Organic Hydroponics

Abstract

At first sight “organic hydroponics” appears to be an oxymoron.

There is considerable interest by consumers world-wide to purchase organic vegetables, but a considerable antagonism by organic producer councils to accept that it is possible to use hydroponic systems and still conform with the organic philosophies.

In fact by using nutrient solutions derived from acceptable organic sources, and using biological pathogen control measures, and recirculating hydroponic systems, a more sustainable system of crop production is possible than when using “classical” soil based organic systems.

Studies at Massey University have shown that it is possible to produce lettuce using NFT with an organically derived nutrient solution, although growth is slower than when using a conventional inorganic hydroponic solution.

The results of our work will be presented, along with a critique of the current New Zealand and Australian organic standards, which permit aquaculture, but disallow hydroponic produce from being labelled organic!

Introduction

Soil is not a good medium in which to grow plants. Most controlled environment agriculture (greenhouse) crop producers have been well aware of this for many years, and this has lead to the development of a range of different hydroponic and soilless culture systems of crop production.

In recent years there has been an increased consumer interest in the production of crops using “organic” methods. For example in New Zealand the Organic Products Exporters of New Zealand (OPENZ) survey showed that New Zealand certified organic exports in 1999-2000, increase by 100% compared to 1998-99 to over $US 30 million, and in the next 4-years are expected to exceed $US 200 million.

There is an increasing interest by consumers for organically produced greenhouse vegetables, but modern greenhouse vegetable production has moved away from using soil to using media such as peat, pumice or rockwool, or even to using medium free systems such as the nutrient film technique (NFT) or aeroponics.

The concept of “organic hydroponics” is to some extent an oxymoron, as theoretically it should not be acceptable to produce organic produce without soil, as a living dynamic soil is considered to be the corner stone for organic production. However two of the major objections by organic supporters to conventional horticulture are the use of artificial pesticides and the use of artificial fertilizer.

Both of these objections can be overcome, by using biological control for pathogen control, and by deriving the nutrient solution from organic animal and plant material.

This thus poses the question of why do organic producers emphasise the importance of soil in the system??

In fact plants do not “grow” in the soil. The roots are found in the pore space between the soil particles. So what magic property does the soil have over an artificial medium?

I suspect that the answer is that the soil is filled with a mass of living organisms, and that these micro-organisms provide a very valuable balance with nature. In this respect I can see little if any difference between the modern practise in hydroponics (where the solutions are re-circulated through a deep sand filter) and growing in the soil.

Hydroponics is NOT a sterile system. In fact it is impossible to keep the system sterile, so the modern approach is to try to control soil borne pathogens by biological means. As the plant roots do not grow in the soil, but in the pore spaces between the soil particles, it is really not important whether the soil particles exist or not except to support the plant.

Public perception

In a survey carried out in New Zealand recently on behalf of NZ Vegetable Growers Federation by Colmar Brunton it was found that in a survey of people who purchase organic vegetables just under 40% purchased them because they had no (or less) sprays applied. Nobody referred to the fact that they were grown without the use of artificial fertilizer. There is a huge consumer misconception regarding spraying. I must add that NZ VegFed are not antagonistic to organics, in fact they see it as a potentially valuable niche market.

In fact the whole organic movement is founded on the premise that:

Healthy soil—healthy plants—healthy people.

Unfortunately none of the links has ever been “scientifically proven,” so the concept that because plants are grown in a healthy soil does not mean that they are axiomatically resistant to pest and diseases, and (to date) nobody has been able to demonstrate any major difference between organic and conventionally produced crops from a human health viewpoint.

Certainly in the realm of pesticide applications the organic people have not been perfect. Copper sprays until recently were acceptable in organic culture, and in recent time have been implicated as a potentially dangerous heavy metal, while rotenone (derris) is now regarded as a risky chemical. Even BT is under fire in Auckland, where it is being used to control the painted apple moth.

