Xref: 127.0.0.1 uk.rec.gardening:160193

On Mon, 3 Feb 2003 20:24:39 -0000, "pp" wrote:


Can you suggest a simple solution to improve the drainage of my heavy clay
soil.
snip

No! How I would like to find one.

Other responses to your message have touched on several possible
methods of improvement of clay soils in general. Your particular
situation of a wet depression over clay definitely needs a field
drain! But that will still leave the problem of soil improvement.
Here are my limited experiences, and a request for information.

A few months back I took on an allotment in the Thames area with
approx 8" of heavy, sticky topsoil over fairly solid clay; the site
does have a reasonable slope to it. My experience of vegetable
gardening on clay many years ago suggests the problem is two fold.
Not only does the clay prevent drainage in wet conditions - to the
extent of drowning roots; when the clay dries to the stage of
'cracking' in dry conditions, it allows the soil to dry out
excessively, providing no moisture retention. So Improving moisture
retention should be considered as part of improving the drainage.

Having 'dug about' for information on how to deal with the clay
problem it would seem first, that there is no real prospect of being
able to deal with the solid clay subsoil layer itself.
A study of Agricultural type improvements to clay soils suggests that
deep ploughing to break the clay layer is of limited use on heavy
soils as the inevitable mixing of the clay with the upper soil layer
is likely to result in the latter become unworkably heavy. It is also
reported that ploughing to 'depths of three feet or less usually do
not accomplish very much.'
http://clic.cses.vt.edu/icomanth/soilmodi.html Not encouraging news
for the small holder!
However if the gardener does not wish to grow deep rooted items (most
vegetables, for instance) it may be that it is sufficient to think in
terms of improving just the topsoil or that and the layer of clay
immediately below the topsoil: perhaps the equivalent of
'double-digging'; and as this lower layer is not usually disturbed,
any improvement there could be long lasting.

The following methods of improving a clayey topsoil have been
suggested

a) sand or grit, but they may just bind with the clay (as in concrete)
unless in sufficient quantity (50 - 80%); they will not have a major
effect on moisture retention.

b) gypsum is said to bind the clay into distinct particles, rather
than remain an amorphous mass. The theory is that this will improve
drainage and moisture retention. Lime is said to have the same
effect, but alters the ph of the soil.

I applied approx 8oz gypsum per sq yd to the allotment in autumn. So
far the effect has not been discernible (perhaps not surprising as the
weather and the soil has been consistently soggy). I am now applying
a further 8oz per sq yd, having found a recommendation for a total
application rate of about 16oz per sq yd. However I do not expect this
treatment to affect more than the topsoil; I certainly hope it does
not take 30 years to show!

c)compost/manure with a lot of organic matter will help provide an
open structure in the topsoil, and retain moisture.

I applied a good dose of at least partially composted manure, some
last autumn, some recently. Of the manure applied last autumn, it has
been noticeable that the manure based on woodshavings has been the
most effective in 'opening' the topsoil and keeping it from
re-compacting. It remains to be seen whether the nitrogen demand when
it decomposes will outweigh its advantages

d) raised beds will provide a deeper level of topsoil and free
drainage to ditches surrounding the beds.

I have divided the plot into four raised beds with sunken paths
between to assist drainage. A ditch at the top end of the lot, to the
depth of the clay layer, should break the flow of water underground,
in wet conditions, from uphill of the plot.

e)calcined clay (clay heated to a degree where it becomes granulated)
provides drainage and moisture retention.

Anyone who has gardened on clay soil will have noticed how clayey soil
that emerges from the bonfire is converted into an amenable gritty,
porous substance. Clay granulate is now often offered as a major
ingredient of potting mixes. I seem to recollect that this was an
early (18th C?) method of improving clay soils agriculturally, but I
cannot now trace any reference to the practice. There are obvious
practical difficulties, and a lot of labour, involved in digging up
enough clay to make a difference and passing it through a fire; but if
you do need to have a bonfire, it could be worth digging up some
subsoil to calcine it.

f)another method I intend to try is the incorporation of wood chips,
either in the topsoil or in the layer below in the course of a
'double-dig'.
Wood chips provide an almost completely open structure. Yet with the
addition of some moisture retainer (such as peat) and a nutrient
source (such as slow release fertilizer) plants will grow in the mix
with astonishing rapidity. My experience (some years ago, in USA) has
been with growing plants in containers in such a mix, but using pine
bark chips; they are preferred as they decompose very slowly, and
therefore have limited nitrogen demand.
In the UK I am not sure if pine bark chips are available; if so I
suspect they would be somewhat pricey. Regular hardwood chips, at
least when raw. should be readily available at minimum cost, as tree
chippers often have to pay to dispose of them. (Three months
composting will reduce the future nitrogen demand substantially.)

Literature dating from the time of my experience with growing plants
in containers (Plant Production in Containers Carl C. Whitcomb) only
provides two references to trials substituting hardwood chips for pine
bark chips. In one, woody ornamentals performed equally with controls
grown in pine bark chips; in a second, geraniums and chrysanthemums
grown in a greenhouse trial, performed poorly compared to their
controls. The reasons for the differing results were not determined.

Used as a soil amendment rather than a growing medium the proportion
of woodchips to soil would probably be sufficiently low that a
negative effect on the growth of plants from incorporating woodchips
would seem unlikely, given that any nitrogen demand were compensated
for.

Woodchips do not seem to bind into clayey soil in the same way as grit
- perhaps because each chip has its own internal moisture channels and
is also continually subject to the changes of slow decay. They would
be much longer lasting than the straw or similar organic matter in
manures or composts. As they decompose they would become even more
moisture retaining. If incorporated in a 'double dig' I can envisage
they could created a transition layer between the topsoil and the
solid clay layer which would mitigate the disavantages of the latter
in both wet and dry conditions.


Because I have not yet conducted any trials with woodchips, I would
much appreciate information from anyone who has done so, in particular
by incorporating raw or composted woodchips in a clayey soil as a
soil amendment (and not merely as a surface mulch).



regards

jt