Leaching Fraction

Every irrigation delivers salts, and the plant takes up water preferentially over those salts, so the medium concentrates with each feed. Left alone, the solution around the roots climbs to strengths that damage the crop even though the feed itself is correct. Deliberate over-watering — applying more than the medium can hold so that a measured fraction drains away — is what carries the excess back out.

Ten to twenty percent runoff is the usual target for container growing, rising where the feed is strong or the source water is hard. The measurement is straightforward: catch and measure the drain from a representative pot for one irrigation, and divide by what went in.

The number that makes leaching diagnostic rather than merely routine is runoff EC compared to feed EC. If runoff reads higher than feed, salts are accumulating and the leaching fraction is too low. Roughly equal means the system is at steady state. Runoff below feed means the plant is taking up nutrient faster than water, which usually indicates it is being underfed rather than over-leached.

The cost is real, and worth stating: leachate carries nutrient out of the system. In a drain-to-waste setup that is money down the drain and, at scale, a nutrient discharge that may be regulated. In a recirculating system it is not lost, but the return solution drifts in composition as the crop removes elements unevenly, which is why recirculating growers monitor individual ions rather than EC alone.