Nutrient Solutions

Greenhouse & hydroponic
systems information

  Day to Day Solution Management  


Simple Chemistry

Conductivity & pH

Laboratory Analysis

Crop & Season Effects

Recipe Calculations

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Practical Tips

Day to Day

Solution Management


Day to day solution management and record keeping

Keeping good records on the behaviour of your recirculating nutrient solution will be useful in tracking down any problems that may occur and is essential for providing the information needed to calculate nutrient uptake. It is just as essential to keep records with automatically controlled solutions as it is for manually controlled systems.

Record keeping is not difficult or involved, it just requires a very little discipline and a little organisation. Memory is never as good as we imagine it to be, and written records are much more reliable.

Crop nutrient uptake can be calculated following solution analysis when reliable records are available. Calculated uptake figures show what the crop is taking from the solution and allow recipes to be fine tuned to suit individual crops at each and every stage of their development and in relation to their particular water supply. If no records are available then recipe tuning is more of an inspired guess than a calculated adjustment. Crop nutrient uptake is calculated from the change in nutrient content of the recirculating solution between two analyses and the quantities of nutrient added to the solution during the interval between analyses. The information needed is the volume of water added to the system and the volumes of A & B solution and acids added.

All of the required information can be gathered in about 2 minutes each day. A water meter is essential in the water supply pipe. If the A & B solution tanks are fitted with a sight tube, then they should be calibrated with a litre scale. If there is no sight tube then a calibrated dip stick should be made. Each morning when first starting work, the pH, and CF  of the solution should be noted, together with the volume of A & B solutions and acid in their respective tanks.

Automatic control failures do occur from time, and can result in solutions with excessively high CF or very low pH. This malfunction may be unnoticed for days if daily checks are not made. The unusual CF or pH will provide an alert to the malfunction, the A & B tank readings or acid tank readings will indicate the nature of the malfunction. These records are equally useful and applicable to both manual and automatic systems. Your own daily checks will alert you to many different problems. Any sudden increase in water use may indicate a leak in the system. Unequal dosing of A & B solutions is not uncommon with automatic systems, the daily records will alert you promptly to such faults. A decrease in acid use may indicate either a reduction in nitrogen uptake or perhaps the onset of root death.

Each solution recipe supplied by me has a individual recipe number, note this number on your record sheets. Each time you have to make a repeat batch solution of the same recipe note the batch number on your record sheet. This then provides a double check on how much solution is being used. The uptake calculations also require that the volume of solution in the system (sump, pipes and gullies) be known. This can be measured by noting water meter readings before refilling the system after dumping, and again when the recirculating solutions starts to flow back into the sump. The NFT system volume will change as the roots fill the gullies and the recorded system volume needs to be updated regularly. This may be difficult for growers who do not need to dump regularly. It is possible to estimate the system volume by measuring the depth in the sump and in the NFT gullies and using the known sump dimensions and known total length of gullies.

Solution Analysis

The recirculating nutrient solution is subject to continuous change as the crop removes nutrients and these changes can only be monitored by solution analysis. Some authorities recommend regular analysis, the Dutch for example recommend analysis every two weeks, but a variable interval with sampling at critical stages of crop development is proabaly even better. Tomatoes typically need solution analysis when the 3 rd, 5 th and 7 th trusses are flowering and then monthly from the mature crop, while lettuce only need ocassional solution analyses to accomodate season changes in growth rates.

Taking solution samples for analysis

A 200 ml sample of the nutrient solution is adequate for analysis by R.J.Hill Laboratories Ltd. Other laboratories may require a different sample volume. The sample should be representative of the whole volume of the solution . New polythene bottles are best, but any clean (plastic) bottle can be used. Make sure that you write your name and the sample description (house number or crop) on the bottle with a waterproof pen before filling. Rinse the bottle with two lots of the sample solution before filling with solution leaving about 10 ml of head space in the top of the bottle. Note the pH and CF of the solution at the time of sampling as the laboratory measurement will then provide a check on your instruments. The CF does not change after sampling, but the pH may change during transit.



Original July 1992 Revised July 1996.Web site version May 2006