HYDROPONICS INTERNATIONAL: IRRIGATION SYSTEM AND CULTURAL PRACTICES FOR CROP PRODUCTION UNDER CONTROLLED ENVIRONMENT PRODUCTION SYSTEMS

By Dr. Mike Nichols

Institute of Natural Resources

Massey University, New Zealand

IT WAS JUST OVER 50 years ago that I entered the University of Nottingham to read for a Bachelor of Science degree in horticulture. Prior to such studies, 12 months of pre-entry practical horticultural experience were required.

I worked at a market garden near my home in Oxford, on which in addition to growing tree fruit, berry fruits, and field vegetables there was a heated greenhouse operation producing tomatoes and chrysanthemums.

By the standard of the day it was a relatively advanced greenhouse operation. Remember, this was 1954. The young tomato seedlings received supplementary photosynthetic light (from mercury vapour lamps) during the winter months, and the greenhouse soil was sterilized by steam annually and heated by low voltage electric cables. It must be noted, however, that all watering was done by hand by hose (drip irrigation had only just been developed by Blaas), and all the plants were grown in soil and fed with "dry" fertilizer. The only exception was at the propagation stage, when the young seedlings were grown in a "John Innes Potting Mix" (still soil-based), which was the first attempt to standardize media in the UK. In the USA the University of California Soilless Mixes were just being developed, but had yet to become routine.

We have come a long way since those days, and I am sure that we have got much smarter in our greenhouse cropping.

When I visited the Cameron Highlands in 1995 my main memory was predominantly of soil-grown plants. I have similar recollections from my visit to Taiwan prior to the symposium on Tropical Greenhouses, although to be fair, we also were shown some very sophisticated soilless culture systems, particularly for orchids. Hydroponics in general had not yet become well established. In most developed countries hydroponics is the norm, and soil-grown crops are the exception.

In fact, for a number of reasons soil is not a good medium in which to grow greenhouse crops. It provides excellent buffering for both water and for nutrients, but in the final analysis it is a difficult (impossible) medium in which to achieve the correct balance of water (and therefore nutrient) availability and aeration for optimum growth.

THE PLANT ENVIRONMENT

The plant environment comprises above-ground and below-ground components (Table 1). In modern greenhouses it is possible to control many of the above-ground factors, such as temperature, humidity, and atmospheric composition; many of the below-ground factors can also be controlled, but it is extremely difficult to modify temperature and atmospheric composition.

The importance of controlling these factors can be shown by the huge increases in yield obtained in the UK (Ho, 2004) when using a controlled environment greenhouse compared to a non-controlled house (Fig. 1).

Clearly, improving the environmental control in a greenhouse has a huge effect on yield. Much of this improvement is due to improving the above-ground environment, with improvements in greenhouse design, temperature and humidity control, and the use of supplementary carbon dioxide. Modification of the below-ground environment (primarily through hydroponics) has also had a part to play, particularly in relation to water supply, nutrition, aeration, and the control of pathogens.

The nutrient delivery system for greenhouses came of age some 50+ years ago, when an Israeli scientist (Dr. Blass) developed the first low-pressure drip-irrigation system. This was very expensive (about U.S. $0.10 /plant in 1950) because it relied on rubber tubing and brass nozzles. But it was well ahead of its time, incorporating a liquid feeding system that allowed the grower to modify not only the ratio of nitrogen to potassium, but also the osmotic level of the feed.

It was not, however, until some 10 years later with the development of cheap alkathene "whiskers" that drip irrigation really took off. This provided the takeoff point for many hydroponic systems.

Table 1: The Plant Environment.

MEDIA

There are essentially two media in which hydroponics is undertaken: solid and liquid.

Solid Media

These media have been the main type since hydroponics was initiated. Originally sand or gravel, they have over the years been replaced with lighter materials in which the aeration and water-holding characteristics can be more precisely controlled. The main types of solid media used are

. organic products such as peat, sawdust, cocopeat, bark, etc.;

. mineral products such as volcanic

tuff, perlite, pumice, vermiculite, etc.;

. manufactured products such as rockwool, foam plastic, etc.

The standard system would undoubtedly be rockwool, which is now a huge industry, particularly in northeastern Europe. However, its value away from Europe is reduced because of freight costs and the need to have a reasonably-sized industry to warrant establishing a local production factory. In New Zealand some growers import rockwool and others use a local product such as pumice or sawdust.

The key with all solid media is to have the correct amount of air space and water-holding capacity (EAW), and this is not simply a consideration of particle size, but also the distribution of the different particle sizes and the type of medium (Table 2, Verdonck and Demeyer, 2004).

Table 2: Physical Properties of Composted Bark Mixtures (Verdonck and Demeyer, 2004).

Liquid Media

Essentially, there are three systems for using liquid media: deep flow, nutrient film, and aeroponics.

Deep Flow System

The deep flow system appears only to work well for leafy vegetables. The plants are commonly grown on floating polystyrene boards (Massantini, 1976) in a large tank of nutrient solution, which is slowly recirculated. Aeration of the nutrient solution is the main problem with this system, as it is physically impossible to load up the solution with more oxygen than the solution will hold at that specific temperature.

