How to formulate a nutrient solution with complex fertilizers

The grower has the option to obtain pre-mixed fertilizers that are already mixed in the right ratio for a specific crop or to use basic fertilizers and to mix them into various tanks and ad them to the fertigation system. The first option is the easiest but slightly more expensive and recommended for inexperienced growers. After a couple of years a grower might decide to have more control over the ratio of the various nutrients provided to his crop and decide to make up his own formulations. He would then need to calculate how much of each basic fertilizer that must be added to a solution. This method is complex and recommended only to those growers that have an understanding of the effect of the various elements on plant growth and the plants requirements of the elements at various stages of growth. It has been proven that costs can be reduced by mixing fertilizers on farm. The next section will describe how to mix and prepare a nutrient solution from complex fertilizer mixes.

Complex fertilizers or pre-mixed fertilizers come in a minimum of two bags. A complex which contains most of the macro elements except calcium (Ca) and another bag containing calcium nitrate (CaNO₃). If crops such as tomatoes, peppers, brinjals and cucumbers are grown, it is good to obtain one more fertilizer that contains extra potassium (K) in the form of potassium sulphate.

We will ignore the values in the table above until later in the chapter. Let’s assume the grower has a 1,000 L tank and needs to mix the fertilizers in the tank to form a 1:1 nutrient solution. A 1:1 or normal solution is a nutrient solution that can be applied directly to a crop. There are other solutions, which will be discussed later in the chapter that are called stock solutions and are mixed from 1:20 to 1:100 concentrations. What makes complex fertilizers easy to mix is that they need to be combined in a 1:1 ratio, i.e. 1 unit of Calcium nitrate is mixed with 1 unit of Chemical X. Most companies provide their complex fertilizer in such a way that this ratio must be applied. Another convenient aspect of using complex mixes is the required amount that must be applied to 1,000 L, this is usually 1kg of calcium nitrate and 1 kg of the complex mix, in this case Chemical X. However, consult your local agent for correct amounts since they might change depending on crop and water analysis. In most cases it is safe to assume 1 kg of each.

Using calcium nitrate and Chemical X will provide the plant with a complete balance of nutrients in order to grow. However it might not be the optimal balance of nutrients, but the plants will grow. The potassium sulphate is there to add additional potassium in early stages of growth and not more than 0.5 kg per 1,000 L of water is required. The table below provides the amounts required for each fertilizer to make up a 1:1 nutrient solution. Potassium sulphate is also added in the first two months of production for crops that produce fruit, such as tomatoes, cucumbers, peppers and brinjals. However, leafy crops, such as lettuce, leeks, spinach and celery potassium sulphate should be omitted.

Each chemical should never be added into the main nutrient tank as a powder. The following procedure should be followed when mixing fertilizers in nutrient tanks:

• Fill 90 % the main nutrient tank with water
• Check if the pH is between 5.5 and 6.8
• Increase or decrease the pH if necessary with either an acid or base
• Start stirring the water in the tank by switching on the pump and closing of appropriate valves so that water does not flow towards the plants
• Dissolve small portions of one bag in a smaller bucket filled with warm water
• As soon as all the fertilizer is dissolved in the bucket, ad the contents to the large nutrient tank
• Repeat this procedure until all the required nutrients are added into the nutrient tank.
• Keep the pump on in order to circulate the nutrient water for at least another hour.
• The solution should be ready for use