To make a nutrient stock solution, you need a minimum of two tanks. Nutrient Stock solutions cannot be made in one tank. The reason is that above a certain concentration, calcium reacts with sulphates and phosphates to form insoluble calcium sulphate () and calcium phosphate (). The two nutrient tanks will be called Tank A and Tank B. The designation of A and B is universal and for a very good purpose, and that is to prevent any confusion as to what is in each nutrient tank. The golden rule is that all nitrogen, potassium, phosphorus, magnesium and micro nutrients , except iron, are added to Tank A and that all calcium and iron fertilizers are added to Tank B.
The procedure in mixing nutrient stock solutions is extremely important. Following the correct procedures will prevent sediments accumulating at the bottom of the nutrient tanks and prevent the formation of insoluble compounds which are not available to plants. The procedure is as follows:
- Fill the two tanks 90 % with water
- Measure the pH and correct with either an acid or base
- Switch the irrigation pump on to agitate the water in the tank. The valve directing water to the plants should be switched off. Water is now circulating from the tank to the pump and back tot the tank.
- Dissolve small portions each fertilizer in a 20 L bucket before adding the contents to the large stock tank. If some of the fertilizer did not dissolve in the small bucket, do not add the un-dissolved sludge in the main tank, rather add more water and dissolve until there is not sludge at the bottom of the bucket.
- Dissolve the macro nutrients first starting with the sulphates such as potassium sulphate and magnesium sulphate (if they are used), then proceed with the nitrates and phosphates. Remember calcium nitrate cannot be added with potassium nitrate or magnesium nitrate since they are added into a separate tank.
- Calcium nitrates dissolves slowly so it is recommended that hot water is used when mixing the fertilizer into the 20 L buckets.
- Check the pH of both tanks and adjust only Tank A between 5.8 and 6.8.
A nutrient stock solution to control and adjust the water pH can also be made. Again use a small nutrient tank; 500 L or 1,000 L is quite sufficient, and fill 95 % with water. If phosphoric acid is used add any amount of acid into the stock tank. Some acids come in 5 L bottles, so the whole bottle can be added into the tank, don’t worry.
When adjusting the pH of the main nutrient tank, small amounts of the pH tank are used. Quite often the amount that is required to adjust the main nutrient tank becomes predictable, for instance, every day the 5,000 L tank requires 10 L of the stock pH solution. It is safer working with the stock pH solution than with the concentrated phosphoric acid or nitric acid.