While Water Only Soils are convenient and can save time, many gardeners I find want to add more to their regimen and push plants to their limit in regards to growth and yield. Here's a few ways to maximize your plant's potential growth and yield. I've loosely organized these by the order in which I would prioritize them based on a cost/benefit analysis.
Mycorrhizal Fungi - Mycorrhizal fungi is a "must have" for me and works well with our soil mixes. It's a one time application early in the plant's life, directly to the root zone. Mycorrhizal fungi will help with nutrient uptake, water retention, and overall plant health and growth. See my blog post on mycorrhizal fungi for more information.
Aerated Compost Tea - Aerated compost tea (ACT) is a wonderful addition to any garden. It's main purpose is to increase nutrient cycling by adding a high diversity and biomass of all major sets of microorganisms (bacteria/archaea, fungi, and protozoa). For more information on ACT, see my other articles here.
KIS Organics Nutrient Pack - Our Nutrient Pack works wonderfully as a top dress, especially if your plant is outgrowing it's container size and running low on nutrients. We recommend a 1/2 cup per plant in a 5 gallon or 7 gallon pot at the end of the first week of flower and the end of the 3rd week of flower. Just sprinkle it lightly on the surface of the soil and scratch it in before watering. For 30 gallon containers or larger, you can go up to 1 cup per plant week. Keep in mind that it can take up to week or so for noticeable plant response.
Earthworm Castings or High Quality Compost - It's important to use high quality compost or castings (worm poop) as the main benefit is to add organic matter and beneficial microbes to soil. Be sure to check that the compost is fully finished and ask for biological and nutrient lab results on the product if purchasing. Just a handful added to the top of the container and watered in can correct many micro-deficiencies.
Other Nutrients - Unsulfured Molasses, Fish Hydrolysate, Seaweed Extract Powder, Humic and Fulvic Acids, Silica, etc....All of these products are affordable and offer a variety of plant benefits. Unsulfured molasses is a wonderful microbial food and also provides a small amount of K. Fish hydrolysate is also a great microbial food (especially fungi) and is a fast acting source of N and P if you notice a deficiency during mid-flower. Seaweed Extract Powder is an affordable way to add many trace minerals and some K to your plants and can be used as a soil drench or foliar application. Humic and Fulvic acids chelate (bond with) minerals in your soil and make them plant available. The humic acids we carry are highly concentrated, requiring only 1/8th tsp. per gallon of water.
Your own bottled nutrient regiment at 1/4 strength - Many growers have a bottled nutrient line or program that they like or have been using for years. There's nothing wrong with using bottled nutrients with our soils, in fact you should see even better results than with traditional potting soils! That being said, you'll want to start our at 1/4 strength and work your way up from there so as not to burn the plants by over applying nutrients. If re-using the soil, I would suggest a $25 soil test from Logan Labs to make sure there is no major excesses or deficiencies developing over time.
The list above is not all-encompassing, but rather a guideline based on our own experiences and knowledge.
The key to any of these is to not getting carried away and keep the mentality that "less is more." Be sure when you do experiment, to only add one variable at a time and keep a control plant so you can evaluate if the variable is having a desired effect on plant health and growth. (see Scientific Method and Scientific Control)
A periodic soil test will also allow you to determine if any excesses or deficiencies are developing over time. We recommend a standard soil test from Logan Labs. If you have any questions about using a particular product or nutrient in our soil, don't hesitate to contact us via email on our contact page.