Nutrient Solutions

Greenhouse & hydroponic
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Practical tips on making nutrient solutions

 

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Making Nutrient Solutions

Nutrient stock solutions for hydroponics have to be made as three separate solutions usually called the A, B and acid or alkali solutions. The A solutions always contains all of the calcium, but calcium phosphate and calcium sulphate are insoluble so sulphates and phosphates must never be put into the A solution. Recipes that I supply will often have calcium nitrate, potassium nitrate and iron chelate and possibly calcium chloride or potassium chloride in the A solution, and all the other fertilisers and trace elements in the B solution.

The acid solution is used to control the pH of the diluted hydroponic solution, and may be nitric acid or phosphoric acid or a mixture of both. The acid solution thus provides either or both nitrogen and phosphorus and must be considered as an important part of the crop nutrition. The recipe sheets that I provide will specify the composition of all three solutions. Including some of the potassium nitrate in the A solution will make it easier to dissolve potassium sulphate or magnesium sulphate in the B solution. The amount of monopotassium phosphate or mono-ammonium phosphate in the B solution will allow for the amount of phosphate in the acid solution.

Volume of A & B solutions. The recipe sheets specify the amounts of fertilisers per 100 litres of solution. Many growers make up solutions in batches of 100 or 200 litres, but some larger scale growers may need to make batches of up to 1,000 litres. The volume made up at any time should not be greater than the amount which is expected to be used within a three or four week period. This is not because of any problem of keeping quality of nutrient solutions, but simply that regular solution or plant analyses may show that a change in feed recipe is needed and it maybe difficult or impossible to adjust an existing solution to a new recipe.

Equipment required. The main fertilisers in the A & B solutions need to weighed out to the nearest 100 grams for batches of 100 or 200 litres. Most growers produce scales will be accurate enough for this. The trace elements in the B solutions need to be accurately weighed out to the nearest gram. Laboratory type triple beam balances with taring are ideal for this. One suitable scale is the Ohaus triple beam scale model 2660T. This sturdy, accurate and reliable mechanical scale should last for many years and costs about $350. It will weigh up to 2600 grams and can weigh to the nearest 0.1gram.Such scales are readily available from laboratory supply companies (see yellow pages for local addresses). Acids are usually bought as concentrated commercial grade acid in 20 litre polythene containers. A polythene siphon pump, polythene 1 litre measuring cylinder, PVC safety gloves and a face shield should be used for dispensing and mixing the acids.

Weighing and checking procedures. Use a check list of fertilisers in each solution and tick off the weights of each fertiliser as they are added to the mixture. All the fertilisers for each solution can be dumped into suitable sized bucket or drum after they are weighed. The recipe sheets provided by me also list the total fertiliser weight in each solution, and if you can it pays to check weigh the dry fertiliser mix. The weights of some of the trace elements required for the B solution are small (especially the zinc sulphate, copper sulphate and sodium molybdate), and the weighing accuracy can be improved by making these into individual stock solutions of ten times the required strengths. Recipes might call for 1 gram of sodium molybdate per 100 litres of B solution, so weigh out 10 grams of sodium molybdate and dissolve in 1 litre of water, and store this in a labelled bottle. When making up the B solution add 100mls (=1 gram of sodium molybdate) of this bottled solution for each gram required in the B solution.

Dissolving the fertilisers. The fertilisers will dissolve very quickly in hot water. About 70 litres of hot water at about 65C will dissolve all the fertilisers for a 100 litre solution in 3-5 minutes with stirring. The solution can be made up to its final volume as soon as the fertilisers have dissolved. Note that the recipes are written for the weights of fertilisers in 100 litres of solution, and that this is quite different to the weights of fertilisers plus 100 litres of water since the dissolved fertilisers have some volume in the solution. Thus the A & B tanks should be calibrated with a fill mark (100 or 200 litres or whatever), the fertilisers should be dissolved and then the tanks made up to the final volume with water.

Making the acid solution. Concentrated nitric acid fumes strongly, and the fumes are corrosive and dangerous. The acids should only be dispensed in a well ventilated place or in the open. Both acids are strongly corrosive and dangerous and can cause severe skin burns, or blindness if splashed into the eyes. They must be handled with great care. Never add water to concentrated acid, concentrated acid reacts strongly with water and can boil and sputter with the risk of throwing acid drops onto the skin or into the eyes. Wear PVC gloves and a face shield when handling the concentrated acid. Small voluimes of acid solution are most easily made up in a translucent polythene 20 litre drum. Calibrate the drum before use with marks at 10 and 20 litres. Put 10 litres of water into the drum before adding any acid. Dispense the concentrated acid with a siphon pump from the original container into a litre polythene measuring cylinder and then the tip the acid into the 10 litres of water in the dilute acid drum. After you have finished adding the acid, make the volume up to 20 litres by adding more water.

Caustic potash solution. Sometimes the pH of the nutrient solution may be too low and it may be necessary to add an alkaline solution to raise the pH. Caustic potash or potassium hydroxide solution is used for this purpose. Pure potassium hydroxide is usually sold as flakes or pellets. Both forms are strongly corrosive and will cause nasty skin burns which are hard to heal, so wear rubber gloves and do not handle the pellets or flakes. A considerable amount of heat is released when potassium hydroxide is dissolved in water and the solution will get very hot. Do not dissolve too much caustic potash in too little water in plastic containers or the heat may melt the plastic, and spill the caustic solution. The usual strength of potassium hydroxide solution for raising the pH of nutrient solutions is 50 grams of potassium hydroxide per litre. If only a small volume is required, weigh out 200 grams of caustic potash and put into a calibrated 4 litre glass flagon, add about 3 litres of water and shake until dissolved, and then make up to the 4 litre mark.

 

August 30, 1991, revisions 7/96, 6/02, 7/06