Preparation of liquid fertiliser stock solutions

Calculating liquid feed solutions

First remember:

• 1ppm = 1mg/L (1 part per million = 1 milligram per litre)

• 1 milligram = 1/1000 of a gram (1000 milligrams = 1 gram).

Decide which fertilisers are to be used for the nitrogen, phosphorus and potassium sources. Urea, potassium nitrate and phosphoric acid have been chosen for the following example.

Worked example

Suppose you need a liquid feed with a final concentration of:

Nitrogen (N)            200ppm

Phosphorus (P)        30ppm

Potassium (K)          156ppm

As nitrogen is supplied by both potassium nitrate (KNO₃) and urea {CO(NH₂)₂}, it is best to start the calculations with potassium nitrate.

Amount of K required = 156ppm which is equivalent to 156mg/L.

% K in potassium nitrate = 38.6%

Therefore, for 156mg/L of K you need:

     156 X 100/38.6mg/L of potassium nitrate

 = 404.1mg/L of potassium nitrate.

Another way to calculate the amount of K is:

More than 156mg of potassium nitrate is needed since it is only about a third potassium, so divide 156 by 0.386 and get the answer immediately.

Just remember to divide the desired concentration by the percentage of the nutrient needed (in this case the percentage K) written as a decimal (that is, 100% = 100/100 = 1, 50% = 50/100 = 0.5 and so on). Dividing any number by a figure less than one will give a bigger answer.

So 404mg of potassium nitrate per litre of water is needed for a concentration of 156mg/L of K, but how much nitrogen has been supplied with that potassium? The amount of N is then deducted from the total requirement so we can work out how much ammonium nitrate is needed.
404mg/L of potassium nitrate is being used

Potassium nitrate contains 13.9% N

Therefore the amount of N in that 404mg is:

404 x 0.139 = 56.2mg/L

(This time remember to look for a smaller number than 404, so multiply by the percentage figure expressed as a decimal, rather than divide.)

The total amount of N needed is 200ppm or 200mg/L, so the amount still needed to be supplied by the urea is:

200 - 56.2mg/L = 143.8mg/L

Urea is 46% N so divide 143.8 by 0.46 to obtain the amount needed:

143.8/0.46 = 312.7mg/L

Phosphoric acid is normally supplied as an 85% solution and contains 26.8% phosphorus (P). This fertiliser mix needs 30ppm or 30mg/L of P.

To calculate the amount of phosphoric acid needed, divide 30 by 0.268:

30/0.268 = 111.9mg/L

Note: Phosphoric acid is a liquid not a solid and is heavier than water, so either weigh the liquid, or if using a measuring cup or cylinder apply the following calculation (in this case):

Divide the weight required by the density of 85% phosphoric acid (density = 1.53 at 20°C) which gives a volume of:

111.9/1.53 = 73.2mg/L

Warning: Be careful when mixing phosphoric acid. Add the acid to the water and not the water to the acid, as the acid will boil and spit.

For a solution of 200ppm N, 156ppm K and 30ppm P using urea, potassium nitrate and phosphoric acid, the following amounts of each fertiliser are required:

• Urea - 312.7mg/L of water (round off to 313mg)
• Potassium nitrate - 404.1mg/L of water (round off to 404mg)
• Phosphoric acid - 111.9mg/L of water (round off to 112mg) or 73.2mL

Invariably, much more than 1L of fertiliser solution is needed. Generally stock solutions are mixed up in 100L batches, so multiply each of the above amounts by 100, that is 42g of urea, 40g of potassium nitrate and 11g of phosphoric acid (note weights are now in grams).

These liquid fertilisers can be concentrated and then applied through a proportioning or diluting device - commonly called a fertiliser injector. There are several types  available which dilute the fertiliser in different ways. A common type has a dial for ‘dialling up’ a dilution rate - often 200:1. When using a fertiliser injector, modify the stock solution again so that when it is diluted by the injector it comes out at the correct strength. Multiply the above fertiliser rates by the dilution factor - in this example, 200.

• Urea: 31.3g x 200 = 6260g or 6.26kg/100L of water (round off to 6.3kg)

• Potassium nitrate: 40g x 200 = 8020g or 8.02kg/100L of water (round off to 8.0kg)

• Phosphoric acid: 11.2g x 200 = 2240g or 2.24kg/100L of water

The amounts of fertiliser (now in kilograms) are needed when making up 100L of stock solution to be injected at a rate of 200:1.

The amounts of each of the fertilisers needed in this example are less than their solubilities. Refer to the mixing section at the start of this page. (The solubility of phosphoric acid is not given, but should not be a problem.)

Solubility is not usually a problem when mixing stock solutions, but the possibility of a fertiliser not dissolving because it has exceeded its solubility must be considered, particularly when using chemicals containing boron. More common problems are the reactions between the fertilisers, especially in concentrated stock solutions. The following fertilisers cannot be mixed together in the same tank:

•        Calcium nitrate and any phosphates or sulphates
•        magnesium sulphate and di- or mono-ammonium phosphate
•        phosphoric acid and iron, zinc, copper and manganese sulphates.

Note:

• The percentages listed above are those usually found in purchased horticultural grade products and may differ from the theoretical percentage.
• Potassium chloride is not recommended for use in liquid feeds.