Making Stock Solutions.
Nutrients for hydroponics have to be made as three separate
solutions usually called the A, B and acid 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. One option is
to place the calcium nitrate, potassium nitrate and iron chelate
in the A solution, and all the other fertilisers and trace
elements in the B solution.
The acid solution
The acid solution is used to control the pH of the diluted
hydroponic solution, and is usually 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. 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. The combined weight of the chemicals in each tank
should be approximately equal.
Volume of A & B solutions.
The recipe sheets we supply 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
because regular solution or plant analyses may show that a change
in feed recipe is needed and it may be difficult or impossible to
adjust an existing solution to a new recipe.
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.
It is worth writing out the list of fertilisers in each solution
and ticking off the weights as they are weighed. All the
fertilisers for each solution can be dumped into a suitable sized
bucket or drum after they are weighed. If you can , 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. Many recipes will
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 labeled bottle. When making up the
B solution add 100mls (=1 gram of sodium molybdate) of this
bottled solution to the B solution.
Fertiliser quality for hydroponic solutions
Most of the chemicals required for making nutrient solutions
are also used by the fertiliser industry. Readily available
fertiliser grade chemicals may be hard to dissolve, or may
contain impurities which have either a nuisance value or may be
harmful in nutrient solutions. Better grade products suitable for
hydroponic use are sold as "greenhouse grade" or
"technical grade" products. Major components of
hydroponic solutions are generally available as listed below.
- Calcium nitrate - greenhouse grade
- Calcium chloride- technical grade
- Potassium nitrate - greenhouse grade
- Iron Chelate - many types of iron chelate are available
at a wide range of prices. FeEDTA (Iron- ethylene diamine
tetra acetic acid or sodium salt) is suitable (provided
that it is a high quality product) and should contain 13%
Fe. The AKSO, Dissolvine and Librel brands are widely
sold internationally and are suitable. Some unbranded
sources of FeEDTA are difficult to use.
- Mono-ammonium phosphate - technical grade
- Mono-potassium phosphate - technical grade
- Phosphoric acid (65%) food grade
- Nitric acid (70%) technical grade
- Potassium sulphate greenhouse grade (fertiliser grade
very hard to dissolve)
- Potassium chloride - greenhouse or technical grade
- Magnesium sulphate - most grades are suitable\
- Magnesium nitrate - technical grade
- Manganese sulphate - technical grade
- Zinc sulphate - technical grade
- Copper sulphate - technical grade
- Borax - sodium tetraborate decahydrate - technical grade
- Boric acid - technical grade
- Solubor and similar 20% boron products - technical grades
- Sodium or ammonium molybdate -technical grade.
- Some fertiliser brands which are sold internationally and
which offer greenhouse and technical grade products
suitable for making hydroponic solutions include Haifa
Chemicals, Kemira, Hydro-Agri Rotterdam.
Dissolving the fertilisers.
The fertilisers will dissolve very quickly in hot water. About 70
litres of hot water at normal domestic hot water temperatures
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
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.
The acid solution is 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
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.
Revised November 1999. ©R.A.J.White November 1999.