Soil provides a basic structure within which various organisms and microorganisms, roots, water, air and a wide range of organic and inorganic chemicals proliferate.
However, there is nothing magical about soil; in fact, it isn’t always the best medium in which to grow high-value horticultural crops because it can be either poorly aerated due to ideal moisture conditions or too dry due to optimum aeration.
The emphasis on soil as the basis of “organic greenhouse cropping” is false and there is no reason why organic nutrients cannot be used in a recirculating hydroponic system.
Organics and soil
Organic greenhouse horticulture is defined by the International Society of 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-controlled greenhouses and tunnels.
There is no mention of soil-based systems in this definition. Much of the organic philosophy appears to be based on the UK Soil Association and the writings of Rudolph Steiner. Both organizations have their origins well before anyone considered growing crops commercially using hydroponic systems, so hydroponics did not get considered.
But what is magical about soil? Soil is normally comprised of inorganic particles derived from rock (such as clay or silt or sand), organic matter (humus), a range of microorganisms, water, air and some nutrients dissolved in the water. In some “soils,” the inorganic particles may be partially or totally replaced by semi-decomposed organic matter (peat).
Well, a recirculating organic hydroponic system can comprise all of the above, with the exception of the solid inorganic particles (although pedantically it would not be too difficult to add a few pieces of rock to the system to fulfill all of a normal soil’s characteristics). In fact, a recirculating organic hydroponic system is also much more sustainable than a soil-based system.
One of the major claims of organic vegetable growers is environmentally friendliness, but from a sustainable viewpoint, a recirculation system is much better in nutrient and water efficiency. This is because considerable quantities of nutrients leach through the soil profile into the water table and aquifers in an intensive traditional greenhouse situation in order to obtain acceptable levels of production. This does not occur with organic hydroponic systems.
In the early 1950s, virtually all greenhouse crops were grown in soil. The only exception was pot plants as for many years it was thought soil was not a suitable medium for these plants (potting compost was developed to sustain these plants).
It was not until the ’60s that researchers began to consider alternatives to soil for the production of other greenhouse crops, such as tomatoes, cucumbers, and lettuce. What started as straw bales, peat base beds, etc. resulted inevitably in the establishment of commercial hydroponic systems like nutrient film technique (NFT) and rockwool.
This had a huge influence on productivity and a marked increase in yield, as the complex balance of aeration and adequate moisture at the roots became much easier to obtain (the control of soil-borne pathogens also became easier due to isolation).
Then, in 2001, a number of greenhouse studies were conducted at Massey University by Kim Atkin to compare conventional and organic hydroponic systems. He found that, in a comparison between an organically derived solution of liquid fish and liquid seaweed and a conventional hydroponics solution, lettuce raised using NFT grew faster in the conventional solution.
This might have been due to the fact that some of the liquid fish solution was in suspension rather than dissolved, which might have caused anaerobic conditions. Nevertheless, the organically grown lettuce reached maturity only one week after the conventionally grown ones. Also, Atkin found that cattle effluent was capable of producing lettuce growth rates similar to those of conventional hydroponic solutions.
Try aquaponics as a form or organic hydroponics
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 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, which is comprised of bacteria that convert fish waste into soluble nutrients for the plant roots (a key conversion is ammonia—which is toxic to fish—into nitrite and nitrate).
In studies recently undertaken in New Zealand that compare the productivity of lettuce and herbs within conventional hydroponics and aquaponics, the aquaponic system proved to be similar (or in some cases superior) to the conventional system. These results depended on the time of the year, as the productivity of herbs in the aquaponics system was reduced during the winter due to the poor feeding of the fish in the cooler conditions.
In trials carried out in Italy, the productivity of aquaponic systems was not significantly different to that of a conventional hydroponic system. In the first experiment, a small quantity of fertilizer was added to the aquaponic treatment; but there was significantly lower yield.
In the second experiment, however, no significant difference was found in the productivity of lettuce (when comparing aquaponics with hydroponics) thanks to the greater quantities of nutrients available in the aquaponics system during this second trial. Aquaponics is probably the ultimate in organic sustainability.
Why choose organic?
In North America and Scandinavia, there is a growing acceptance of the use of organic media (peat) and recirculating hydroponic systems that use organically derived nutrients to provide organically certified produce. (There would appear to me that the potential to use “untreated” coir might provide a valuable alternative if peat supplies ever became limited.)
This makes sense. Soil is not a good medium in which to grow high value crops, because it is difficult if not impossible to provide it with optimum levels of both moisture and aeration. Also, producing high yields of greenhouse crops (a necessity in expensive capital structures) requires considerable inputs of nutrients. In a non-recirculation system, this poses considerable problems in terms of leaching into the water table.
As such, the insistence on the use of soil rather than other media for organics and the objection to recirculating system is illogical in terms of sustainability. Also, to suggest that hydroponics is unnatural (as has been suggested by some) is to limit our future to being “hunter gatherers” rather than farmers.
In my view, the key factor for the future must be sustainability and soil-based organic greenhouse systems are not sustainable in practice, whereas organic hydroponic systems are far more sustainable.