Gardeners ask how to improve clay soil more often than 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.
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.
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.
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 coarser 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.
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 coarser 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 coarse 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 coarse sand or even gravel, the finer soil must become very 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 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 coarse and fine soils together.
The answer is the same for both: organic matter.
Compost is what I’m generally referring to. Amend soil with 6 inches of good compost. Work it right into the top of a clay soil and it will improve infiltration and will probably improve the amount of air and water available to your plants.
(I should mention that no amount of organic matter, rototilling and aeration will fix a serious drainage issue such as flooding. That needs to be addressed by installing drainage, or even better, work with nature and put in a pond.)
Back to compost. Improving sandy soil with 6 inches of good compost will drastically improve the water-holding capacity of the soil.
Since we’ve been learning about how how to improve clay soil and improving sandy soil and specifically about water moves through the soil, I’d like to mention a couple of other important organic gardening implications of what we’ve learned about amending soil.
Take patio pots for example. Gardeners will often put a layer of gravel in the bottom of a container to improve drainage, but as we have seen it actually does the opposite. The soil on top has to be very saturated before the water will drain through. This is not necessarily a bad thing with the right plants, but it is important to know how this works.
The other main implication is with regards to organic matter. If you amend soil by turning in (burying) course organic matter such as leaves or straw and they end up buried in a layer, or if you drop 6 inches of soil on top of a big layer of course material such as sticks or straw, you could be creating a course layer underneath a fine layer.
This is the same situation where that soil will get very wet before water will drain out. If straw is incorporated very well, this will not occur, but if it is just turned under, it will.
If you need to bring soil into your yard in order to build your garden, you must make sure to rototill it in to the existing soil extremely well. If you just drop it on top, you are inevitably creating an interface that will slow drainage.
And try to find a soil that has a similar texture to your exiting soil (i.e. if your soil is clay, bring in more of a clay topsoil than a sandy topsoil). Also, use compost for perhaps 25% of the mix.
Is there anything organic matter can't do?