Make this sowing medium for maximum plant health

Just as prospective mothers are told to stop smoking and drinking during pregnancy, and are then exhorted to provide the best possible nutrition for their newborns after the happy event, gardeners won’t enjoy the best possible fruits of their labours if they try to sow seeds in unsuitable soil.

So, you wish to ensure the best possible germination of your seeds, and the strongest, best formed seedlings. What characteristics should you look for in a good germination medium?

Moisture and warmth

Generally, moisture and warmth are key to germination itself. Your sowing medium should be able to take up moisture and make it available immediately around the seed. And the whole process is enhanced by a warm environment.

But the sowing medium must also be soft and light, allowing good airflow to the seed and nascent roots. It should also be light and soft enough to enable the tiny, delicate roots that emerge from the seed to develop and penetrate. Also, the soft stem of the seedling needs to emerge unhindered to push its way upward towards the light.

Balanced nutrition

But coupled to these characteristics, the medium needs to provide the seedling with an adequate balance of nutrients. This encourages strong root and stem development.

Finally, just as you wouldn’t knowingly expose a newborn baby to viruses and bacteria, so you don’t want these pathogens present in your sowing medium. Neither do you want the presence of weed seeds.

How, therefore, can you best assure yourself that your seeds are sown in the most conducive medium?

For smaller quantities, many nurseries sell ready-mixed seedling medium but as serious smallholder growers we often require larger quantities.

Plus, many of the ingredients of the optimum sowing mix are available in a normal smallholder growers’ arsenal.

Four simple ingredients

The recipe is simple, and contains only four ingredients.

In equal quantities, a thoroughly-mixed pile of worm castings, sieved compost, perlite and potting soil will give you a sowing mix suitable for most purposes.

Let’s look at the role of each.

Worm castings (available from better nurseries) are packed with nutrients, in a readily available form, to feed your seeds as they germinate.

Compost, if well-made and sieved to remove any remaining big pieces (eg uncomposted twigs etc), is rich in organic matter, and is soft and light, allowing easy penetration of roots, while facilitating the uptake of moisture.

Compost is also full of micro-organisms that help break down the nutrients in the soil and make them available to the seedling.

Perlite is a white, granular mineral derived from volcanic matter that has been expanded by the application of heat. It is inert and sterile, and more importantly porous, helping to capture and retain moisture, while also helping with aeration. In South Africa it is commonly available in three grades, depending on the size of the particles, from 1mm dia to 3mm.

Potting soil, bought from a nursery forms the base of the mixture. Look for a fine textured version, rather than coarse regular potting soil.

Optional additions

This four-part mixture will work just fine. However, there are two additions which can be incorporated if necessary. They are vermiculite and peat moss.

Before expending money on them, however, check the ingredients in your potting soil as these sometimes contain one, or both, rendering any further addition unnecessary.

Vermiculite is another sterile mineral that aids with water retention and aeration.

Peat moss, which is partially decomposed sphagnum moss, is great for moisture retention and also enhances the soil structure. (Sphagnum moss is the plant-like material used by florists and nurseries in presentation pots.

Main image: Healthy seedlings in a good sowing medium. Image: Brianna Larow, via Wikimedia Commons