I know a lot of people that enjoy gardening from time to time and they all say “I hate our clay soil! it’s impossible.” Yes. Clay soil can be a complete pain in the ass to work with. But, I liken it to those who have thin hair and those who have thick hair — each person wants what they don’t have. Sandy soils drain well, warm up faster in the spring, and are easier to dig. But sandy soil also, has a hard time retaining moisture, doesn’t aggregate well (bind together in nice crumbs), nor is it nutrient rich. Clay soil is the exact opposite. It’s difficult to dig, drains poorly (as you have probably found that the soil stays wet for a long time and puddles when it rains), and it compacts as soon as you reach for any kind of garden implement. So what’s best? It really depends on what you are doing and trying to grow. Many CA natives and mediterranean plants will perform better in a sandy soil. But if I’m growing vegetables or annuals, I’d take a clay soil any day over a sandy one. Why?
It really just comes down to two things: nutrients and water. Annuals and vegetables perform a lot of work in one season. They have to grow fast, make flowers, and set seed so they can reproduce. All of this work takes a lot of nutrients and moisture. Clay soil can hold so much more water and plant nutrients than a sandy soil it’s almost unfair.
There’s a great amount of science underneath it all and I am by no means a soil scientist. I do have a decent understanding of the processes at work and I find it fascinating. I took a wonderful soils class a couple years ago at Merritt College in Oakland, I have read up on the subject, and have performed some of my own experiments over the years. I will explain it as I know and understand.
Particle shape and size is directly related to the total surface area of a given soil volume
If you have ever taken a pottery class, the first thing you do with the clay before you actually make something is ‘wedge’ it, or knead it a bunch of times, to get the flat particles to align with each other. This work compacts the clay and makes it denser and stronger (absolutely not what we want in a garden but true to the point). Individual clay soil particles are very tiny and shaped like thin, flat, sheets. Since clay particles are small and flat in shape, clay soils have a tremendous amount of surface area and places where charged particles can reside. Sand particles, on the other hand, don’t really compact, are larger, and spherical in shape. So, for any given volume of soil, a sandy soil will have less particle surface area than a clay soil. I think it helps to think of two large bowls. Imagine that one is filled with marbles and that the other is filled with fish food flakes. The total combined surface area of all the flakes (if you laid them out end to end) would be much greater than the total surface area of the marbles.
The total surface area of the soil particles is related to the number of Cation Exchange Sites (places where nutrients can be held on the particles)
The surfaces of the soil particles are charged based on their molecular components (chemistry here — think of electrons and attraction) and contain cation exchange sites. These sites attract and accept nutrient molecules. You can read more about CEC (Cation Exchange Capacity) but what is important here, is that clay has a tremendous amount of cation exchange sites compared to sand. Charged nutrient molecules in the soil solution (such as nitrate, phosphorus, and potassium) become attracted and bound to these sites to be later taken up by plant roots. More sites means more nutrient holding capacity and ultimately more stored plant nutrients in the soil.
Lots of smaller air channels (micropores) can collectively hold more water than a few larger ones
Holding water is another thing that clay does well. There are three forces that we care about acting on water in the soil profile; gravity, adhesion, and cohesion. Gravitational forces are what causes water to fall toward the center of the earth, (the same forces that makes us stick to the ground). Adhesive force is the attraction of water to the soil particles (like water beads stuck to your shower door). And then there is cohesive force, which is the ability of water to molecularly bind with itself and can be seen when water beads up or pools together.
If we think back to the bowls of flakes and marbles try and imagine what the spaces around the corresponding particles looks like. There are many more and much smaller spaces around and between the fish food flakes. Whereas the marbles have fewer and much larger spaces between them. The spaces between the marbles will lose a lot of water due to gravitational force (water will fall through the marbles) and less will stick around because the surface area is smaller. With clay soils, the greater surface area means more more water will adhere. And rather than falling through the soil, water will actually be pulled along through the small channels via capillary action. This results in more water being retained per a given volume of soil.
So generally, if you hear that a soil is clayey it means it has the ability to hold lots of nutrients and lots of water… perfect for vegetables. A well amended clay soil (amended with lots of compost) can’t really be beat for tomatoes. And if it happens to be insanely compacted — well, then there are always mud pies.