Nutrient uptake in recirculating solutions.
Recirculating nutrient solution maintenance using the regeneration principle
Day to day management of recirculating nutrient solutions ( recirculating in NFT systems or in bag growing systems) is achieved by keeping the conductivity and pH of the recirculating system approximately constant by adding appropriate amounts of the A & B stock solutions and acid or alkali solutions.
The assumption underlying this method is that nutrients removed from the solution by the plant roots are replaced by nutrients added in the stock and acid or alkali solutions, so that an approximately constant concentration of nutrients is maintained in the recirculating nutrient solution.
Conductivity measurements are extremely useful in managing nutrients solutions, but their use must be properly understood. Conductivity meters measure the flow of an electrical current between the meter's two electrodes immersed in a test solution. Nutrient ions in the solution (for example K+, Ca++ , NO3- and SO4= ions) are charged with one or two electrons and carry the current between the electrodes. Thus the conductivity (or rate of flow of the current) is almost directly related to the total number (or concentration) of ions in the solution. Because conductivity only measures the total number of ions in a solution, nutrient solutions of quite different composition can have the same conductivity.
The balance of nutrients (that is the ratio of the individual nutrients to each other) is not the same in the recirculating solution and in the A & B stock solutions.
A typical recirculating system might contain 1 m³ of solution with the solution containing 9 mols K+, 3 mol Ca++ 2 mols Mg++ and the equivalent number of NO3-, H2PO4- and SO4= ions, at CF 19. If the crop removes 1 mols K+,0.5 mol Ca++ and 0.25 mol Mg++, the solution will then contain 8 mols K+, 2.5 mol Ca++ and 1.75 mols Mg++, and the conductivity will have fallen to 16.5 CF. The solution CF is then restored to 19 CF by adding A & B stock solution.
The stock solutions are usually 100 or so times more concentrated than the recirculating solution. Adding 2.63 litres of A & B stock solutions with the same ratio of nutrients as in the recirculating solution but 100 times more concentrated will restore the nutrient solution to CF 19 and will add 1.18 mol K+, 0.395 mols Ca++ and 0.263 mols Mg++ . The recirculating solution now contains (approx.) 9.18 mols K+, 2.895 mol Ca++ and 2.01 mols Mg++, and has a slightly different balance of nutrients to the original solution. The difference will increase with each successive addition, so that after a period of time with many additions the nutrient balance will be different. These changes are illustrated in the graph below, which follows the changes when the CF is allowed to fall by 2.5 CF before adding sufficient A & B stock solution, with the same balance of nutrients as in the recirculating solution, to restore the CF to 19 over a period of 15 days.
The potassium concentration increased from 9 mols (352 ppm) to 11.7 mols (460 ppm), the calcium concentration fell from 3 mols (120 ppm) to 1.4 mols (57 ppm) while magnesium remained nearly constant (2 - 2.2 mols or 48- 53 ppm). The increase in potassium concentration will tend to force a higher rate of potassium uptake by the crop, but at the same time the calcium uptake rate will be decreased because of the low concentration in the solution, and by competition from the high potash level. Magnesium was being supplied at a rate which was very close to the uptake rate and hence the concentration in the recirculating solution was nearly the same at the end of the 15 days.
This illustrates the very important principle of regeneration of nutrient solutions, that is if the nutrients are supplied in the same balance or ratio as they are being taken up by the crop, then the initial balance or ratio of nutrients in the recirculating solution will be maintained. Neither uptake rates nor the balance for any crop is constant as both are affected by the greenhouse environment and hence rate of crop growth and by the stage of development of the crop.
Environmentally induced changes can be large on a day to day basis, whereas changes due to the stage of crop development and season are more gradual and more predictable. The most reliable method of formulating the A & B stock solutions is by using estimates of the uptake rates for each obtained by regularly analysing the nutrient solution. The estimates are calculated from the change in nutrient quantities in the recirculating solution and quantities of A & B stock solutions, water and acid or alkali in the period between successive analyses. These estimates assume that no nutrients are precipitated within the system or lost other than crop uptake.
When the principle of regeneration is followed in formulating stock solutions the balance of nutrients in the recirculating solution and in the stock solution may be quite different. Using this principle in this way also makes good allowance for the different environments and crops and allows balanced solutions to be maintained for new crops about which there may be little or no information on nutrient uptake balances or recommended solutions.
More information is available in the section on Nutrition
© R A J White November 1999