What is the difference in results between soil and hydroponics?

Rick Donnan

Rick Donnan

I have noticed many different claims of the advantages of hydroponics over soil growing, especially a wide variation in figures for relative yields and water use. Are there any standard and reputable data available?
In general, this is a “how long is a piece of string?” type of question.

There are many different hydroponic system types and management techniques. Growing in the open field has severe variations, especially due to the outside climate. There will also be huge differences in added water depending upon rainfall and irrigation techniques, for example, the difference in water usage between flood and dripper irrigation.

I also say this because the environment has a major impact whether growing in soil or hydro. The biggest influence is having a greenhouse—the more sophisticated, the greater the influence. Most claims of the yield advantages of hydroponics over soil unfairly ignore the major impact the use of protected cropping has upon the hydroponic yield.

It is easy to find statements that hydroponics uses far less water than soil growing. The most common seems to be 10% or 20%, although some claims go as extreme as 1%. In my opinion most of these figures are opinion rather than having a factual basis.

Table 1An actual example
Actual measurements are very rare, but here are some figures from a presentation by Graeme Smith ‘Overview of the Australian Protected Cropping Industry’. This is one specific case of tomatoes grown hydroponically in a glasshouse compared to soil growing in the open field. The figures are all based upon one hectare.

One result that would surprise many is that the total water usage per hectare for hydroponics was almost double that of soil growing. There are several aspects to this—the length of the hydroponic crop was far longer. Importantly, a major reason that hydroponic production and quality is better is its higher water uptake for transpiration in order to cool the plant in hot conditions. This significantly reduces the stress on the plant.

In terms of the efficient use of water, hydroponics produced just over five times the quantity of first-class produce per unit of water used. That is, for this example, the water use in hydroponics was about 20% of that used in the field. This figure would be higher if compared to a crop grown under cover in the soil, but this is now quite rare.

If a hydroponic farm does the sensible thing it will invest in collecting rainwater. In this case the amount of water needed from an external source is significantly reduced, or in high rainfall areas possibly even eliminated.

Commonly quoted by some aquaponicists is that hydroponics requires only 20% of the amount of water relative to growing in the soil and that aquaponics requires only 10%. That is, they are claiming that aquaponics is twice as water efficient as hydroponics. In my opinion, this claim is difficult to justify. The hydroponic component of aquaponics would have no different water usage to a standard hydroponic system growing the same crop, and evaporation from the fish tanks would contribute significantly to the total water usage. There is a need for some measurements to be made on actual commercial scale operations.

Overstated yields
In common with sometimes overstated water savings is the much more serious aspect of grossly overstated yields. This was common in Australia, especially through the early 1990s. There were numerous schemes promoted, which promised totally unrealistic yields, combined with claims that no knowledge was needed, plus a very low workload. Of course, many of those who bought these turnkey operations failed. Unfortunately, in terms of commercial failures, the US is the classic case with commercial failures swamping successes going back to Dr Gericke in the 1930s and frequently since then.

An example of the adverse impact of this was a class action in Australia where over 30 growers took the promoter of a turnkey scheme to court. What happened was that the defence quoted from a well known and otherwise reputable hydroponic book, first published nearly 40 years ago, which contained a table purporting to compare yields in the soil and hydroponics. Typical figures were for tomatoes 60-300 tons per acre in hydroponics (when the best commercial yields were actually under 150) versus five to 10 in the soil. Although not one grower achieved other than a fraction of the promised yields, their case was lost because of the exaggerated figures published in this book.   Ω

PH&G August 2014 / Issue 146