Tuesday, May 31, 2011

Here's the Dirt: Part 2 - Different Types of Soils

When we talk about types of soil, we are usually talking about whether the soil is predominantly sand, silt or clay. It is important to know the difference because gardening in sandy soil is quite different from gardening in soil that is mostly clay.

The reason for this is that the size of the soil particles is very different. A particle of sand is much bigger than one of silt, which is much larger than a particle of clay. To illustrate this, I've lined up some items on the windowsill to help you get a visual sense of the differences.
The cherry tomatoes on the left represent the large particles found in sandy soil, the rice (center) is like the much smaller particles of silt and the flour, at right, has tiny particles similar to clay. (Don't hold me to these exact proportions: this photo is just to give you way to think about this.)

Now imagine what happens when it rains. If you have sandy soil, it's like putting the cherry tomatoes in a colander and running water over them. As soon as you turn off the faucet, water runs right through those big pore spaces, leaving just a thin coat of moisture on surface of the tomatoes. If you are gardening in sand, you know this - without organic material added to the soil to soak up and hold onto water, it dries out in no time. 

You can think of silty soil as being like the grains of rice. When you run water over rice, it drains slowly, leaving quite a bit of moisture both coating the grains and held in the small spaces between them. 

Clay particles behave a lot like flour. They are so tiny that they pack in tightly together with very little space between them. If you pour water on flour, it will just sit there until you mix it in. Once you've mixed it in, however, it is very hard to completely dry it out. 

OK, so that's how soil particle size affects water retention. Particle size also matters when it comes to nutrients. Ions of nitrogen, potassium, phosphorous, calcium, magnesium, sulphur, etc. cling to the outer surface of soil particles where they can be taken up by plants. Smaller particles, like silt and clay, have far more surface area per volume than those big particles of sand and therefore can hold a lot more nutrients. 

Bear in mind that this is a very simple explanation of soil types. It is enough information to help you make some reasonable decisions about how to improve your soil, which I'll talk about in the next post on soil amendments. But it by no means covers the subject. Healthy soil is more than particles, it includes organic material and many living organisms working together. I strongly recommend that gardeners learn more about that fascinating world under our feet. James Nardi's book, "Life in the Soil: A Guide for Naturalists and Gardeners," is a good place to start. 

Remember that unless you are right on a sandy beach, your soil is likely some combination of particles. If you aren't sure whether it is mostly sand or clay, you can do the "squeeze" test. Pick up a handful of moist soil and try to make it into a ball. If it packs nicely into a ball, it has a lot of clay in it. If it crumbles without holding a shape, it is mostly sand.
Wondering how much mulch, soil or compost you should buy for your gardening project? Check out this handy table.
Read part 1 of this series - Here's the Dirt: Introduction

Monday, May 30, 2011

Here's the Dirt: on Soil, Compost and Mulch - Introduction

When you go to a garden center, you'll find lots of bagged goods with names like potting soil, planting soil, top soil, bark, fine bark, play chips, compost, mushroom compost, planting compost, etc. There's chicken manure and steer manure and bat guano (more manure). Some companies sell three-way and five-way mixes of various ingredients that claim to be excellent growing media.

With so much available, how do you choose which, if any, of these products are right for your garden? The first step is to understand what these materials really are. There's a lot to say about some of them, so I'm going to do a series of posts to tell you about soil, soil amendments, compost and mulch.

First up - soil. 
Soil, also called dirt, is the foundation of your garden. The health of your soil determines the health of the plants you grow. If you are at all serious about gardening, particularly if you plan to grow some of your own food, you owe it to yourself to learn about the soil you have and how best to maintain its tilth or health.

Technically, soil is a combination of sand, silt and clay particles with some amount of organic matter mixed into it. There is no precise recipe for soil. The soil you have is the product of the wind, weather and geology of your area.

Thanks to the action of glaciers long ago, soils in the Seattle area are varied. Some of you are gardening in sandboxes. Some of you are trying to break up clay. If you are gardening in an area along a river, you might have silt. Many of you are finding rocks the size of your fist nearly everywhere you dig. All of these scenarios are the gifts of what is called "glacial till."

The combination of sand, silt and clay particles you find in your garden is what makes up the structure of your particular batch of soil. Organic material comes and goes over the seasons. Nutrients and water are taken up by plants. Depending on the health of your soil, microscopic organisms thrive or struggle. Through it all however, year after year, those sand, silt and clay particles remain, providing the stage where all the rest is played out.

There are many things you can do to improve your soil, but there is nothing you can do that will change the type of soil you have. A clay soil, for example, cannot be changed into sandy loam, no matter what you add to it. (In fact, as I'll explain in a later post, adding sand to clay is a bad idea.) Your best bet is to know what type of soil you have, what its characteristics are and how to make the most of what you have.

In Part 2 of "Here's the Dirt," I explain more about soil types.
Wondering how much mulch, soil or compost you should buy for your gardening project? Check out this handy table