Conditioning your soil

17:00, Apr 14 2011
Southland Times photo
BOOST: Clay soils can be improved by adding compost, aged manures and leaf mould. Coffee grounds can also dish out a useful amount of nutrients to the soil. They are high in nitrogen and also contain a few trace minerals and a small amount of phosphorous and potassium.

If gardening throughout winter doesn't appeal, preparing the ground for a flurry of spring action might be a better option.

Adding a layer of organic mulch to your garden is the easiest solution – it will break down on its own accord and boost soil health and improve structure.

Or you can plant a green manure crop. Green manure is a cover crop that is grown primarily to add nutrients and organic matter to your soil. The crops are typically grown over winter as beds become available (once you've dug up your summer/autumn veges, for example) and then dug into the soil at the end of winter in time for the new growing season.

Leguminous crops, like alfalfa, clover and lupins, are ideal because they add large quantities of nitrogen to the soil along with organic matter. Compare a planting of legumes to a layer of compost: compost returns to your soil around 98 per cent of the nitrogen you originally started with (remember that continuous cultivation depletes your soil of nutrients), whereas a green manure crop replenishes what's been lost plus adds considerably more nitrogen.

Leguminous crops should be cut when young, before they become woody and before flowering. That's because at this stage the nitrogen content is at its highest.

Buckwheat and mustard are also good green manure crops, although these two are generally grown in spring and summer. All these cover crops are available from Kings Seeds.



Clay soils can be greatly improved by adding compost, aged manures and leaf mould. A mulch applied now will also act as insulation over the winter months.

Adding a good sprinkling of gypsum (calcium sulphate) to clay soils can help break it up, but I'd recommend a soil test before adding gypsum. Your garden soil may contain adequate calcium; too much calcium can interfere with the uptake of other nutrients.

Compost and leaves (gather your autumn leaves now!) work just as well. It's what I used on my clay soil when I first moved onto my property, and I now have a beautiful, crumbly soil mix.


If your soil is acidic (another soil test is warranted here), add lime to raise the soil pH. Certain veges (beans, brassicas, celery, onions and peas) grow better in soil that has been limed – although it's not the lime itself they like but an alkaline soil. Just to confuse things, there are two types of lime you can buy: ground limestone (or just plain lime) and Dolomite lime. Ground limestone contains calcium in the form of calcium carbonate, while Dolomite lime contains calcium (calcium carbonate) and magnesium (magnesium carbonate).

If your soil is already high in magnesium (again, a test is required), use plain lime.

Autumn through to early winter is the best time to apply lime since water is required for lime to react with the soil, and change in pH can take several weeks. And New Zealand generally gets a lot of rain over winter.


Coffee grounds can, surprisingly, dish out a useful amount of nutrients to the soil. They are high in nitrogen, which promotes good leaf growth, so they're great for leafy crops and fast growing vegetables. They also contain a few trace minerals and a small amount of phosphorous and potassium.

The acidity level, however, is confusing. While roasted coffee grounds are acidic, tests have shown that some of that acidity is removed in the brewing process (but not all). And the amount of acidity that remains depends on a number of factors: the beans themselves, the temperature of the water used in the brewing process, how long the grounds were exposed to that water, whether the water is soft (acidic) or hard and what other compounds are in it.

So while the drink itself is fairly acidic, used coffee grounds are not necessarily high in acidity. Their pH of decomposing coffee ground also changes over time.

However, as far as fertilisers go, coffee grounds are not particularly "slow release" (the nitrogen becomes available immediately). So small amounts added to your garden plot regularly is better than one huge dumping.


Too much nitrogen in your soil? Overzealous applications of high-nitrogen fertiliser or high-nitrogen mulches such as animal manure and grass clippings can cause an abundance of leaves at the expense of flowers and fruit.

One way to rebalance excess nitrogen is to apply a fertiliser that's high in phosphorus and potassium. Bone meal, which is high in phosphorus, might also help. A better alternative though is to apply a carbon-rich mulch, such as crushed bark, wood chips, sawdust, straw or newspaper.

As they break down, organic mulches cause a temporary boom in the population of soil micro-organisms. These microbes, which drive the breakdown process, require lots of nitrogen, and effectively "rob" the surrounding soil of nitrogen until decomposition is complete.

As these mulches have a high carbon-to-nitrogen ratio, excess nitrogen in the soil is used up. Or you can plant a "mop up" crop. Farmers often plant squash to mop up excessive nitrogen in the soil. It can take just a matter of weeks for squash to exhaust the nitrogen supply.Easy composting

If you're after a simple fix, simply dig your kitchen scraps straight into an empty bed and let them decompose over the cooler months. Add a layer of compost, then sit back indoors and swot up on your spring planting choices.


Question: Last year I planted tulip bulbs but none came up. In fact, the bulbs seem to have disappeared. Would something have eaten them? Susan Cooper, Invercargill

Answer: It could be mice. They often dig up and steal newly planted bulbs. One trick you can try is to construct a chicken wire cage to surround your bulbs and plant this.

The Southland Times