Commercial Use of Soilless Culture in New Zealand

Media Based systems for greenhouse vegetable production are well developed system in New Zealand but are high technology crop production systems, and requires both skill and experience from users if the best results are to be obtained.

Introduction

Increasing difficulties with greenhouse vegetable production is often due to soil borne root diseases. Yields can be limited even in the crop following immediately after soil sterilisation as the disease organisms are persistent in the soil below the depth reached by normal doses of soil sterilants. This situation is common in old greenhouses. These problems can be avoided by changing to growing with soilless growing methods which isolate the crops from soil borne diseases. This can be growing in planter bags full of pumice or sawdust , or for example, rockwool slabs or pumice or sawdust modules. Here we will deal with planter bags or modules as they are they are more common in New Zealand than Rockwool slabs due to the higher cost of the rockwool. The first part of this report details the work that needs to be done and the equipment which has to be installed to make the change.

Advantages and disadvantages of the NFT system versus bag growing systems

The operating cost of NFT systems is less than the operating cost of bag growing systems, as there are no bags or pumice or sawdust to purchase each year though new gullies do need to be purchased each year. Less labour is required for crop turn around as the used gullies are more easily removed than used bags, and laying out new gullies is much easier than carrying in new bags. The cost of power for pumping nutrient solutions for bag growing will only be about one third of the pumping cost for NFT as the pumps run continuously for NFT but only intermittently for bag growing. Fertiliser use in run to waste systems can be 30% greater than that in NFT systems, but should be similar for recirculating bag systems and NFT. Liquid feed dosing systems are generally much simpler for NFT than for bag systems.


Both systems should have a low dry plant risk. Each bag is typically watered by one or two small bore microtubes (1.27 mm nominal bore) and if the nutrient solutions are not properly filtered or if the nutrient solution is not properly balanced or kept at the proper pH, then the risk of blocked microtubes increases. Only the plant irrigated by the blocked microtube is affected. Each NFT gully is supplied with nutrient solution by one or two 3 or 4 mm bore leader tubes and the risk of blockage of these tubes is very low, but if there is blockage then a whole row of plants may be damaged by dryness.

A major advantage of the NFT system is the ability to easily change shoot density, between or within seasons. This is not easy with the bag growing systems as changing between seasons involves remaking the microtube trickle irrigation system with microtubes spaced to match the planter bag density. Growers using modules with say four plants per module can more easily alter the shoot density when growing tomatoes.

Changing density in this way in bag crops is difficult as the plants can only draw the same amount of water from each bag, and when density is increased and some bags are supporting two shoots the water supply from the bag becomes limiting.

Roots system in NFT can be inspected at any time, and this leads to the assumption that root problems are greater in NFT than in bag systems, and especially so for bags in run to waste systems where there is no recirculation of nutrients and reduced risk of spread of pathogens via the nutrient solution. In fact root diseases are not uncommon in run to waste bag growing system, and are often not detected as quickly and can be just as severe as disease outbreaks in NFT systems.

The minimum temperature in the root zone in NFT systems is usually controlled by solution heating. Root temperatures in bag growing systems are not controlled and follow air temperatures. This does not seem to limit tomato production in heated houses but may be limiting factor in cold houses. A disadvantage to NFT system is that solution temperatures may be too high for crops planted between December and February (NZ summer time), when small plants provide no shade for the gullies and solar heating causes a rise in solution temperature.

Nutritional management in NFT systems is relatively precise and easy with regular fortnightly or monthly solution analysis, immediate correction of any nutrients at low concentrations, and calculation of uptake rates which allow the stock solution to be balanced to meet expected crop nutrient demands. Management in run to waste systems is not as easy. Weekly checks of the total salt concentration in the bags should be made by the grower with monthly analysis of the sawdust or pumice media for estimation of nutrient availability in the root zone, followed by alteration of the feed composition to attempt to produce the target nutrient concentrations in the root zone. This is much less direct and less reliable than NFT system management. With recirculation in bag systems, it is possible to monitor the nutrient solution by regular analysis and to monitor the total salt level in the bags and calculate crop nutrient uptake for adjustment of feed recipes, and this should then provide a nutritional management systems which is just as precise as that for NFT.

