IMPROVING SOIL FERTILITY

There is a saying ‘healthy soil = healthy plants’, so it is no surprise that garden centres and nurseries offer a wide range of products to improve soils. Why do we need to focus on soil? How can one improve life in the soil and how often is it necessary to fertilise or add compost? 

First it helps to have a basic understanding of soil biology. Soil is not just dirt: there is a whole world under our feet, teeming with life which is invisible to the naked eye. There are micro-organisms such as bacteria, fungi, protozoa and algae which all play a role in making nutrients available to plants. Plants live in a mutually beneficial relationship with these micro-organisms: the plant roots give off substances which the soil microbes absorb and convert into nutrients, vitamins, growth regulators and antibiotics, which are taken up by the plants. So healthy soil is actually an underground grocery store and pharmacy for plants and trees.

Nearly all plants rely on fungi in the soil called mycorrhizae for nutrients and moisture. The fungi produce fine threads which increase the plants root surface area by 300 to 1000%. It is astounding to think that a tree may have a delicate network of fungal filaments that covers the size of a tennis court! Therefore it’s not surprising that digging or turning the soil causes major setbacks to plants and disrupts the ecosystem of the soil. So the best thing a gardener can do is spread a generous amount of organic matter on the soil surface and then do nothing. Earthworms will feed on the organic material and convert it to humus, and the soil micro-organisms will do the rest.  By feeding the soil, you are effectively feeding the plants.

How to improve soil?

There are multiple ways, 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 carboard, 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 – i.e. 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 simply to mulch with organic matter – i.e. 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?

Here you need to consider the type of plant and its requirements. Vegetables require ongoing and regular inputs of organic matter and fertiliser. Remember that every time you pick a tomato, cauliflower or pepper you are in effect removing a concentration of macronutrients (carbon, nitrogen, phosphrous, potassium) and micronutrients (e.g trace elements) 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 (e.g. 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 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 your organic matter (leaves, branches, 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 (e.g. leaves, straw, 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 – e.g. veggies need a lot and some plants don’t need much

 

Text and pics by Marijke Honig

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