One of the major claims of organic growers is the reduce leaching of nutrients through the soil. Recirculating hydroponic systems (with no leaching) are clearly much more efficient from this point of view!!!

The Soil

One of my pleasures as a University teacher has been to observe students shock when I tell them that “soil is not an ideal medium in which to grow plants.” Many have always considered soil as the only possible medium, and it takes some discussion to explain that although soil has some admirable properties, not the least being it’s ready availability and its ability to provide excellent water and nutrient buffering, it is not ideal, because it is impossible to provide optimum levels of water and aeration at the same time. It is mainly for this reason that high value (mainly greenhouse) crops tend to be grown hydroponically in “artificial media”.

The emphasis by organic producers that the soil is an essential component for organic production needs to be questioned. Plant roots grow in the spaces between the soil particles, which are inert. By using a recirculating hydroponic system soil micro-oganisms will become established in the media, or where no media is being used (NFT, aeroponics) the solution can be cycled through a slow sand filter, which will essentially have the same effect. It is difficult to see how this differs essentially from a “soil grown” production system.

Pest and disease control

There is unlikely to be any huge difference in the control of pests and diseases in a conventional or in an organic greenhouse production system. Few greenhouse growers use pesticides these days, as they rely very much on the control of the plant environment to reduce fungal diseases to a minimum (by reducing humidity), and various biological agents, such as Encarsia formosa to control pathogens such as white fly. Conventional growers dislike using pesticides just as much as organic producers. In fact as most conventional growers use bumble bees for pollination of tomatoes, the use of most (if not all) pesticides is extremely counterproductive.

Grafting onto resistant root stocks is always an option for greenhouse tomato or cucumber growers IF soil borne pathogens are a problem.

Nutrient and water delivery

Hydroponics involves a range of different systems to deliver water and nutrients to the roots. From media based systems using peat, sawdust, bark, pumice, perlite, or rockwool, to systems without any media such as the Nutrient film Technique (NFT) , deep flow or aeroponics.

One of the key factors of most (if not all of these systems) is that it is possible to use a recirculating system, in which the “drainage” solution is collected, and further nutrients and water added, and then returned to the plants roots. Such a system is extremely nutrient efficient, and is therefore currently the most sustainable and environmentally friendly use of nutrients that exists. In the field there are very real problems in some countries from the excessive use of organic matter in the soil, leading to a build up of nutrients in subterranean water. Pig manure has been a major environmental problem in the Netherlands for example, and even within New Zealand a lot of the water pollution can be laid at the unsatisfactory disposal of dairy farm waste—very organic, but still a major problem!!

Nutrients

The major challenge facing the “organic hydroponic” producer is to develop a satisfactory nutrient solution. Two readily available organic products—fish meal and seaweed contain large quantities of sodium and chloride, which is not a problem in the field, as they are easily leached out by rain, but in a hydroponic system this can not be easily done. Vermicaste derived solutions have very low levels of nutrition, and probably the major source of organically derive solutions will come from animal manure (probably dairy waste). Unfortunately plant roots tend to absorb ions and not molecules, so it is essential that the nutrient solution is completely broken down (oxidized) and that will involve oxidation tanks and then holding tanks for the concentrated solution. Finally, of course, it will be necessary to monitor the nutrient solution to ensure that the nutrient levels remain in balance, and are at the optimum for good plant growth.

New Zealand Standards

Currently the Standards Association of New Zealand, and the equivalent body in Australia are developing a set of Organic Standards. In fact, the proposed New Zealand standards are very similar to the Australian ones.

Both of these standards prohibit the use of hydroponics, without providing any explanation, but permit aquaculture. I am well aware of the aquaponics research conducted at Curtin University in Perth by Marios Vasiliou, and I find it very difficult to see how such a system can not comply with any organic standards.

I must add that the committee which established the New Zealand organic standards did not include any representation from mainstream New Zealand horticulture (such as VegFed) and comprised a majority with strong “organic” affiliations.