Hops are a very chlorine-sensitive crop. Hops varieties such as Cascade, Sorachi Ace, Sterling, Horizon, H. neomexicana and many other aroma hops commonly exhibit chlorine toxicity symptoms in many American soil types if fertilized improperly. Many other varieties of hops may not show clear toxicity symptoms but are held back and produce less than an optimal crop.
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.
Lynn, the head hop grower at Great Lakes Hops has over 30 years of experience in the horticultural field. Browse the blog articles here to find useful growing information for humulus lupulus, based on personal experience and observations at Great Lakes Hops.