Nutrient Film Technique

In the mid-1960s, Cooper (1979) developed the nutrient film technique (NFT), whereby a thin film of nutrient solution flowed down a gulley in which the plants were grown. It was a unique system because it attempted to overcome one of the major problems of using a liquid medium (that of adequate aeration) by having a very shallow film. Nevertheless, it was not very suitable for cucurbites, which demand well-aerated roots, and has never become as popular as rockwool because the system had nothing to sell! It introduced the concept of the recirculating system in a time when all watering was to "waste." In the light of present knowledge the concept was perhaps too simplistic in using conductivity and pH to control nutrient levels in the solution.

Aeroponics

Aeroponics involves suspending the plant roots in a light-proof box and spraying them with a nutrient solution. It is one of the few hydroponic systems that permit some control of the root temperature and also of the root gas environment. In recent years it has been used in the tropics to produce lettuce and in North Korea to produce high-health seed potatoes. It provides not only a production system, but also a unique research tool (Christie and Nichols, 2004)

WATER SOURCES

It is desirable that the water source for both recirculating and non-recirculating systems is free from pathogens and contains no toxic chemicals. Although lakes, rivers, dams, tube wells, and streams are a logical source, they may not necessarily be ideal (Schwarz et al., 2004), and rainwater from the roof of the greenhouse is now considered the most acceptable source. If other sources of water are used, then it may be necessary to undertake some form of sterilization (e.g., using ozone or hydrogen peroxide) to remove pathogens, and reverse flow osmosis to remove excessive "salts."

Non-recirculating Systems

By far the simplest way of producing plants hydroponically is to use a non-recirculating system. This overcomes many of the nutritional and root pathogen difficulties. It is relatively easy to ensure that the nutrient solution being supplied to the plants is free from pathogens and also contains the required nutrients at the required levels and ratios for the crop at it's particular stage of development.

It is also relatively easy to ensure that excess nutrients are leached from the medium by applying 10-20 per cent more water (as nutrient solution) than the plants transpire. The difficult part is disposing of this leachate in a sustainable manner, without causing pollution problems. In fact, where greenhouses are the exception, it is probably acceptable to use the leachate as a liquid fertilizer on extensive agriculture, but in many parts of the world this is not an option, and it has become a legal requirement that only recirculating systems be established.

Recirculating Systems

As soon as we move towards a fully recirculating system, we introduce potential problems of nutrient management and pathogen control.

Nutrient Control

Cooper (1979) proposed that the objectives must simply be to keep the pH between 5.8 and 6.3 and to ensure that the nutrient solution was at the correct conductivity, and the crop would decide which nutrients to absorb. This oversimplification has now been superseded, and the objectives now are to monitor the major elements in the nutrient solution and adjust the levels if the concentration (of a specific ion) moves outside the acceptable range. The acceptable ranges are crop specific (Bugbee, 2004).

Pathogen Control

With the move towards recirculating systems, there are potential problems with pest and disease management. These can be approached in a number of different ways:

. Ensure that only "sterile" nutrient solution is recirculated.

. Use "biological control" via a deep sand filter.

. Use "grafted" plants.

Sterile System

At first sight, the idea of establishing a completely sterile system is very attractive. The water used to top up the system can be made free from pathogens by means of ozone or hydrogen peroxide prior to being added to the nutrient solution. Furthermore, the recirculating solution can be treated in a number of ways to eliminate any pathogens from the initial water supply, including

. ozone treatment,

. hydrogen peroxide treatment,

. use of chlorine compounds.

These are not so useful for sterilizing recirculating nutrient solution because they are potentially phytotoxic to plant roots. In order to free the recirculating nutrient solution of pathogens the normal techniques are to use either UV light (Runia and Boonstra, 2004) or heat pasteurization.

Using UV light has two potential problems:

. If the nutrient solution is turbid, there may be problems

with the UV actually sterilizing the solution.

. UV may precipitate iron from the solution.

It is possible to add a low concentration of hydrogen peroxide to the solution prior to passing it though the UV source to remove some of the turbidity. There is, of course, a high capital cost and running cost of the UV treatment. Although turbidity is not a problem with pasteurization, this technique is also both expensive and may precipitate iron as well.

Heat pasteurization is also not cheap in terms of both energy and capital, and it has similar problems with iron precipitation.

Biological Control Using a Deep-Sand Filter

The principle of the deep-sand filter is that it is impossible to ensure complete sterility in hydroponic systems, and it may, therefore, be preferable to go for some form of ecological balance. This ensures that the nutrient solution not only contains pathogens, but also that it contains a broad pectrum of biological control organisms (Postma, 2004). In order to achieve this, the nutritional solution is put onto a deep-sand filter that is charged with a wide range of biological control organisms such as Trichoderma sp , non-pathogenic Fusarium oxysporum , Gliocladium virens , and Coniothyrium minitans .

This technology now appears to be gathering favour in many intensive greenhouse areas in northeastern Europe.

