There are multiple ways to improve soil fertility, and they usually involve the introduction of organic matter in one form or another.
In newly-made plant beds one can dig in compost, or adopt the “lasagne-style” method of preparation, where one layers organic matter such as corrugated cardboard, newspaper, kitchen waste, manure and straw or leaves.
Another method used in permaculture, is to bury logs and branches – hugelkultur – which slowly decompose and provide humus and moisture to plants growing above. Sowing green manures is a time-honoured way of improving soil fertility.
Fast-growing plants such as legumes, oats and/or mustard are sown to cover bare soil, smother weeds and prevent soil erosion.
When dug into the ground while still green, they return valuable nutrients to the soil and improve soil structure.
Legumes are particularly valuable, as they have nodules on their roots with specialised bacteria that convert nitrogen from the air into nitrogen compounds; fertiliser for free.
Using green manures is widely used in organic vegetable gardening, and would be a cost-effective method of soil preparation for larger landscapes and developments.
In established plantings, it is preferable not to disturb the soil and to simply mulch with organic matter; spread a layer of chipped garden material, straw or leaves.
For most gardens and landscapes this is sufficient to meet the needs of plants.
How often to compost or fertilise the soil?
One should consider the type of plant and its requirements. Vegetables require ongoing and regular inputs of organic matter and fertiliser.
Remember that every time a tomato, cauliflower or pepper is picked, a concentration of macronutrients (carbon, nitrogen, phosphrous, potassium) and micronutrients (trace-elements) are removed, so these need to be replenished.
Soil drenches and foliar feeding with “teas” made of compost, manure, comfrey, seaweed and earthworm castings, are all highly beneficial.
Organic fertilisers made of plant, animal and mineral material provide a concentration of nutrients that become available to plants in a slow and sustained way, so there is no leaching or waste.
Plants growing in forests have an ongoing supply of leaves, twigs and branches landing on the forest floor which decompose and become compost.
So forest trees, shrubs such as plectranthus, and bulbs all enjoy regular dressings of compost.
On the other hand, Strandveld species grow in the sandiest of soils, and fynbos is adapted to nutrient-poor soil that is low in organic matter, so these plants require minimal or no composting when planted in “clean” soil.
Plant growth will be slower but more sustained without compost, which requires less pruning and results in a longer lifespan.
However, cement and plaster from building activity changes the pH of the soil and requires generous amounts of compost to undo the effects of contamination.
Fertilising in a nutshell
Feed the microorganisms in the soil, and you will be feeding your plants.
Save all organic matter (such as leaves, branches and trimmings, and recycle it on site by chipping and composting, or spreading as a mulch. No more exporting bags of green waste.
Kitchen waste and cardboard can be composted or processed in a worm bin or in a compost heap.
Always mulch with an organic mulch like leaves, straw or chippings, as it will decompose and feed the soil.
Use leaf blowers only on paved areas. Ensure it doesn’t blow away mulch from plant beds, as this will lead to soil encrustation and depletion.
Feed and improve the soil according to plants’ specific needs; for example, veggies need a lot and some plants don’t need much.
Marijke Honig is a botanist, landscape designer and author of Indigenous Plant Palettes – a guide to plant selection. She is passionate about creating more resilient, water-wise landscapes.