Agriculture Reference
In-Depth Information
How to Improve Heavy Clay or Sandy Soil
Gardeners often ask how to amend a heavy clay or sandy soil to improve water infiltra-
tion and drainage, air content and nutrient-holding capacity. In many gardens, clay doesn't
infiltrate and drain fast enough, while sand drains too fast. Clay may have a good cation ex-
change capacity, but not enough air and increased potential of compaction. Sandy soil may
have plenty of air and resist compaction, but not hold onto nutrients long.
The common advice is to add clay to the sandy soil, or add sand to the clay soil. My ex-
perience has been that both of these approaches are generally bad ideas. Before we go into
why, we need to take a quick look at how water moves through the soil. After rain or irriga-
tion, water moves both downward to the water table and upward to eventually evaporate
from the soil surface. This water flows through the open pores between soil particles. In
any soil that isn't dominated too much by sand, silt or clay, roughly half the soil volume is
pore space, and water and air share this pore space.
When soil is entirely saturated with water, gravity forces the water to move down
quickly through the big pores, but the rest of the time, gravity doesn't play a big role in
how water moves through the soil. Most of the time, adhesion and cohesion govern the
movement of water. Adhesion is how water molecules tend to stick to other surfaces and
cohesion is how water molecules tend to stick together.
We definitely don't want to bring in a layer of topsoil and just spread it on top of the ex-
isting soil. Let's look at what would happen if we did that. Say you have a clay or silt loam
soil that doesn't infiltrate or drain well. What happens if you add six inches of a coarser
soil such as a sandy loam on top of the finer soil? When it rains, it may infiltrate better, but
as you might imagine, the water slows down when it hits that fine soil layer, although it
does continue to move through the soil. Still, it slows down, which is the opposite of what
you were going for.
Let's reverse it and say you have a sandy soil that doesn't hold water. This part is really
interesting. What happens if you add six inches of a finer soil on top of coarser soil below,
or if the original garden builder brought in some topsoil that was clay based and put it on
top of your sandy subsoil? When it rains, you might think the water would speed up when
it hits the coarse sandy layer, but in fact, water movement stops until the new soil becomes
nearly saturated above. Even more interesting, if the finer soil is on an especially coarse
sand or even gravel, the finer soil must become extremely wet before water will move
down through the coarse layer. In this case, the overlying soil can hold two or three times
as much as it normally would.
These same principles are often used when making golf greens. A layer of gravel is used
underneath the sandy soil for the green in order to create a situation where water will stay
in the upper, relatively fine layer of sandy soil and be available to the short roots of the
