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Posted by David Hare-Scott on February 21, 2010, 5:11 pm
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anthony123hopki wrote:
> Hi
>
> I am preparing my raised beds for some early spring sowing. I put in
> some manure a few months ago. Over the last year or so the beds have
> really settled down and so there is plenty of room to add more to
> them.
>
>
> I was wondering if it is better to add pure compost or topsoil or a
> mix of both. Does pure compost have all the nutrients a vegetable
> needs, and do the nutrients last very long, or is topsoil longer
> lasting? Also, what is a "soil improver"?
>
> My soil is tending towards to clay side of things.
>
> Thanks for any advice.
It depends on the composition of what is there and how much you need to add.
If what is there is still heavy, tends to set hard and has low organic
matter add compost or manure. If it is friable and has good organic matter
but needs to be topped up quite a bit add topsoil or topsoil and
compost/manure. If it just needs a bit add compost/manure.
Whether pure compost has all the nutrients depends on its sources, it is
possible that it does but it is not certain. A compost made from just one
source probably is not complete. The time that the nutrients in any soil
last depends on the situation. The one thing that you can say about
gardening that is almost always true is "it depends".
This is a vry complex subject so I am skimming here. Nutrients do not last
long if they are lost quickly to the environment. The main method of loss
is running off when dissolved in water but evaporation can play a role too.
Run off happens more when nutients are present as very soluble compounds or
ions and when the soil has little capacity to bind them. Binding takes
place on the surface of colloids which are found in clays and in broken down
organic matter. It is not the source of the nutrients that determines how
long they last but the environment that they are in.
What you should be aiming for is balance in tilth and infiltration, and
retention of water and nutrients. Very sandy soil has little binding power,
pouring on chemferts will have only a short term effect, most of it will end
up in the nearest waterway helping the water plants to grow. Heavy clay has
much binding power but it is so impervious that there is no room for air and
plant roots and it takes ages for water and nutrients to infiltrate. A soil
that has a balance of clay, sand and organic matter is workable and can
carry nutrients, air and water. Such soil allows water to infiltrate and to
drain away reasonably quickly but holds some.
David
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