Gardeners ask how to improve clay soil more often than asking about improving sandy soil, but the reasons are generally the same, and the main reason has to do with water.
In many gardens, clay doesn’t infiltrate and drain fast enough and sand drains too fast. One of the most commonly given pieces of advice on how to improve clay soil is to add sand. When it comes to improving sandy soil, the advice is often to add clay.
Both of these are poor organic gardening practices. Before we go into why, we need to take a quick look at how water moves through the soil.
Trust me, this is really good to know and quite interesting.
How Water Moves Through Clay And Sandy Soil
Water moves downward after rain or irrigation and upward to eventually evaporate from the soil surface.
This water flows through the open “pores” between soil particles. In any soil that is not dominated too much by sand, silt or clay, approximately half the soil volume is pore space. Water and air share this pore space.
When soil is entirely saturated with water, gravity forces the water to move very quickly through the big pores, but the rest of the time, gravity doesn’t play as big a role in how water moves through the soil.
The rest of the time, adhesion (how water molecules tend to stick to other surfaces) and cohesion (how water molecules tend to stick together) govern the movement of water in the soil. Interestingly it moves out in all directions fairly equally - up, down and horizontally. It moves downward only slightly more due to gravity.
Adhesion and cohesion in action
So let’s say it’s a beautiful Saturday morning and you are doing some organic gardening. Let’s look at what happens when you have layers in your soil.
How To Improve Clay Soil
Let’s say you have a clay or silt loam soil that doesn’t infiltrate or drain well. What happens if you add 6 inches of a courser soil such as a sandy loam on top of a the soil? When it rains, the water slows down when it hits that fine soil layer as you might imagine, although it does continue to move through.
Still, it slows down, which is the opposite of what you were going for. If you instead rototill the sand into the clay, it doesn’t create a nice soil texture like you would think. The sand just gets embedded in the clay and often forms a soil environment that is like concrete.
When deciding how to improve clay soil, adding sand is not the answer.
Improving Sandy Soil
This part is really interesting. Let’s reverse it and say you have a sandy soil that doesn’t hold water. What happens if you add 6 inches of a finer soil on top of a courser soil below? This also may happen if the 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 course sandy layer, but in fact, water movement stops until the soil becomes nearly saturated above.
Even more interesting, if the finer soil is on an extremely course sand or even gravel, the finer soil must become very wet before water will move down through the course 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 layer of sandy soil and be available to the short roots of the grass on the green, rather than draining away.
But doing this in a home organic garden is dangerous because you may create the opposite problem, which is a very waterlogged soil, or you may make a soil that is like concrete if you rototill the course and fine soils together.
In my next article, I outline what you should do instead.
