Prof Walker tells
us how to build up and maintain a healthy soil.
we cultivate, we're growing plants more intensively than nature
ever meant, and this presents a problem.
If we continually crop
our soil, the soil organic matter slowly declines as the soil microbes
decompose it. Every time we remove a crop from the soil, whether
it be carrots or cabbages or potatoes, we also remove the nutrients
with those crops, and our soil can become slowly depleted.
So we have to make sure
that, firstly, we maintain organic matter, and secondly, we maintain
the supply of nutrients.
WHAT IS SOIL ORGANIC
Soil organic matter includes
the fresh organic plant material that's being added to the soil
the decomposing remains of spent plants or kitchen scraps
for example. And when it's thoroughly decomposed and formed what
we call humus, that's soil organic matter.
WHY IS IT SO IMPORTANT?
It contains nitrogen,
and that nitrogen is going to be liberated and supply nitrate for
plants to grow on. It also greatly improves the soil structure.
WHERE DO YOU GET IT
We've got lots of sources
of organic matter in New Zealand. Firstly, we can bury our kitchen
waste in our soil, or make it into compost. We can bury our crop
residues as well, or make it into compost. Sawdust is a good source
of organic matter, but it takes a lot of nitrogen to break it down,
so it should be composted beforehand.
I like to dig my crop
residues straight into the soil let the composting take place
there. It's going to be decomposed by earthworms, beetles, microbes
of all kinds, bacteria, fungi. When they decompose it, they excrete
ammonia, which gets converted to nitrate by soil bacteria. Ammonium
and nitrate are the two forms of nitrogen that all plants that can't
fix their own nitrogen from the air require from the soil if they're
to grow. That's why it's so vital to keep organic matter levels
up in the soil.
I find that digging in all my crop residues and growing green manures
is still not enough to maintain my soil in good physical condition,
so I do have to bring in compost.
We've got a good range
of composts in New Zealand. We've got compost like Living Earth,
made from garden waste mainly. We've got compost made from the gut
contents of slaughtered animals, composted with bark. We've got
compost made with sawdust and cow manure; sawdust in pig slurry.
We've got poultry manure, collected on litter of various kinds.
We've got spent mushroom compost. We're fortunate in having such
HOW MUCH ORGANIC MATTER
DO YOU NEED?
To get that lovely friable
condition, I find that, in addition to my green manures and my crop
residues, I need to put on about a 5 cm depth of compost over the
whole garden. That means a cubic metre will do about 20 square metres
And when do I know that
I need it? Well, my soil will become sticky, lumpy, cloddy. It becomes
difficult to dig, difficult to make seedbeds. You'll know when your
soil's poor. Get lots of organic matter into the soil.
Whilst there are many
things we can use to add organic matter to our soil, there's one
problem they can all differ very much in their chemical composition.
They improve the PHYSICAL condition of our soil, but they can claim
very different amounts of nutrients. Some will supply a lot of nitrogen.
Some will supply very little. Some will be rich in potassium.
Poultry manure is very
rich compared with most. Why? Because, as far as I know, birds have
only got one hole. So it ALL comes out the liquid and solid
all mixed up together. (It is a very rich material so it probably
needs composting with sawdust or straw before use).
Whereas if you just collect
horse manure without the urine or cow manure or sheep manure
without the urine you've got valuable nutrients missing.
Because in the urine of mammals like us, we find nearly all the
potassium we excrete and 75% of the nitrogen we excrete is in our
urine as urea. 75% of the sulphur we excrete is in our urine as
sulphite. So urine is a very valuable liquid fertiliser.
Sheep pellets straight
from the shearing shed with the urine mixed in would be a rich source
of organic matter and nutrients.
I often use a mushroom
compost made with poultry manure. I like it because of the lime
it contains, and I've used it a lot in my veggie garden.
FIXING NITROGEN IN
Legumes are nature's
way of building up organic matter in the soil. But we did not know
until 1885 why legumes were so important. They can fix their own
nitrogen from the air, and the reason they can do it is because,
if you dig up a clover plant an invaluable legume
you will see, scattered over the root system, little nodules about
the size of pinheads. Those nodules are filled with thousands of
bacteria which fix nitrogen from the air.
Clovers have been vital
in our agriculture in New Zealand. We've depended on them in our
pastures to provide lots of nitrogen and food for our animals.
In Marlborough they're
beginning to grow clovers between their rows of vines to supply
not only nitrogen and organic matter, but also to attract the hoverfly,
which controls aphids.
And we can put legumes
to use in our gardening too. We can grow peas, beans, clovers or
lupins to build up nitrogen and organic matter.
This year I've decided
to grow red clover as a green manure. We've got a cultivar named
Pawera that's a winter-growing clover, and I want to see if it does
well in my soil over winter.
Sow it directly into
the hoed soil and rake the seed in. Then gently compact the soil.
That's to keep the moisture rising.
Remember, there's an
important place for legumes like lupins and clovers for us to dig
in and add organic matter and nitrogen to our soils. I can strongly
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