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Home NFT System Management Soilless Culture System Management
Introduction
System Design
Plant Propagation
Water Quality
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Pre-plant Routine
Post-plant routine
Quality Control
Media Sampling
Root Disease
Nutrient Levels
Nutrient Uptake
Greenhouse management Tomato crop management Management of alternative greenhouse food crops. Crop Nutrition

Nutrient uptake and nutrition management in run-to-waste bag culture

The nutrient demands of greenhouse tomato crops must be very similar in any growing system, and information on nutrient demand obtained by research or experience with NFT or rockwool systems must have some relevance to the nutritional requirements of run-to-waste bag grown crops in pumice, sawdust or other media. This contention is supported by the close similarity of total nutrient uptake in peat modules, NFT and rockwool .

Published estimates of nutrient uptake by glasshouse tomatoes in soilless media.
Nutrient uptake per crop kg/ha
Worker Year N P K Ca Mg Method*
Maher 1964 612 90 961 330 120 peat, DM
White 1992 790 170 1415 606 112 NFT, Dep
Voogt 1993 720 140 1162 523 85 RW, Dep
Voogt 1993 599 145 1125 378 82 RW, DM
* method, RW= grown in rockwool, Dep = uptake calculation by depletion,
DM= uptake calculated from analysis of dry matter.

The major problem is how best to monitor the nutritional status of crops growing in bag systems. None of the media currently in world-wide use (rockwool, polyurethane foam, perlite, scoria, pumice or sawdust ) has any appreciable cation exchange capacity, and all have very limited anion exchange capacity. In this circumstance there seems to be a general agreement that the most valuable measurement for nutrition management is the nutrient concentration in the root environment. The moisture content close to the bottom or rockwool slabs typically varies between 40 and 80% v/v and so collection of samples of the nutrient solution surrounding the roots is possible and commonly recommended (Smith 1987). Samples of 30-40 ml from each of 15 -20 slabs are withdrawn with a syringe and pooled for laboratory analysis. Recommendations for nutrient concentrations in the root zone have been made by Sonnevelde (1992, 1994).

Pumice and sawdust used for tomato growing in New Zealand has a total water holding capacity in the range of 30 - 45% v/v. It is not possible to collect samples of nutrient from within the bags with a syringe, but it was felt that analysis of the nutrient solution within the root zone would offer the best method of nutrition management using Sonneveld's recommendations for interpretation. The method of we recommend for sampling is to take 20-25 mm diameter core samples extending from the top to the bottom of 20 bags. A sub-sample is then taken after thoroughly mixing the 20 cores, and an extract prepared by shaking 1 volume of media with 1.5 volumes of distilled water. The filtered extract is then analysed in the same manner as any other nutrient solution. This method has now been in use since 1992. It is important that the samples are from the full depth of the planter bags as it reasonable to expect gradients in nutrient concentration and solution conductivity within the planter bags.

The laboratory reports the pH and conductivity of the extract and the concentrations of N, P, K, Ca, Mg, S, Na, Cl, Fe, Mn, Zn, Cu and B in the extract. These are not the concentration in the root zone, they are concentrations in a diluted extract of the solution in the root zone. Pumice and sawdust bags are kept close to the maximum water holding capacity by frequent irrigation with small volumes of nutrient solution. If the water holding capacity of the media is known, then the dilution factor is known and the concentrations in the root zone can be estimated. The average water holding capacity (WHC) of pumice samples that we have had analysed is 38% with a range of 30-44%, the average for sawdust samples was 39.5% with a range of 35 -52%. When the WHC is not known we assume a WHC of 37.5% and use a multiplier of 5 for converting the concentrations in the extract to concentrations in the nutrient solution in the root zone.

Growers are recommended to take media samples for analysis at no more than monthly intervals, and to control total nutrient concentrations between sampling dates by measuring the conductivity of 1:1.5 water extracts of core samples at regular (preferably weekly) intervals, and then control the total nutrient concentration by adjusting either the conductivity of the nutrient solution or the proportion of run off so that the conductivity of the extract (and hence total nutrient concentration in the root zone) remains at the desired level.

Nutrients accumulate in the media in bags in run-to-waste systems as result of an imperfect balance between nutrient supply and nutrient uptake. The accumulation should be limited by constant leaching of nutrients in the run off solution. High run off rates and low accumulation rates are economically undesirable and hence relatively low run off rates (20-30%) are common in run-to-waste systems, resulting in considerable accumulation if the system is not well managed. The principle function of the analysis and interpretation is to check the balance of nutrients in the root zone and to adjust the balance of nutrients in the irrigation solution so that the desired balance can be maintained as far as possible in the root zone. The exact nutrient concentrations are not important as the main concern is with balance and hence knowledge of the exact WHC of every sample and use of a precise dilution factor is not critical.

Some nutrients appear to leach more readily than others, or conversely to accumulate more readily than others in pumice and sawdust. Potassium and sodium accumulate very readily, calcium and magnesium much less easily. Phosphorus leaches very readily. Somewhat free interpretation of Sonneveld's recommendations appear to give good results, allowing for somewhat higher potassium concentrations and lower calcium concentrations. Manganese, and to a lesser extent zinc, appears to accumulate readily and persist in sawdust and pumice. The manganese content of applied nutrient solutions frequently needs to progressively reduced through the growing season because of this and if manganese becomes excessive in the media manganese can be omitted quite safely from the applied nutrient solution for a month or more.

Some variation in the composition of the applied nutrient solution through the course of the growing season is needed to match the changing pattern of nutrient uptake. Low potassium, high calcium solutions are best for young plants prior to development of a significant fruit load, with increasing potassium supply as the fruit load develops, but the extent of the necessary increase can be reduced by potassium accumulation in the media.

Conclusion.

More refinement of methods for managing nutrition in pumice and sawdust run to waste growing systems is needed, but the methods described here appear to be the best available at present. Bag growing systems with recycling of nutrient solutions appear to offer greater possibilities for more accurate control, with additional fertiliser cost savings by avoidance of run off.

References

Maher, M.J. (1976) Growth and nutrient content of a glasshouse tomato crop grown in peat. Scientia Hort. 4:23-26
Smith, D.L. (1987) Rockwool in Horticulture. Grower Books, London.
White, R.A.J. (1992) Nutrient uptake by tomatoes grown in NFT. Proc 8th Int.Congress Soilless Culture, Hunters Rest, South Africa: 483-496
White, R.A.J. (1997) Management of greenhouse tomato nutrition in hydroponic growing systems. In Nutritional requirements of horticultural crops (Eds L D Currie & P Loganathan). Occasional rep't no.10 Fertilizer and Lime Research Centre, Massey Univ. NZ.
Voogt, W. (1993) Nutrient uptake of year round tomato crops. Acta Hort. 339:99-112.

November 1999. Error in table- nitrogen uptake corrected 27/10/08

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