Contents
Introduction
System Design
Plant Propagation
Water Quality
Stock Solutions
Pre-plant Routine
Post-plant routine
Quality Control
Media Sampling
Root Disease
Nutrient Levels
Nutrient Uptake
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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.
©:R A J White 2 July 2007
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