Soil normally comprises solid rock particles (e.g. sand/clay/silt) plus air, organic matter, water, micro-organisms, and nutrients. The proportion varies considerably with the soil type, from mineral soil (with low organic matter) to peat soils with little or no mineral content.
Soil is not an easy medium to provide the crop with the ideal combination of moisture, aeration and nutrients. When the moisture content is ideal, the aeration tends to be inadequate, and when the aeration is ideal then moisture tends to be the limiting factor. Growing crops in the soil poses major problems, not only in terms of moisture and aeration, but also in terms of nutrition. For example, in order to grow producers need to apply considerable quantities of nitrogen, phosphorous and potassium if they are to be productive. These quantities far exceed (are actually double) the maximum levels of nutrients that can be applied according to the EU directive of 170kg/ha/year of N and 200kg/ha/year of P. Much of these chemicals will eventually (inevitably) find their way into the ground water in a soil-based system and considerable quantities of nutrients are lost by leaching. If one uses the quantities of nutrients approved by the EU, there is a marked loss of yield.
Pest and disease control in soil can be another problem with crop production, where crop rotation choice is severely limited. Most soil-borne diseases can be controlled by grafting onto disease resistant rootstocks, but, to date, the satisfactory control of nematodes has eluded us, and the only satisfactory control measure, which is used is to steam sterilize the soil. This action is (surprisingly) accepted for organic production, even though it appears to go completely against the tenets of organic production — healthy soil — as virtually everything is killed, and the vacuum is filled by the first micro-organism to re- infect the soil. It’s not a very satisfactory solution.
Organic greenhouse horticulture is defined by the International Society for Horticultural Science (ISHS) as the production of organic horticultural crops (vegetables, ornamentals and fruits) using inputs derived only from natural, non-chemical sources, in climate-controllable greenhouses and tunnels.
The International Federation of Organic Agricultural Movements (IFOAM) not only require the production system to be soil based, but also make it very clear that ‘hydroponics’ is not permitted, but there is no clear definition of hydroponics given by IFOAM.
There is a clear discontinuity between these two definitions, as the ISHS definition makes no mention of soil, which is considered by IFOAM to be an essential component of an organic system, but emphasises the natural (non-chemical) source of the inputs.
Worldwide, the majority of greenhouse crops are grown in hydroponic systems. In developed countries there was a move away from soil-grown greenhouse crops in the early 1960s, which, combined with improvements in the control of the greenhouse environment, has resulted in considerable increases in productivity compared with soil-based systems. In fact, 60 years ago the best greenhouse tomato growers were only achieving 20kg/sqm/year, and today the best growers harvest 80kg/sqm/year.
Achieving such productivity in soil is virtually impossible, and thus the premium required for organic greenhouse tomatoes will need to steadily increase if organic producers are to remain in business.
Controlled Environment Agriculture is capital and labour intensive, and to be profitable it is essential that productivity is as high as possible. Improving the root environment is an essential part of providing the plant with an optimum environment, and soil is not an easy medium to provide the crop with the ideal combination of moisture, aeration and nutrients. When the moisture content is ideal, the aeration tends to be inadequate, and when the aeration is ideal then moisture tends to be the limiting factor. For this reason most greenhouse crop producers in developed countries have tended towards using media (such as rockwool, coir, and peat) that have more ideal (and forgiving) physical characteristics than soil, combined with irrigation and feeding by hydroponic systems.
Growing greenhouse crops in the soil poses major problems, not only in terms of moisture and aeration, but also in terms of nutrition. For example, in order to grow greenhouse fruit vegetables (tomato, cucumber and pepper), producers need to apply considerable quantities of nitrogen, phosphorous and potassium if they are to be productive. These quantities far exceed (are actually double) the maximum levels of nutrients that can be applied according to the EU directive of 170kg/ha/year of N and 200kg/ha/year of P. Much of these chemicals will eventually (inevitably) find their way into the ground water in a soil-based system. If one uses the quantities of nutrients approved by the EU, there is a marked loss of yield.
This must have serious implications for the long-term viability of the soil-based organic greenhouse crops industry. The simple choice will be either to accept a significant reduction in productivity, with a need to markedly increase the premium on organic greenhouse grown crops or, alternatively, pollute the ground water with excessive nutrients.