There are some differences in capital costs for the two systems. NFT crops are at more at risk from power supply failures, than bag grown crops. A four hour interruption of power supply during summer could damage a bag grown crop, but an NFT crop could be damaged by a one hour power failure. Thus most growers using NFT have a small standby generator with sufficient capacity for operating the nutrient solution pumps. Fewer growers using the bag system have standby generators, but if there is a significant risk of power failures longer than two hours then a generator is highly desirable.



Comercial Use Of Soilless Culture for Tomatoes in New Zealand

R.A.J.White,New Zealand.

Introduction
More than 95% New Zealand's greenhouse vegetable growers have changed to soilless cultures in preference to growing in the soil during the last 15 to 20 years. The change to soilless culture typically results in a 20 to 25% increase in tomato yields as longer growing seasons can be used when soil sterilisation is no longer required during the summer months. Technical improvements in crop productivity when using soilless cultures can further increase yield for skilled growers.
Soilless culture methods used commercially in this country have included strawbale culture, peat and bark module growing, sawdust bag culture, pumice bag culture and nutrient film technique (NFT), with the last three being most important today. Rockwool is now commercially available in New Zealand, but is imported and very expensive and little used except for propagation purposes.

HISTORY OF SOILLESS CULTURE IN NEW ZEALAND
Strawbale culture was introduced to NZ following soon after its initial development at the Lee Valley EHS in the UK. It was used especially in the Oamaru district where soils were difficult to sterilise safely and where growers had previously used soil in containers as the main method of tomato production. Its use was never widespread and is very rare now as it is increasingly difficult to obtain straw from cereal crops which have not been treated with persistent herbicides.
Overseas developments of peat modules( Moorat 1975,Gallagher et al, 1977) were followed by research into the use of peat modules in NZ ( White et al 1978, Dennis 1979) which in turn led to the development of methods for using composted bark modules (White 1983). Culture in factory filled modules of fertilised peat or Pinus radiata bark enjoyed a limited popularity for about 5 years but has been largely replaced by various techniques for growing in 10 litre polythene planter bags filled with peat, bark, sawdust or pumice. Bag culture in sawdust was introduced following developments of techniques in Canada (Maas et al, 1971) and quickly adapted for local conditions (Mavromatis 1979, Jamieson 1979,Longley 1983).

Commercial use of NFT started in the mid 1970's following Cooper's work in the UK. There had been a few short lived commercial hydroponic operations prior to this. One large commercial NFT operation suffered severely from disease ( Pseudomonas solanacearum) in 1975/6 and there was little further use of NFT and no official research on NFT. Cooper visited NZ in 1977 and his visit was followed by more rapid adoption of NFT by commercial growers.
There are no official statistics on the current use of soilless culture methods in NZ, but my opinion, based on my client sample, is that more than 95% of the greenhouse vegetable crop area is grown in soilless cultures with pumice or sawdust bag culture used on a larger area than NFT.

PRACTICAL ASPECTS OF CHANGING TO SOILLESS CULTURE
Although growers are quick to see yield and profit gains in changing to soilless culture, practical considerations of costs and difficulties in making the change often prevent them from doing so. Sawdust bag culture and module culture techniques used fertilised substrates and allowed growers to use existing trickle irrigation and liquid feeding equipment previously used for soil culture. Nutritional requirements were relatively simply met by liquid feeding. Thus changing from soil to these systems was relatively easy and not especially expensive. Bag culture is practiced today with sawdust or pumice but without base fertiliser as a hydroponic system and with all of the crops nutrient requirements have met by liquid feeding with complete nutrient solutions.

