Nutrients for Rockwool Seedlings

Growing hydroponic tomatoes provide you with fresh fruit even in winter.

Plants need a variety of nutrients, not just the big three advertised on commercial fertilizers -- nitrogen, phosphorus and potassium. The three numbers on the bag indicate the percent of nitrogen, phosphorus and potassium contained in the fertilizer. For instance, 20-10-5 fertilizer has 20 percent nitrogen, 10 percent phosphate and 5 percent potassium. In terrestrial systems, the rest of the nutrients are usually provided by the soil, but in hydroponic systems, using rock wool as the growth medium, you must provide all the nutrients. Therefore you need to either buy special hydroponic fertilizers or mix your own.

Macro Nutrients

Seedlings need fairly large amounts of the big-three nutrients. Your hydroponic solution should include 70 ppm of nitrogen, 50 ppm of phosphorus and 120 ppm of potassium. Along with carbon, these nutrients are the main structural components of a plant. But seedlings need other macro-nutrients for healthy development. These nutrients and their recommended levels include: calcium at 150 ppm, magnesium at 150 ppm, sulfur at 50 ppm and iron at 2.8 ppm in your system water. You'll need micronutrients such as copper (0.2 ppm), manganese (0.8 ppm), zinc (0.3 ppm), boron (0.7 ppm) and molybdenum (0.05 ppm).

Hydroponic Fertilizer

To mix your own fertilizer, you'll need access to the raw chemicals. Purchase the materials through chemical or biological supply houses. You'll need a 30 gallon stock bottle to hold the solution and a gram scale to weigh these ingredients. You'll also need a small graduated cylinder and weighing paper.

To save room, you can make a concentrated stock batch 10 times the strength you will actually use by using the same amount of chemicals but only diluting to 3 gallons. Simply add a gallon of the nutrient solution to 10 gallons of water when you're ready to use it.

S.T.E.M. Solution

Soluble Trace Element Mixture, better known as S.T.E.M, is a commercial product. Trace elements, used in such small quantities, are nearly impossible to accurately weigh out by a home grower or experimenter. It's also impossible to buy these materials in the tiny amounts that would be used and you would end up with a lot unwanted chemicals to store.

Mixing

Mix together 1 lb. each of potassium nitrate, ammonium nitrate, potassium chloride, magnesium sulfate, dry calcium nitrate, mono-potassium phosphate and calcium chloride in a 30-gallon container.

Add 1 pint of liquid calcium nitrate, 1 quart of phosphoric acid, 1 gram of Solubor, 0.5 lbs. of Fe 330 chelated iron and 100 grams of S.T.E.M.

Weigh out 1 gram of copper sulfate, 1 gram of manganese sulfate and 1 gram of zinc sulfate on the scale using weighing paper and add to the bottle. Add 1 ml. of liquid sodium molybdate.

Fill the container to the 30-gallon level and stir to dissolve completely before using.

References

• University of Florida: Nutrient Solution Formulation for Hydroponic (Perlite, Rockwool, NFT) Tomatoes in Florida1
• Colorado State University: Understanding Fertilizers
• Arizona University: Transplant Production

Resources

• Scotts: S.T.E.M. MSDS