Grafted Plants

Although the benefits of grafting vegetable seedlings onto resistant rootstocks were first published in the 1920s, it was not until the 1960s that it gained any impetus (Lee, 2003). The technique was time-consuming, however, and it was not until recently, with the development of grafting machines and the increasing interest in producing crops without using pesticides, that grafting got a new lease on life.

Like all "new" technology there is a learning phase, and I well recall my first effort to graft greenhouse cucumbers onto Cucumis ficifolia rootstock. We used the "Princess" variety (the first of the gynoeacious only types) and the stress of the graft switched the plant into male flower production!

Essentially, grafting requires the production of two lots of seedlings, the provision of a high-humidity greenhouse area, and a skilled labour force. It must also be remembered that the scion takes on some of the characteristics of the rootstock in terms of vigour (vegetative or generative) and comes into production a little later. But if the appropriate rootstock has been selected, it will be resistant to a range of soil-borne pests, diseases, and possibly some viruses.

ORGANIC HYDROPONICS

Organically produced greenhouse crops fill a valuable market niche, but as many organic certification methods require the crop to be grown in the soil, yield is immediately reduced compared with conventional hydroponically grown crops. Although organic organizations are very happy with the IPM technology used in greenhouses, they appear to be unable to accept hydroponics, even if the nutrient solution is made up from organic material such as fish meal and sea weed (Atkins and Nichols, 2004).

A compromise solution currently being used in New Zealand is to grow the plants in pots using a peat/pumice medium (certainly no one could say that this was not soil-based), and then water and feed the plants with an organically-derived nutrient solution using an ebb and flow system.

AQUAPONICS

A potential approach to getting more "bangs per buck" is to combine fish farming with hydroponic crop production. The fish live in the nutrient tank and are supplied with food. The waste products then provide the nutrients then circulated over the roots of the crop. This system has been used successfully in the Virgin Islands (Rakocy et al., 2004) to produce 87.4 t/ha of tilapia fish and 5.0 t/ha of basil per year.

THE FUTURE

We have probably gone almost as far as we are able in modifying the aerial environment without considering going into growing rooms (Nichols, 2004), and attention should perhaps now be directed at the below-ground environment. In this respect, aeroponics probably offers the most potential, because it provides an opportunity not only to modify the root temperature (He &Lee, 1998), but also the prospect of improving growth by modifying both the oxygen and carbon dioxide levels in the root zone (Nichols et al., 2002).

References

Atkins, K. and Nichols, M. A. (2004) "Organic Hydroponics." Acta Hort 648: 121-127.

Bugbee, B. (2004) "Nutrient Management in Recirculating Hydroponic Culture." Acta Hort 648: 199-112.

Christie, C. B. and Nichols, M. A. (2004) "Aeroponics-A Production System and Research Tool." Acta Hort 648: 185-190.

Cooper, A. J. (1979) The ABC of NFT. Grower Books. 181 pp.

He, J. and Lee, S. K. (1998) "Growth and Photosynthetic Responses of Three Aeroponically Grown Lettuce Cultivars (Lactuca sativa L.) to Different Root Zone Temperatures and Growth Irradiances under Tropical Aerial Conditions. Journal of Horticultural Science and Biotechnology 73 (2): 173-180.

Ho, L.C. (2004) "The Contribution of Plant Physiology in Glasshouse Tomato Soilless Culture." Acta Hort 648: 19-26.

Lee, J. M. (2003) "Advances in Vegetable Grafting." Chronica Horticulturae 43 (2): 13-19.

Massantini, F. (1976) "Floating Hydroponics: A Method of Soilless Culture." Proceedings 4th International Congress on Soilless Culture, Las Palmas, Canary Island, Spain: 91-98.

Nichols, M. A. and Christie, C. B. (2002) "Towards a Sustainable "Greenhouse" Vegetable Factory." Acta Hort 578: 153-156.

Nichols, M. A., Woolley, D. J. and Christie, C. B. (2002) "Effect of Oxygen and Carbon Dioxide Concentration in the Root Zone on the Growth of Vegetables." Acta Hort 578: 119-122.

Postma, J. (2004) "Suppressiveness of Root Pathogens in Closed Cultivation Systems." Acta Hort 644: 503-510.

Rakocy, J. E., Shultz, R. C., Bailey, D. S. and Thoman, E. S. (2004) "Aquaponic Production of Tilapia and Basil: Comparing a Batch and Staggered Cropping System." Acta Hort 648: 63-70.

Runia, W. T. and Boonstra, S. (2004) "UV-Oxidation Technology for Disinfection of Recirculating Water in Protected Cultivation." Acta Hort 644: 549-553.

Schwarz, D., Grosch, R. and Gross, W. (2004) "Water Quality for Hydroponics: Nutrients, Bacteria and Algae in Rainwater Ponds." Acta Hort 644: 533-540.

Verdonck, O. and Demeyer, P. (2004) "The Influence of Particle Sizes on the Physical Properties of Growing Media." Acta Hort 644: 99-102.