Considerable capital investment is needed for accurate fertiliser dosing and watering systems, although many growers believe that it is less expensive to convert to bag growing systems then to NFT. Modern bag culture methods and NFT require much more technical expertise than soil growing if the best results are to be obtained. The choice of solid soilless media is based on cost and availability as well as on technical factors. Extensive forest industries in NZ ensure a plentiful supply of sawdust and composted ground bark from Pinus radiata .
TECHNICAL COMPARISONS OF SOILLESS CULTURES
There are complex interactions between water and nutrient supply in all of these systems, and module culture, bag culture and NFT provide an interesting set of contrasting problems which have to be understood to solve the practical difficulties of using these techniques.
Modules typically contain 8-10 litres of fertilised peat ,bark or sawdust for each plant, with typical water holding capacities exceeding 20%. From 1 to 8 waterings are required each day to meet transpiration demand of the crop with no leaching. Watering can be controlled by solarimeters but manual methods using estimates based on weather conditions and simple evaporimeters can also be used (White 1989). There is very little media surface exposed to the air and practically all water loss is by transpiration. The shape of the module and diurnal redistribution of water (White et al,1980) ensures that of all the media in the module is kept at reasonable moisture without any dry spots (provided that the media is not allowed to dry to the extent that it becomes water repellent). Over watering and drainage from the modules is avoided, as otherwise base fertilisers would be lost by leaching. Only simple liquid feeds are needed, typically solutions of potassium nitrate and urea supplemented for various periods with magnesium sulphate, mono-ammonium or mono-potassium phosphate and occasionally with a boron salt. All of the ions supplied in the liquid feeds can be absorbed by the crop and salt accumulation is avoided by regularly checking the electrical conductivity of media extracts and using more or less liquid feed to provide the desired salinity in the media. The presence of lime added in the base fertiliser provides buffering to keep the acidity typically in the range between pH5 and pH6. Fritted trace elements provided in the base fertiliser provide slow release of micro nutrients and avoid risks of leaching micro nutrient reserves.
Current bag culture typically uses 10 litres of unfertilised media. Fine grained pumice or sawdust are the most popular media and have about 35 to 40% total water holding capacity and 20-30% air filled pore space when measured by the Australian standard method for soilless media. Up to 20 waterings per day, using trickle irrigation, are typically applied to pumice or sawdust bag cultures in summer. Complete nutrient solutions have to be used and are usually obtained by dosing A (calcium nitrate plus potassium nitrate) and B (monopotassium phosphate, potassium sulphate or nitrate, magnesium sulphate, or nitrate, iron chelate and trace element salts) stock solutions and acid or alkali into the water supply at controlled conductivity. Trickle irrigation does not wet the pumice evenly, but results in a cone shaped wetted volume with dry areas near the top of the bags. High water application rates (at about 6 litres/hour per bag) minimise the formation of dry areas or alternatively a layer of very fine sand (dune sand) will provide water spread over the top of sawdust and more even wetting. This problem is greatest in sawdust and pumice but less in bark or peat bags which offer greater capillary movement of water. The open top of the bag results in surface evaporation and water loss with visible salt accumulation on the surface of pumice bags and in the dry areas. Liquid feeds are applied at conductivity between 1,800 (summer) and 6,000 microS/cm (during dark winter weather) and with an excess of water so that between 10 and 30% of the applied volume drains through the bottom of the bags at each watering. Most growers regularly monitor the EC of the drainage water and use changes in the drainage water EC as an indicator of salinity build up or leaching of the media. The EC of the drain water is usually more than the EC of the feed, but bag to bag variability in drain water EC is very high. Occasional analyses of the drain water have shown that the drain water is often richer in some nutrients than the feed and poorer in other nutrients, so that the salinity control problem arises to some extent because there is a lack of close balance between supply and uptake of individual nutrients.