Pest and disease control in soil can be another problem with organic greenhouse crop production, where crop rotation choice is severely limited. Most soil-borne diseases can be controlled by grafting onto disease resistant rootstocks, but, to date, the satisfactory control of nematodes has eluded us, and the only satisfactory control measure, which is used is to steam sterilise the soil. This action is (surprisingly) accepted for organic production, even though it appears to go completely against the tenets of organic production — healthy soil — as virtually everything is killed, and the vacuum is filled by the first micro-organism to re- infect the soil. It’s not a very satisfactory solution.
There is another possible alternative to the nutritional problem, provided that IFOAM regulations would permit the use of re-circulating hydroponic systems. It all depends on how one defines hydroponic. If hydroponic is defined as the provision of inorganically derived nutrients to the roots in a solution, then there is a major problem, but if it is permitted to have a re-circulating system based on nutrients derived from organic sources, then all becomes possible.
Thus, one possible solution is to use hydroponics, to ensure that the plants receive adequate nutrients, combined with a re-circulating system so that little (none) of these nutrients reach the ground water. This would appear to be achievable, even under current IFOAM regulations, by using only natural (non-chemically derived) nutrients, such as seaweed, fish manure, etc., to provide the plants with nutrients.
In fact, any requirement to be soil-based poses questions of what is soil. Soil normally comprises solid rock particles (e.g. sand/clay/silt) plus air, organic matter, water, micro-organisms, and nutrients. The proportion varies considerably with the soil type, from mineral soil (with low organic matter) to peat soils with little or no mineral content. The only thing missing in a re-circulating system using naturally derived nutrients is that the growing medium is not connected directly to the earth and that by choice the growing medium would be a well-aerated material such as peat or coir.
The huge advantage of such a system would be that it is highly sustainable in that all the inputs are used by the crop, unlike the soil where considerable quantities of nutrients are lost by leaching.
The ultimate of such a system is aquaponics, in which the waste from fish is converted by bacteria in a bio-filter into plant soluble nutrients, which are then provided to the plant roots in a re-circulating system.
Aquaponics can best be defined as a combination of aquaculture and hydroponics. In aquaponics the fish and plants are produced in a single integrated system, where the fish waste provides a food source for the plants and the plants provide a natural filter for the water in which the fish live. A key factor is the bio-filter, between the fish and the plants. This comprises bacteria which convert fish waste into soluble nutrients for plant roots. The key conversion is ammonia (toxic to fish), where nitrite is converted to nitrate. It is possible to produce organically certified salmon in New Zealand, and yet, illogically, if the waste water from the salmon was used to grow vegetables, the vegetables would not be organically certified because they would not have been grown in soil! It goes without saying that with fish in the system, the possibility of using any toxic pesticide is virtually eliminated.
Aquaponics is probably the ultimate in organic sustainability.
Much of the ‘organic philosophy’ appears to be based on the UK Soil Association and the writings of Rudolph Steiner. Both organisations have their origins well before anyone considered growing crops commercially using hydroponic systems, and so hydroponics did not get considered. To suggest that it is unnatural (as has been suggested by some) is to limit our future to being hunter gatherers rather than farmers.
The key factor for the future must be sustainability. Soil-based organic greenhouse systems are not sustainable in practice, whereas organic hydroponic systems are far more sustainable.
One of the major claims of organic vegetable growers is that it is environmentally friendly, but from a sustainable view point a recirculation system is much more efficient in both nutrients and water use. In fact, to obtain acceptable levels of production in an intensive soil-based greenhouse situation, it is highly likely that considerable quantities of nutrients will leach through the soil profile into the water table.
In North America and Scandinavia there is a growing acceptance of the use of organic media (peat) and re-circulating hydroponic systems using organically derived nutrients to provide organically certified produce. It would appear to me that the potential to use untreated coir might provide a valuable alternative if peat supplies ever became limited.
Unless organic greenhouse producers develop satisfactory growing methods that produce similar yields to conventional greenhouse producers, then their long-term future would appear to be at risk, because only a limited number of the population are able (or willing) to pay a high premium for organically grown greenhouse produce. With the steadily increasing productivity of conventional growers, the premium required by organic greenhouse crop products would further increase.