Nutrient solutions.
What are the mineral elements ?
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 sulphur (S) are required by plants in
large amounts.
The rest are required in trace amounts (micronutrients). Essential
trace elements include boron (B), chloride (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 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 aluminium
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 Sulphate is plaster of Paris and will
precipitate when calcium salts are mixed with sulphate salts in
concentrated form.
Note: (1 mg/litre = 1 ppM)
| Element |
ppM |
Range
in ppM |
Part A |
Part B |
| From |
To |
| K - potassium |
279 |
200 |
400 |
|
|
| N - nitrate NO3 |
196 |
70 |
200 |
|
|
| N - as ammonia NH4 |
0 |
0 |
31 |
|
|
| P - phosphorous |
31 |
30 |
90 |
0 |
|
| Ca - calcium |
160 |
150 |
400 |
|
0 |
| S - sulphur |
64 |
60 |
333 |
0 |
|
| Mg - magnesium |
48 |
25 |
75 |
|
|
| Fe - iron |
0.8 |
0.5 |
5 |
|
|
| Mn - manganese |
0.25 |
0.1 |
1 |
|
|
| B - boron |
0.06 |
0.1 |
1 |
|
|
| Zn - zinc |
0.05 |
0.02 |
0.2 |
|
|
| Cu - copper |
0.02 |
0.02 |
0.2 |
|
|
| Mo - molybdenum |
0.04 |
0.01 |
0.1 |
|
|
| Chlorine |
0 |
0 |
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 sterilisers 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 management 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.
Copyright © 1996, 1997, 2001 TPS Pty Ltd
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