Choosing Your Growing Medium

Last summer holidays, I visited Nelson, one of New Zealand's sunshine cities, but this particular year it let me down, with several wet days, so I ended up visiting second-hand bookshops.

Among the books I purchased was the early edition of a book titled Seed and Potting Composts by W.J.C. Lawrence and John Newell of the John Innes Horticultural Institute. This 136-page book is probably the father of the modern growing media we use today, and yet it was only published in 1939-a mere 70 years ago. We have come a long way in such a short time, but there is still a lot to learn.

The book's authors state that in 1939, the potting media used depended purely on tradition. These traditions were passed down from head gardener to head gardener in the large houses, as any study of 19^th century gardening books will demonstrate. Specific mixtures were established for a wide range of crops, such as pineapples, tomatoes, melons, etc., with each potting medium subtly different.

Lawrence describes this as "the old complexity" in which seed and potting composts (media) included materials such as loam, leaf soil, manure and sand in widely varying proportions depending on plant species, age and the inclination of the head gardener.

The magic (and uniqueness) of the work by Lawrence and Newell was to demonstrate that it is possible to grow a wide range of plants in the same growing medium, and that there are certain basic principles that apply to every growing medium. These principles include freedom from diseases and pests, stable physical conditions--good aeration and good moisture-holding characteristics-and an adequate and balanced supply of nutrients at every stage of the crop's development.

The key to the John Innes potting composts was standardization. For the J.I. seed compost:

• 2 parts loam + 1 part peat + 1 part sand

And for the J.I. potting compost:

• 7 parts loam + 3 parts peat + 2 parts sand

The loam was a sterilized medium loam, the peat was moss or sedge peat, and the sand was clean and evenly graded. With these formulas, a pathogen-free medium with stable physical attributes was established.

The nutrient supply was provided by superphosphate plus chalk in the seed compost, with chalk, superphosphate, sulphate of potash and hoof and horn for the potting compost. The precise components are not important now because we have moved on, but the key factor is that the mixtures were standardized, and worked well for an extremely wide range of crops.

In the 1950s, there was a movement away from soil-based media and towards a range of soilless media, including the Cornell mix (peat plus perlite), the University of California (UC) mix (peat plus sand) and the UK mix (developed by Bunt) similar to the UC mix.

But over time it was discovered that the peat supplies marketed had a finer particle size and that this resulted in reduced porosity and poorer root growth than when the original mixtures were being developed.

In addition, there was pressure on the peat producers for environmental reasons to reduce (or even cease) harvesting peat for horticultural purposes, so alternative sources of materials for soilless media were actively being sought. Soilless media are divided into a number of distinct classifications, which include:

1. Manufactured products such as rockwool (stone wool) or polyurethane foams.
2. Natural inorganic products such as pumice, perlite or sand.
3. "Waste" organic products such as peat, coir, bark or wood pulp.

The whole exercise comes down to one of local availability and economics and this is a good example of one solution not fitting all cases. A current gold standard for growing media is rockwool, which can be manufactured to precise aeration and moisture-holding characteristics.

Polyurethene foam media will have similar attributes. The problem then becomes a question of economics-both products are relatively light but bulky, and freight costs become a major consideration. There is also a disposal problem with rockwool.

The naturally occurring organic and inorganic materials can usually be disposed of locally as soil amendments, so pose no real problems in that respect, but there is an increasing demand (and therefore competition) by other users for many of the so-called organic waste products and there is the problem of weight to consider in the case of many of the inorganic products.

With the exception of hydroponic systems such as nutrient film technique and aeroponics, all greenhouse plants are grown in a medium and at this point in time, the only fully standardized medium is rockwool, where the amount of pore space is precisely defined.

Grow Media Particle Size Considerations

Omer Verdonck from Belgium has demonstrated the wide range of particle sizes that can occur in different commercial samples of peat, bark or perlite. He has also studied how these different fractions determine the aeration characteristics of the medium and the amount of easily available soil moisture that these fractions can hold.

The situation is further complicated by the fact that we do not generally use precisely size-graded media, but a mix of different particle sizes, and this mix can have a huge effect on the physical characteristics of the medium.

The pattern is not necessarily what one might imagine because the smaller particles take up some of the aeration space between the larger particles; therefore, a small increase in the proportion of small particles can have a large influence on the reduction in air space. There is a whole science to the way particles of different diameters (and shapes) pack together, as it has real relevance in the construction industry and also in determining the stability of land.

Sadly, this is not the end because we do not usually use a medium comprising a single material, we tend to use mixtures and this further complicates the picture.

The addition of perlite improves the aeration characteristics, but at the same time reduces the water-holding characteristics of the medium. These results are pertinent both for the greenhouse vegetable producer and also for the cell seedling grower, as they suggest methods that could be used to improve productivity.

In the case of the tomato/cucumber/capsicum grower, considerable money has been invested to ensure that the above ground environment is near optimal for the crop, and yet little information is available on the appropriate media characteristics to ensure optimal aeration and moisture holding characteristics.

In the case of pumice, what information is available on the appropriate mix of particle sizes for growing these crops in greenhouses over the long term?

Source: This article has been posted with permission from Practical Hydroponics & Greenhouses.