winter. Also, manure applications should be switched to cattle or horse manures. If poultry or swine manures are used; it is advised to apply them to alternate rows in alternate years. Fall applications of manures that are tilled into the soil and allowed to fallow over the winter are the most effective.
So, in summary, if hop plants show poor color, vigor, fertilizer response, and symptoms similar to those pictured:
- Do a mid-season soil test and/or foliar sample to see what levels of chlorine and potassium are. Don't just guess.
- If high levels of chloride salts are present leach them away from the root zone by irrigating heavily with clear water. This is a “do-over” like erasing a chalkboard. New amounts of nitrogen will have to be applied to replace what leached away with the excess salts.
- Consider switching to an alternate source of potassium such as potassium sulphate
- Switch to cattle manure instead of poultry manure
- Maintain a soil pH between 5.8 and 6.5 for most hop varieties to keep nutrients available and balanced
- If the hop cone alphas are low, resin &oils high, aromas low double check soil tests again. Look for out of balance cations (calcium) and anions (chloride). Fall applications of corrective nutrients can stop a repeat next year.
Great Lakes Hops January 6, 2014 reprints by permission.