Module culture, bag culture and NFT pose entirely different problems of nutritional management for growers. The original module culture represents a system which is lies between soil culture and hydroponics in its nutritional aspects. The media used contain solid fertilisers, especially lime and superphosphate, which provides nutrient reserve and equilibrium between nutrients in solution within the media and those being absorbed by the plants. This buffering effect may be particularly important in control of pH in the media. The organic media all have exchange capacity providing further nutrient reserves. Nutritional management by growers is relatively simple but effective. Frequent sampling of the media and measurement of conductivity on 1:1.5 water extracts gives an indication of whether a sufficiency or excess of nutrients is being supplied by the liquid feeding programme but does not provide any indication of adequacy of individual nutrients. Laboratory analysis or analysis using simple test kits for pH, EC, nitrate nitrogen, ammonium nitrogen, phosphorus, potassium, calcium, magnesium and sodium is needed on an occasional basis to ensure that there is no unbalance of nutrients and well tested guidelines for acceptable nutrient concentrations are available (Prasad et al, 1986).
NFT, rockwool and perlite bag systems contain all their nutrients in solution with no solid nutrient reserves or any appreciable exchangeable nutrient capacity. Rockwool and perlite systems contain a small pool of reserve nutrient solution in the bottom of the bag. The nutrient supply rate is determined by measuring the EC of either the recirculating solution for NFT systems or the retained nutrient solution for rockwool or perlite systems. Full analysis of these solutions supplies the required information on the balance of individual nutrients with any shortage or excess of individual nutrients indicating an unbalance between supply and plant uptake. Recommended values for nutrient concentrations in the recirculating or retained solutions are widely available (Smith 1987, Sonnevelde et al, 1987).
Planter bag culture systems and current module culture systems provide a number of difficulties for on site nutritional control. The nutrient solution flows through the bag with an excess being lost by drainage. The media have no solid fertiliser reserves and very little buffering capacity in the case of pumice (about 20 meq/litre). Measurement of EC of the drainage solution only provides very limited information on the adequacy of nutrient supply as the solution is always concentrated to greater or less extent by extraction of water by the crop. When a considerable excess of water is applied to provide a low EC in the drainage water, there is very little change in nutrient content to be detected by analysis of the drain water, and conversely if a low drainage rate is used the high nutrient concentrations tend to mask unbalances.
It is difficult to obtain samples of the solution retained in the media between waterings, but it is possible to sample the media and obtain a 1:1.5 water extract for analysis. Published guidelines for results of 1:1.5 extract analyses for fertilised peat and bark modules (Prasad et al, 1986) may not be correct for bag cultures in peat or bark and seem unlikely to be correct for pumice. Conversion of results by multiplication using the factor of extract volume over known water holding capacity yields values much higher than those recommended for rockwool retained solution analyses. The bag sampling technique and analysis of the 1:1.5 water extract may well provide a more reliable and simple management method than the more usual drain water analysis. Comparison of nutrient concentrations in the applied liquid feed and in the water extract (feed composition calculated at the same EC as the water extract) may indicate which nutrients are being supplied at greater or lesser rates than they are being taken up by the crop. The quality of the nutrient solution, in terms of the balance of nutrients relative to plant uptake, is a key factor in nutritional management with all of these soilless systems. Internal buffering in fertilised peat ,or bark modules allows more tolerance in nutrient solution quality than do other systems. Careful interpretation of analytical results allows a near perfect balance between supply and uptake for NFT systems, but there is no obvious method of reaching this balance with once through nutrient solution for planter bag systems. The need for excessive supply and regular excess drainage is a consequence of this lack of balance. Control of pH in planter bag media can be difficult at times when N supply needs to be restricted.
An ability to provide more balanced solutions for planter bag growing systems might improve crop nutrition and yield, reduce costs and discharge to the environment. A very few growers are now recycling nutrient solutions through bag and module systems and these systems may offer greater opportunities for control as well as savings in water and fertiliser use.
Non uniform drying out of the media within planter bags and local accumulation of salts can provide additional problems. These accumulated salts may form a nutrient reserve with some sort of equilibrium with the solution, but the situation is likely to be complicated by differential accumulation related to ionic mobility and other factors.

