The reason is that clay contains a lot of colloidal (ie very very small)
particles that have a high surface area and a surface charge. This has two
One that they bind water, this accounts for behaviour of clay soils in that
they swell when wet and shrink when dry. So a soil with a modest amount of
clay will hold water much better than just sand or pebbles whose surface
area is much smaller and don't bind water. If you have a reasonable amount
of clay this is advantageous to growing, especially active annuals like
cucurbits that wilt on hot days if the soil doesn't supply enough water. In
my case I have another benefit that the solid clay sub-strata acts a
reservoir evening out the erratic rainfall of the region. It takes up water
during wet spells and gives it back in dry.
Two that clay binds minerals. The cation exchange capacity and total
exchange capacity is much higher than sand. So clay soils don't leach
minerals nearly so easily and stay more fertile for the application of a
given amount of fertiliser than sand. The disadvantage is that if you have
undesirable minerals such as sodium ions these will bind and displace
desireable ions like calcium, magnesium etc. This sodised soil is unfertile
and very hard to remedy.
Provided you take care not to work it when too wet or too dry and deal with
drainage issues soil with a reasonable amount of clay will be more
productive than that with very little or none. I think Billy has some
tables of the desireable amount of clay in garden soil.
(Awed) I sit at your feet!
Question: Why equate clay to sand? Isn't that going to extremes? My
soil is, AFAIK, a good loam (I see that term is still used). The clay-
ey (adobe) soil characteristic of this area has been greatly modified
by application of various Good Things over time. I inherited pretty
good dirt from the sellers [censored] years ago, and have continued to
apply compost and worm castings, as well as small amounts of Other
Stuff, so it's nice and friable.
Equate no, contrast yes. Yes they are extemes, I said that to illustrate
the difference not to recommend either. They are ends of the spectrum in
particle size but also in minaral composition. Sand is largely silica which
is almost inert with respect to water and minerals. Good soil is neither
extreme but has both (as well as organic matter) avoiding the drainage and
workability problems of too much clay and the poor water and mineral holding
power of too much sand.
Have a look at this:
Which shows the situation in three dimensions instead of the two that I
used. It also has some practical tests so you can work out what you have.
It is well worth learning and performing such tests on your soil.
"Clay soils have over 25 percent clay. Also known as heavy soils, these
are potentially fertile as they hold nutrients bound to the clay
minerals in the soil. But they also hold a high proportion of water due
to the capillary attraction of the tiny spaces between the numerous clay
"Sandy soils have high proportion of sand and little clay. Also known as
light soils, these soils drain quickly after rain or watering, are easy
to cultivate and work. They warm up more quickly in spring than clay
soils. But on the downside, they dry out quickly and are low in plant
nutrients, which are quickly washed out by rain. Sandy soils are often
very acidic "
IME there are certain areas of sandy soil in various parts of the world
all well-known for nutrient problems. I believe the Pine Barrens in NJ
have a very select flora, and most certainly the lateritic sands of
Western Australia are extremely low in phosphates. So much so, in fact,
that if normal (balanced) fertiisers are used on Proteaceae from this
region you might just as well have given them a herbicide!
With my grey clay and rock soil, I fond a marked improvement adding
organic material (5%), and sand (<30%). Till that in, and then grow rye,
or buckwheat as a cover crop. I tilled once, have kept it mulched, and
when I pull up plants, most of the root system follows (the soil is very
friable now, and full of worms). I only grew those cover crops once. If
you grow annuals, mulch during the off season.
Welcome to the New America.
No - not a good idea unless you go up rather than dig down. If you jsut dig
down and replace what is already there but leave allt he surroudnign clay,
you will in effect be building a bog garden if you get lots of rain.
You've probably been cutting it too short. People who are into 'lawns'
often do that because they don't think of it as being made up of 'grass'.
Set your mower at it's highest level and give the poor grass a chance and if
anyone starts making silly comments, tell them it's grass, and the funtion
of grass is groudn cover, weedy species suppression, dust abatement.
Dunno, but I'd suggest 2 things - a walk round the neirghbourhood with a
notepad and one of the Hessayan books on shrubs would be the way to go given
that you are int he UK - a local library would probably have one or be able
to get one for you.
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