Dirt music: how to make your soil sing

The mantra of gardening experts is “Feed the soil, not the plant” – but how exactly do you make your soil healthy?

Healthy soil: but how do you achieve it? Picture: Getty Images

Many of the questions sent in to this column have something to do with soil health. The mantra of experts is “Feed the soil, not the plant” – but how exactly do you make your soil healthy?

Soil provides plants with water, nutrients and oxygen, as well as anchorage. Soil chemistry and soil physics are basic characteristics but soil biology is where the magic happens. Healthy soil is a complex biosphere of organisms that includes fungi, microbial life such as bacteria and protozoa, nematodes, earthworms and insects. Together they decompose plant residues, cycle and regulate nutrients, improve soil structure, degrade chemicals and suppress pathogens such as Phytophthora root rot. The larger organisms help keep the soil open and aerated, which also allows water to percolate freely. Many organisms form complex relationships with each other and with plant roots, which increase roots’ ability to take up nutrients.

All soil organisms need a food supply in order to thrive, and that comes from organic matter such as compost, animal manures, soft mulches, worm castings, biochar, green manure crops, seaweed, spent mushroom compost, fish wastes, and blood and bone. The best compost is your own, which you get for free by recycling your house and garden waste such as lawn clippings, fruit and vegetable scraps, paper products, fallen leaves, prunings and spent coffee grounds. Over a year, about 80 per cent of organic matter is used up or breaks down, so it needs constant replenishing. However, you can add too much rich compost to soils, causing overly high nutrient levels; plant symptoms include reduced growth, yellowing and “burned” leaf edges.

Garden products containing beneficial microbes are becoming more common, such as Troforte fertilisers, SoiLife and Grow Safe products, and Neutrog’s GoGo Juice and Popul8. Every keen gardener should have a soil pH testing kit, as pH affects the availability of soil nutrients to plants. Although a pH of 6-7 suits most plants, some have special preferences. Soil pH varies locally but can be affected by building works (lime in mortar increases alkalinity) and fertilisers (most are sulfates that increase acidity).

For those growing food, soil contamination by heavy metals – particularly lead – is a concern. A recent study showed eggs from chickens kept in inner-city backyards had an average 40 times the lead levels of commercial eggs. Free soil testing for metals is available from VegeSafe.

Top tips to encourage soil organisms

Keep soil moist

Mulch soil to keep it cool

Allow plant litter to stay on the surface

Apply compost and manures

Bury kitchen scraps

Don’t dig the soil

Avoid chemical fertilisers and pesticides

Q&A

My huge elkhorn fern growing around a tree is starting to block a path. What’s the best way to reduce its size? Christine Potter, Gold Coast

Mature clumps consist of many plantlets that can be cut or prised off, or you can divide it into just a few pieces; each needs an “eye” or rosette of foliage. Use a serrated knife to cut away the dead leaf bulk and separate divisions. Mount onto hardwood boards, tree fern trunks or in hanging baskets, using coir, compost or leaf litter to pack behind the plants. Secure with flexible ties.

To grow blueberries I’m trying to reduce my soil pH from 8-8.5 to around 5. I’ve added sulfur according to directions and tested the pH every four weeks since autumn, but it hasn’t changed. Noriko, Perth

On the pH scale of 0 to 14, below 7 is acidic and above 7 alkaline. But it’s a logarithmic scale, so a pH of 5 is 10 times more acidic than pH 6 and a hundred times more acidic than pH 7. Trying to reduce pH from 8 to 5 is unrealistic. Soils over 8.4 usually contain several per cent limestone, all of which must be dissolved by acid before the pH starts to drop. Sulfur will lower pH but it acts slowly. You might need to buy in new, acidic soil.

Send your questions to: [e-mail] or Helen Young, PO Box 3098, Willoughby North, NSW 2068. The best question for October wins a hamper of organic products from Eco-Organic Garden worth $110.