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Solution Management

Optimising hydroponic crop nutrient uptake.

Regular analyses of recirculating hydroponic solutions and adjustments of the stock solution recipes allows crop nutrient uptake to be optimised for both yield and product quality. Estimating crop nutrient uptake rates provides a better control than adjusting recipes according to set target concentrations in the solution.

Adjusting Stock solution recipes according to analysis results

Target method. The common method of adjustment following a recirculating solution analysis is to compare the nutrient concentrations in the recirculating solution with some set of standards, and then increase or decrease the amount of each nutrient in the stock solution recipe according to whether the concentration is above or below the target. The nutrient concentration shown in the analysis does not give a very good idea of how much change should be made in the recipe, this is a guessing game and even the most experienced users of this method find it takes several successive analysis with over and shoot in the adjustments before the proper balance is reached. This process is far from ideal for crops whose uptake demands change with growth and development or according to seasonal changes in the environment.

Nutrient uptake method. Two successive analyses of the recirculating nutrient solution with proper records can be used to estimate a crop’s nutrient uptake rate. This method allows the crop to provide feedback on how much nutrient is being taken and allows a logical adjustment to the stock solution recipe to be made. The uptake rates can be compared with some past history of uptake rates and adjustments can be which recognise both current crop demands and expected future demands due to growth stage or environment. The uptake method is not only more logical than the target method but it also helps to avoid the overshoot undershoot approach to the central target.

Some other advantages of nutrient uptake based solutions. The calculated uptake is feedback from the crop on what nutrients are actually be taken up and can provide valuable guidelines for crops where target values are not known. Target values for tomatoes for example are for ordinary round and beef tomatoes, they are not as reliable for acid free, Roma or cherry tomatoes. 

There is plenty of data on standard round tomato crop nutrition and solution target concentrations,but




but little data for plum, Roma or acid-free tomatoes. Consumers look for sweet low acid fruit.  These tomatoes are very susceptible to  blosson end rot and should probably be grown with less K and more  Ca than round tomatoes.


Cherry tomatoes should have outstanding flavour with extreme sweetness  and hihg acidity.  They are not susceptible to blossom end rot and can be grown with more K and less Ca than usual.

Uptake calculations can often show that a crop is well supplied with a particular nutrient even though the concentration of that nutrient in the solution is somewhat different to the published target value. This means that in this particular set of circumstances the published target value is incorrect and the uptake calculations are providing a better basis for solution formulation than target values.

New Hydroponic Crops
There is an abundance of data for the major greenhouse vegetable crops under European and North American conditions, but for minor greenhouse crops and new crops there may be a dearth of information and no reliable solution target concentrations. Uptake calculations and the resulting feedback from the crop can help considerably in overcoming the lack of data for new and minor crops

Nutrient uptake calculations

Calculating nutrient uptake rates is basically an accounting operation but with amounts of nutrients instead of amounts of money. Two solution analyses are like two bank statements, when nutrient concentrations multiplied by the solution volume equals the amount of each nutrient in the system at the times of the first and second solution analysis. Records of water use, A & B stock solution  and acid or alkali added show how much of each nutrient has put into the system (how much has been paid into the bank) in the time between the two analyses. The difference between the amount of each nutrient in the first analysis plus the amount of all the additions and minus the amount at the final analysis must equal the withdrawls from the system. In a well run recirculating nutrient solution, the only withdrawls will be the uptake of nutrients by the crop. Leaky systems have uncontrolled withdrawls, but it is sometimes possible to estimate the volume of solution leaked and hence the loss of nutrients.

Making Uptake Calculations
The calculations are a little tedious as they have to be repeated for each of the 13 elements included in the analyses but the process is simple if a spreadsheet is used on a computer.

Excel spreadsheets for making the uptake calculations have been used and continuously improved in this practice over the past 16 years. Greenhouse tomato, pepper, cucumber and lettuce growers in New Zealand have been using our services for many years to calculate uptake rates and revise nutrient solutions and a few growers are now using our software to make their own calculations and adjustments to their recipes.

We can make uptake calculations and suggest recipe adjustments for growers in New Zealand and overseas, if they are able to get quality analyses of nutrient solutions and keep proper records and can send and receive data in comma separated value (.CSV) files. Use the form below if you would like a quote for the cost of this service.

Spreadsheets for uptake calculations can be supplied in Microsoft Excel workbooks for growers who would prefer to make their own calculations. The cost of this workbook is $152 including Nutricalc spreadsheet. Subscribers to this website who have already downloaded Nutricalc from the website can purchase the Uptake workbook for only $117.00. The workbook can be emailed to you as an Excel File attachment.
The Uptake workbook contains the following pages:

      • Advice Note data base; this is information on water, stock solution and acid use.
      • Crop data base: this data includes the crop area, planting date and variety
      • Laboratory analysis results data base
      • Uptake calculation worksheet
      • Uptake history data base: stores the uptake rates

Use the form below if you would like to receive more details by email (Office Word documents) or use the GHVI subscription form to order and pay for the Uptake calculation work book.


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