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Nutrient solutions.


There are twenty mineral elements considered necessary or beneficial for plant growth. Carbon (C), hydrogen (H), and oxygen (O) are supplied by air and water.

The six macronutrients, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) are required by plants in large amounts.

The rest are required in trace amounts (micronutrients). Essential trace elements include boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), sodium (Na), zinc (Zn), molybdenum (Mo), and nickel (Ni).

Beneficial mineral elements include silicon (Si) and cobalt (Co). The beneficial elements have may only be essential for some plants. Cobalt for instance is essential for nitrogen fixation in legumes. It may also inhibit ethylene formation and extend the life of cut roses. Silicon, deposited in cell walls, has been found to improve heat and drought tolerance and increase resistance to insects and fungal infections. Excess silicon can cause strawberries to remain white when ripe. Silicon can help plants deal with toxic levels of manganese, iron, phosphorus and aluminum as well as zinc deficiency.

The most productive approach includes mineral elements at levels beneficial for optimum growth.

An typical nutrient formulation is listed below. To the right is the general range of nutrient elements used by plants. Part A and Part B refer to the requirement to keep particular nutrients separated in the concentrated form because of the fact that many nutrient combinations will precipitate. For Instance Calcium Sulfate is plaster of Paris and will precipitate when calcium salts are mixed with sulfate salts in concentrated form. Note: (1 mg/liter = 1 ppM)

ElementppMRange from To in ppM Part APart B
K - potassium279200 400
N - nitrate NO319670 200
N - as ammonia NH400 31
P - phosphorous3130 900
Ca - calcium160150 4000
S - sulphur6460 3330
Mg - magnesium4825 75
Fe - iron0.80.5 5
Mn - manganese0.250.1 1
B - boron0.060.1 1
Zn - zinc0.050.02 0.2
Cu - copper0.020.02 0.2
Mo - molybdenum0.040.01 0.1
Chlorine00 to 350

As plants grow their requirements change and so the nutrient solution must also be changed to keep the plants growing at their optimum. Ammonia has an adverse effect and needs to be kept to a minimum.

Iron and Manganese ions oxidize and precipitate out of solution causing problems of staining and nutrient deficiency. This process is exacerbated when the water is sanitized with UV light or chemicals. Magnets in the nutrient flow will attract and remove Iron oxide (it is magnetic) keeping the plant root systems and sterialisers cleaner.

Plants use some nutrients like nitrates at a faster rate than other nutrients. This causes the solution pH to increase requiring the addition of acids like Phosphoric Acid or Nitric Acid.

Nutrient Management ideals

A major difficulty in using recirculating solutions, is managent of the nutrient balance. This is much more difficult than with non-recirculating systems.

Open systems do a run to waste with the nutrient solution. Closed systems collect the nutrient solution and reuse it. For closed systems a wide range of collection systems are in use, many developed and sold by supply companies. They can vary greatly in initial cost and no type yet dominates the market. The basis of the operation of a typical closed substrate system is as follows: about 30% excess is run off from the medium, collected in channels and stored in a tank. This is then pumped through a steriliser. The sterilised solution is mixed with fresh nutrient solution to make the feed to the drippers.

Once a week, the run-off solution is sampled for complete analysis. The analysis service includes advice on what fertiliser balance should be used for the next week. The fresh nutrient solution composition is changed to follow these recommendations. Fertilisers can be purchased in concentrated liquid form. Commercial growers like those in Holland have an eight tank system, into which the concentrated fertilisers are pumped. Supplying fertiliser in this form is possible only in Holland because of the compact nature of the industry. In other countries, the much smaller size of the industry and the distances involved in cartage, would make the cost of this method prohibitive.

Most growers still use A and B concentrated nutrient tanks, but using the individual liquid fertilisers makes changing formulations much easier. Using separate nutrient tanks makes it possible for nutrient formulation to be remotely controlled by computers.

This information is given as a guide only - different crops, growing environments and growth stages will require different nutrient formulations.

Prepared by Trevor Croll,

42 Pearse Street, Keperra, Queensland Australia, 4054.
Phone 61 7 3855 1115