Striving for a happy medium

There is no single ‘ideal’ method for growing seedlings successfully and economically – conditions vary too much – but there are guidelines that can help a newcomer get it right in a shorter time.

Compost from horse manure in wood shavings makes an excellent growing medium.
Photo: Bill Kerr

A major aspect is the growing medium. This accounts for a large part of the cost of growing seedlings, and nearly every grower tries to obtain a suitable medium that can cut that cost.

Some years ago, I carried out trials for a seedling nursery that used a pine bark medium. I was asked to help the owner reduce the need for chemical fertilisers in order to cut cost and reduce the risk of staff making mistakes with the applications.

I used a 25l container for each batch of growing medium; this was sufficient to plant a tray of each of the species grown in the nursery: cabbages, peppers, tomatoes and lettuce. All the batches had various combinations of phosphate, calcitic lime, potassium sulphate, trace element mixtures and urea formaldehyde (slow-release hydrogen).

To half of the batches, I added 30% sifted cattle manure. The results were revealing. All the plants with cattle manure had a substantially better root system; the trace element mixtures had absolutely no effect.

Another outcome was that different mixtures suited different species, not drastically, but in a way that could be easily observed. One mixture, however, produced the best seedlings in all species; it required no additional fertilisation, saving costs and making it easier to run the nursery.

As not all pine bark is equal, prospective seedling producers need to conduct their own trials. It’s easy to do and takes very little time and effort, given the potential benefits. Imported peat, pine bark, compost and various mixtures from agricultural companies are all suitable as a growing medium. Vermiculite, pearlite and polystyrene can be added for more aeration and to save cost.

Manure for smaller growers
Horse manure makes a highly effective and economical growing medium when composted. I use it for my own seedlings, without any additives, except for a little nitrogen towards the end of the seedling growing cycle, and obtain healthy seedlings with good roots.

It is not for large commercial growers – there are simply not enough horses available – but it may be a good option for a small grower if horse manure is locally available.

Various composts are suitable as is, or they can be used in combination with other ingredients. The compost should be turned at least twice and must generate heat to kill off weed seeds, eelworms and pathogens in the compost material. The ideal temperature is about 65°C.

Properly aerated growing medium
A final point: the growing medium should not be too dense or compacted, as air is crucial for the proper development of roots. Apart from this, a medium that is too fine and compact may become waterlogged after irrigation, which encourages damping off and loss of nitrogen.

Just as soil is the backbone of a farm, the growing medium is the backbone of a seedling nursery and must be tweaked according to your needs.

Bill Kerr is a vegetable specialist and a breeder of a range of vegetables.