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NFT Solution Management and Record Keeping.

Benefits of Accurate Records

Keeping records of the behaviour of your NFT solution will be useful in tracking down any problems that may occur and is essential when calculating the crops nutrient uptake. It is just as essential to keep records with automatic systems as it is with manual systems. Maintaining a record system is not difficult or involved it just requires a little discipline and organisation. The human memory is never good as we imagine and written records are therefore much more reliable.

Information to collect from the NFT System.

Keeping accurate records enables the crop nutrient uptake to be calculated. These figures show what the crop is taking from the solution and so allow recipes to be fine tuned to suit individual crops at each stage of their development and in relation to the particular water supply. If no records are available then recipe tuning is more of an inspired (and experienced) guess than a calculated and logical change. Crop uptake is calculated from the change in nutrient content of the recirculating solution between two analyses and from the quantities of nutrient added to the solution during the interval between the analyses by the addition of water,A&B stock solution and acids or alkalis added. The information required to be able to calculate the uptake rate is:

  • Water meter readings at each sampling
  • Volume of A & B stock solutions used between samplings
  • Volume of acid or alkali used between samplings

Keeping daily records of solution pH, conductivity, temperature, water meter readings, A & B stock tank volumes and acid or alkali use is strongly recommended. Experienced NFT managers can often notice iregularities or inconsistencies in such records that can alert them to incipient problems before they become severe.

Basic Guidelines for solution Nutrients for Tomatoes.

NFT solution analysis - simple interpretation guidelines for use when data is not available for uptake rate calculations. The correct levels are very dynamic depending on crop grown, the time of year, crop load and age, the greenhouse environment maintained and the locality and its climate.

1. CF. The best CF (Conductivity Factor) at any time depends very much on the crop age, season, and crop load. The CF will generally need to be greater than 20 and could be as high as 60 for cherry tomatoes or during winter with table tomatoes.

2. pH. The pH often increases by as much as 0.3 pH during transit to the laboratory. The maximum pH for NFT solution we suggest is pH 6.3 but prefer to keep the pH between 5.5 and 6.1. If the pH is allowed to rise to pH 6.5 or greater then there is a risk of precipitating calcium phosphate within the NFT system and consequent phosphorus deficiency in the crop.

3. Nitrogen concentration. This depends on tomato crop age, CF and relativity with other nutrients. Young crops need more Nitrogen, crops close to picking and for some time after the first pick need less Nitrogen, but Nitrogen needs to be increased again later in the life of the crop if there is a decline in vegetative vigour. Generally higher Nitrogen levels are required in summer compared to winter.

4. Phosphorus. Tomatoes are extremely efficient at removing phosphorus from NFT solutions and phosphorus at 32 ppm to 64 ppm is more than adequate regardless of solution CF or the concentration of other nutrients. Phosphorus levels greater than 64ppm are unnecessary and increase costs and high phosphate may reduce fruit quality.

5. Potassium. High K levels are required for tomatoes with set fruit. A target range of 350 to 800ppm depending on CF is often suitable.

6. Sulphur. A target range is 64 to 350ppm is adequate, 64ppm will normally be adequate, but concentrations up to 350 have to be used to allow high K and Mg concentrations without excessive nitrogen concentration. Prolonged high sulphate concentration can reduce fruit quality.

7. Calcium. The calcium concentration needs to be about 50% of the potassium concentration.

8. Magnesium. Magnesium uptake is subject to competition from the other cations in the solution. We aim to keep magnesium at 16% of the total cations in the solution, i.e. moles Mg++/ (K+ + Ca++ + Mg++ + Na+)= 0.16

9. Sodium. Sodium can be allowed to accumulate too as high as 400ppm as long as the solution CF is adjusted upwards to compensate for the CF contribution of the sodium. This does depend on the variety grown and the time of year with lower CF levels in summer required to avoid restricting water uptake by the plant due to a reduced uptake rate.

10. Chlorine. Moderate chlorine (chloride) levels do not appear to cause any problems, but excessively high chloride levels may cause some leaf scorch. Chlorine should be included in the analysis of NFT solutions so that the cation/anion balance of the solution can be calculated for checking the analysis.

11. Iron. A old target for tomatoes is 5-10ppm in summer and 10-15ppm in winter, but observed uptake rates suggest much lower solution concentration are adequate.

12. Manganese: The target range is 0.3 -0.5ppm and upto a maximum of 1ppm. Higher levels of manganese compete with iron for uptake.

13. Zinc: target 0.6ppm

14. Copper: target 0.2-0.5ppm

15. Boron: target 0.8 -1 ppm. The margin between boron and deficiency and boron toxicity is very narrow. Some research at the Levin HRC in New Zealand suggested that tomato growth and yield could be reduced with solution B at 3ppm and that visual toxicity symptoms could occur with boron at 5ppm.

More reliable interpretation is available for our clients keeping accurate records, which enable the crop nutrient uptake to be calculated.
Revised June 2007. ©R.A.J.White 26 June 2007