Liquid feeds: buy or make your own?
By far the easiest way of preparing a liquid feed is to buy a package of mixed nutrients with the required proportions of major nutrients, dissolve it in water in your stock tank and apply it through your injector and irrigation system. However, this method is far more expensive than if you made up your stock solution from individual fertiliser salts.
Packaged soluble fertiliser mixtures cost approximately $7-10/kg. There are many excellent products among them, with a wide range of nitrogen (N), phosphorus (P) and potassium (K) proportions. However, quite a few contain unnecessarily high concentrations of P. With many, a high proportion of the nitrogen is present as ammonium or urea, therefore they are highly acidifying. With most mixtures you pay for trace elements that your plants do not require when your growing media has had them added. All – other than Peters Excel – do not contain calcium (Ca). Some supply magnesium (Mg); many do not. Most do not supply sulphur (S).
By making your own solution, you can easily vary the composition of the solution to suit your plants and seasonal conditions. By having a higher nitrate content than many bought mixtures, you can create less acidifying feeds. In the process you will be spending much less on your fertiliser inputs.
Liquid feeding via sprinklers can only be successful if your irrigation system has a high coefficient of uniformity, preferably higher than 90%. Low uniformity will produce uneven growth across the batch of plants being grown.
Fertiliser salts to use
If you make your own stock solution, you will use one or several of the salts in this table. There are others, but you can make any liquid feed needed with those listed.
|Fertiliser salt||Approximate cost ($/kg)|
|Secure- AN28 (200 L drum)||2.1 ($/L)|
Calculations for making your own liquid feed
These calculations assume that the potting mix contains all trace elements in adequate amounts for the duration of the crop being grown. To make up 200 L of stock solution according to the calculations below would cost approximately $19.60, or $4.0 per kg of salts used – half the cost of a bought mixture.
- Potting mixes formulated according to the Australian Standard will have adequate amounts of S, Ca and Mg for many months of plant growth. Inclusion of medium coarse gypsum (2-5 mm chips) will ensure adequate supply of S and Ca for up to a year. Dolomite used to adjust pH will supply Mg for many months. Coarser dolomite (0.5-2 mm) included for pH buffering will ensure adequate Mg supply for a year or more.
- If the irrigation water contains more than about 15 mg/L S, 20 mg/L Mg and 40 mg/L Ca, no further additions of these nutrients are needed within the liquid feed, irrespective of what sources of these nutrients were included in the mix.
- Decide which major nutrients you want to supply to your plants: N only; N and P; N and K; N, P and K. Generally, all three should be included.
- Decide on the ratios between these three nutrients. For most situations, a P/N ratio of 0.15 and a K/N ratio of 0.75 (concentrations in mg/L) will be suitable, although for P-sensitive plants the P/N ratio would be something like 0.05 and for seedlings that must remain squat it will be in the range 0.07-0.1. The K/N ratio could be lower when the potting mix supplies K and higher when there is a need to use the extra K to toughen plant tissues. This example uses a P/N of 0.15 and a K/N of 0.75.
- Decide on the amount of N to be supplied at each fertigation. Suppose you intend to feed three times a week and the total amount of N needed per plant each week is 50 mg. This is a typical need for a woody ornamental plant. Each fertigation is to supply 50/3 = 17 mg N.
- Decide on the volume of fermentation solution that is to be applied to each container. The following example uses 130 mm pots, each of 1 L capacity, that will receive 200 ml of solution at each application. (You need to measure how long to leave your sprinklers on to achieve this application). Hence the solution will contain 17 mg N/200 ml, which is 5 x 17 = 85 mg N/L. Hence, the P concentration will be 85 x 0.15 = 12.75 (13) mg/L and the K concentration will be 85 x 0.75 = 63.75 (64) mg/L.
- Choose monopotassium phosphate (KH2PO4) as the source of P. From page 201 of the 4th edition of Growing Media for Ornamental Plants and Turf, to make a solution that contains 100 mg/L P takes 0.39 g monopotassium phosphate per litre. So a concentration of 13 mg/L will require 13 x 0.39/100 = 0.051 g/L. Also from page 201, this amount of monopotassium phosphate will supply 0.051 x 100/0.31 = 16 mg/L K
- Choose potassium nitrate to supply the rest of the K (64-16 = 48 mg/L). From page 201, this will require the addition of 48 x 0.26/100 = 0.125 g/L of potassium nitrate. This will also supply 0.125 x 100/0.72 = 17 mg/L N.
- Choose Secure-AN 28 (ammonium nitrate) to supply the rest of the N (85-17 = 68 mg/L). This product contains 28% w/v N (28 g N/100 mL = 280 mg/mL), so to get 68 mg N, you need to use 68/280 = 0.243 mL Secure-AN28 per litre of final solution. Hence, each litre of fertigation solution will contain: 0.051 g monopotassium phosphate, 0.125 g potassium nitrate and 0.243 mL Secure-AN28.
- If your tank holds 1000 litres, you must add to it: 51 g monopotassium phosphate, 125 g potassium nitrate, 243 mL Secure-AN28.
- If you use a fertiliser injector (Dosatron, etc) and it has been set at a dilution of 1:100, a container of stock solution with a volume of 200 litres (This makes 200 x 100 = 20,000 L of fertigation solution.) will require the following amounts of nutrient salts: 1020 g monopotassium phosphate, 2500 g potassium nitrate and 4.86 L Secure –AN28.
To use calcium nitrate to supply 40 mg/L Ca, you need (from p. 201) 40 x 0.59/100 = 0.24 g/L calcium nitrate. This will supply 0.24 x 100/0.84 = 28 mg/L N, which has to be subtracted from the N added via Secure-N28. This will have to be made up in a separate stock solution, so as to prevent precipitation of calcium phosphate
To use magnesium sulphate to supply 20 Mg and extra S, you need 20 x 1.01/100 = 0.202 g/L magnesium sulphate. This will supply 0.202 x 100/0.77 = 26 mg/L S. Magnesium sulphate can be included in with the NPK stock solution.
These amounts have to be multiplied up for your stock solution volume and injector setting, as detailed above for the NPK solution.
For more information, please contact one of our grower services representatives.
Please note, any advice displayed is of a general nature only, and an intending user of a product should obtain and only rely on professional advice particular to their intended purpose, climate, soil conditions and other factors.