NUTRIENT DISCHARGE TO THE ENVIRONMENT
Nutrient discharge to the environment has not been of any concern of growers in NZ in the past, but greater awareness of environmental pollution is now being considered by growers and is causing considerable concern to local bodies responsible for maintaining quality environments. The original module growing systems provide little discharge, there is very little nutrient drainage and the media itself can be recycled by other users. Nutrient discharges by bag growing systems seem to be greater than those indicated by Smith for rockwool systems. Most growers water with 10 to 30% drain through volumes so that a 50 week crop might result in 200 litres of drain water discharge per year per m² of greenhouse area and if the drainwater analyses above are indicative of average composition this could result in discharges of about 85g N, 100g K, 56g S and 66g Ca per m²/year or between about 25% and 50% of the nutrients applied in the liquid feeds.
Nutrient discharge from NFT systems is the result of dumping the solution when its composition becomes unmanageable, usually because of sodium accumulation. Nutrient discharge is thus directly related to water quality, with good quality water (30 ppm sodium or less) discharge is very low, less than 5% of applied nutrients, but this could rise to about 12.5% with water containing 50 ppm sodium.

CHANGE IN RELATION TO RESEARCH AND TECHNICAL SUPPORT
The greenhouse industry in NZ is small and not an important export earner. Publicly funded research support is currently minimal for the greenhouse industry, which is made up of competing individual businesses unlikely to co-operate sufficiently to fund significant research. Strong public research programmes were in place during the 1960-1980 period, and were an important factor in the limited change to soilless systems that occurred at that time. There was virtually no government research in NZ on NFT, but this technique has been readily implemented by growers using information from overseas. The modern bag culture systems which have developed locally appear to have no exact overseas counterpart, no research support and only limited technical support from government and private consultants. Development has been the result of grower ingenuity in adapting prior experience with unfertilised sawdust bag growing and some application of nutrient solution recipes from rockwool and other soilless culture methods.

CONCLUSIONS
NFT and pumice or sawdust bag or module culture have become the predominant methods of soilless culture for tomatoes and all greenhouse vegetable crops in NZ. Economic forces will continue to increase the importance of soilless cultures relative to growing in the soil over the next few years. New techniques used for greenhouse vegetables have generally been extended fairly rapid to greenhouse cut flowers and the bag culture methods are already being used to a limited extent for flowers. NFT has developed with straightforward application of overseas research to the NZ situation, but pumice bag culture has developed with limited input from overseas. Pumice and sawdust culture provides a robust growing system possibly with considerable tolerance to poorly defined nutritional requirements associated with its use. The ease of application of the method and the low cost of the pumice and sawdust are likely to lead to much greater use in the future.

REFERENCES
Dennis,D.J. (1979) Cucumber Production in peat filled bolsters. Seminar papers: Development and use of soil-less media for horticulture, Horticultural Research Centre, Levin:25-25.7
Gallagher,P.A., Maher, M.J., & Mahon,M.J. (1977) Programme for early tomato production in peat. An Foras Taluntais, Dublin.
Jamieson,A. (1979) The move into sawdust culture in Oamaru. Seminar papers:Development and use of soil-less media for horticulture, Horticultural Research Centre, Levin:24-24.2
Longley, B. (1983) Growing greenhouse tomatoes in sawdust. Ministry of Agriculture & Fisheries, Auckland, NZ:19pps.
Maas,E.F. & Adamson,R.G. (1971) Soilless culture of commercial greenhouse tomatoes. Canada Dept. of Agriculture, Ottawa, Publication no. 1460
Mavromatis,G. Sawdust culture of glasshouse tomatoes in Canterbury Seminar papers:Development and use of soil-less media for horticulture, Horticultural Research Centre, Levin:23-23.4
Moorat,A.E.(1975) Growing tomatoes in peat substrates Acta Horticulturae 51:117-121
Prasad,M., Brice,I.S., Thomas,M. & Wood, R.J. (1986) Protected Crops Fertilizer recommendations for horticultural crops. Ministry of Agriculture & Fisheries, Wellington:p54
Smith,D. (1987) Rockwool in Horticulture Grower Books, London
Sonnevelde,C & de Kreij, C. (1987) Nutrient solutions for vegetables and flowers grown in water or substrates Proefstation voor Tuinbouw onder Glas, Naaldwijk. Series:Voedingsoplossingen Glastuinbouw No. 8
White,R.A.J. & Brundell,D.J. (1978) Experiments on formulation of peat modules using native New Zealand peat Acta Horticulturae 82:179-190
White, R.A.J. & Prasad,M. (1980) Nutrient, salt and pH distribution in soil and in peat modules used for tomato growing Acta Horticulturae 99:167-178
White,R.A.J. (1983) Growing in Bark Modules Granulated Bark Supplies Ltd., Auckland, NZ. 16pps
White,R.A.J. (1989) Production systems. In Proc. Greenhouse vegetable crops short course. Massey Univ. Dept. Horticultural Science;103-105
White, R.A.J. (1997) Managing greenhouse tomato nutrition in hydroponic growing systems. In Nutritional requirements of horticultural crops. (Eds L D Currie & P Loganathan. Occasional report No.10 Fertilizer and Lime Research Centre,Massey Univ.Palmerston North,NZ.

October 5, 1999 This report has been updated for this website from a paper originally published in Proc. 8th International Congress on Soilless Culture, Hunter's Rest, South Africa, Oct 1992 :471-481.

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