Gardening: Cloak your soil in green

Phacelia, which can be used as a green manure crop, also attracts beneficial insects tot he garden. Photo / Meg Liptrot

A cover crop will inject a good dose of nutrients into your veg patch, writes Meg Liptrot.

If you have a veg garden, you may be wondering what to do with it as the weather cools and we head towards winter.

Many vegetables are well-suited to the cooler temperatures, but if you are less likely to get into the garden, you may choose to leave a bed fallow by sowing a green cover crop.

This is a great way to protect your soil and let it rest after intensive vegetable gardening.

When we grow vegs and frequently harvest crops, the soil nutrients and organic matter are slowly depleted. Incorporating compost into the soil is one way to replenish the soil with carbon and nitrogen. Another method is to sow a green cover crop, with the intention to dig the plants back into the soil several weeks before planting your veges in spring.

Cover crops are also a great way to reduce weeds, making the garden easier to manage.

I enjoy growing green manure crops. A swathe of silvery green lupin with their small blue flowers is lovely to look at, and the soil also has a protective blanket over it for winter. You're giving something back to the soil, more than you can achieve by adding store-bought fertilisers.

Green crops have the added benefit of keeping soil aerated over winter while they're growing; their roots help provide soil structure, reducing the likelihood of wet, compacted soil in spring. They also help prevent the leaching of nutrients which happens when heavy rain hits bare soil.

Legumes such as clover, lupins, and broad beans fix nitrogen from the atmosphere and are excellent green manure candidates.

Clover can be left as a semi-permanent cover crop, lupins should be chopped and dug into the soil in spring, just after flowering (buy blue lupin seed in bulk for green manure, not the ornamental lupin).

Broad beans can be chopped into the soil when you've finished harvesting the produce. The chopped-up green manure should be left to decompose in the soil for about three weeks, as planting straight into a freshly incorporated green crop can adversely affect the growth of your spring veges.

This year we've had a much wetter summer than usual, and a number of people I know have had issues with their tomatoes because of the reduced sun hours.

The wet soil has contributed to problems with rot or blight.

Some organic fertilisers include beneficial bacteria and fungi. Digging this into your garden will help prevent the damaging effect of soil pathogens and also stimulate root growth for healthier plants. Environmental Fertilisers has developed an organic product which incorporates these beneficial ingredients. This is like us taking a good dose of yoghurt to keep our gut healthy. Another organically approved material, basalt rock dust, helps to re-mineralise the soil after heavy cropping, and digging in lime or dolomite lime helps to offset the acidifying effects of organic matter and heavy rain.

Green manure seed options

A number of green manure and cover crops are available for different purposes and situations.

From Kings Seed in Katikati (mail order):

* Spring Manure Mix: a combination of nitrogen-fixing plants including pea and lupin, with the addition of oats which provide organic matter and bulk (carbon).

Best for spring sowing, but early autumn in warmer climates shouldn't be a problem.

* Mustard helps clear up harmful soil fungi, and will help with weed seed control. (Note: avoid sowing mustard after having just had a brassica crop in the same ground.)

Kings Seeds also stocks lucerne for breaking up worn-out soil, buckwheat for beneficial insects and carbon building (organic seed available), and blue lupin for nitrogen.

From Kaiwaka Organics (mail order or direct from shop):

* Green Manure Collection: mustard, phacelia and buckwheat - great for attracting beneficial insects and building soil.

* Compost and Carbon Crop Collection: an assortment of seed chosen as the best soil builders after a 25-year trial by Ecology Action.

* Subterranean clover: grows right through winter and suppresses early spring weeds.

From Kings Plant Barn:

Burnet's Horticulture supplies a range of green crops including lupins, mustard and a green manure mixture (not organic).

* For more on the autumn veg garden, soil care and green crops, a one hour workshop will be held with organic horticulturist Carl Pickens at the Sustainable Living Centre, Auckland, on Saturday, May 12, $10. For info and bookings phone (09) 826 0555.