I've been struggling to find the answer to a question about feed strength. I've noticed most feed schedules for hydro mediums like coco or rockwool call for much higher electrical conductivity (EC) rates than soil. I realize some soils are nutrient rich, but even some like Sunshine Advanced #4 do not have much fertilizers inherent in the soil. What is the science behind this? Why can I feed an EC of two in rockwool, but an EC of two in Sunshine Advanced #4 would be detrimental? Any input would be greatly appreciated!

Sunshine Advanced #4 contains some added fertilizers (as well as naturally occurring salts in peat and coir) to give early plant growth a boost. In the case of Sunshine Advanced #4, it has had lime (calcium hydroxide) and a starter organic fertilizer added. Lime is used to help correct the naturally low pH of peat. This mix is designed to be used without any added nutrients for the first seven days, as the nutrients in the product should be sufficient for early growth. After that, the grower irrigates with their own feed solution to supplement those fertilizers in the substrate. Because lime and fertilizer are already present in the soil mix, if a full-strength nutrient solution is given in this early phase, particularly to young, sensitive plants, then the EC around the roots could rise to higher than optimal levels. However, after the first week or two, a dilute-strength nutrient can be applied to supplement the fertilizers in the product and, as the plants grow (and fertilizers become depleted), the EC of the hydro solution can be brought up in full strength.

So, the main reason for the lower recommended EC when using these types of potting mix soils is to prevent salt build up in the root zone from both the fertilizers (which are often slow release forms) and the hydro nutrient solution. This process is easily monitored and adjusted–just check the EC of the solution draining from the base of your growing bags/beds/pots (the leachate solution). It is the EC in this drainage which is important–so if it is becoming higher than the recommended EC for your crop and stage of growth, drop the EC back in the feed solution. The combination of fertilizers already in the soil mix and nutrient solution will both add to the EC of the drainage. The fertilizers in the soil mix will gradually become depleted over time, so checking the drainage solution EC gives you an indication of when you need to increase your feed EC to full strength in the mix being used.