When I first moved into my current home a little over 15 years ago, the half acre of land around my house was a wild tangle of oaks and plum trees and ivy, wild roses, California sage, blackberry bushes and assorted unidentified brambles and native grasses. Undaunted, I began clearing the first area I planned to transform into gardens. What I discovered, almost immediately, was a soil so compact and hard it resembled cement. Yikes! Like many in the Bay Area, I had come face to face with our local CLAY SOIL.
In the years that followed, as I gradually tilled my soil and transformed some of the wild tangled spaces into colorful perennial beds, I learned a great deal about clay soil and how to utilize it’s strengths to enhance the garden.
In order to understand how to work with clay soil, it is important to know a few things about soil in general. The mineral part of any soil (as distinct from the organic matter) is composed of sand, silt, and clay particles. The “texture” of soil refers to the relative size of the particles: sand is the coarsest of the three components, while clay particles, reduced to microscopic size by weathering, are the smallest. Most soils are a mixture of sand, silt and clay, and ideal soil texture for a fertile garden is considered to be somewhere in the neighborhood of 10-20 percent clay, and about 40 percent each silt and sand. This type of soil, called garden loam, tends to be open and porous with crumbly, pea-sized particles. Clay soil, that is, soil that is composed of over 40% clay particles, tends to be sticky and dense and doesn’t drain well. If your soil forms a hard crust when dry, and you find persistent puddles on the surface after a rain, then it has a higher than ideal proportion of clay.
It is not possible to change the basic texture of your soil, except by extreme measures such as total soil replacement (not recommended). The earth outside your window has offered up clay, and it is clay you will have to work with. (Incidentally, because individual clay particles are negatively charged, they have the capacity to attract and hold onto other elements such as potassium, calcium and magnesium, making clay soil more fertile than sand). Instead of bemoaning the clay, a better idea is to work on improving soil structure. Structure refers to the ways soils hold together: clay soil particles typically arrange themselves along a horizontal plane in plate-like structures, making the soil tight and sticky. Your aim in improving the structure of clay soil is to achieve a looser, more crumbly or granular aggregation.
So how do you transform that impenetrable expanse of rock-hard clay into a rich, fertile bed of loamy earth ready for planting? In a word: DIGGING. Soil cultivation is hard labor, and it is good to realize this before you begin. Start with an area that isn’t too large for the condition of your back or the constraints of your calendar – in other words, start small. You can always expand your cultivated area next month, or next year.
The basic idea in improving clay soil is to incorporate as much air and organic matter into the soil as possible, breaking it up and developing over time a soft, loose structure. Organic matter improves the soil in a couple of ways. First, the organic matter coats soil particles, physically separating clay particles and aggregates from each other. Second, and more important, microorganisms that degrade organic matter produce byproducts called glomalin that bind individual clay particles together into aggregates. Particle aggregation in the topsoil reduces crusting, increases the rate of water infiltration, and reduces erosion and runoff.
So here is what I do:
When the clay soil is neither too wet nor too dry, I use a gardening fork to break it up to a depth of approximately 1 foot. (To determine if your soil is right for tillage, roll some soil between your palms into a ball about an inch in diameter – if the moisture is right, a pinch between thumb and finger causes the ball of soil to fracture, break, or crumble.) A rototiller can also be used to break up the soil at this stage. Once the clay soil has been broken up, I lay down a 2-3 inch layer of organic matter and work it deeply into the soil. Compost is good, and excellent organic soil amendments are available from American Soil and Stone as well as local nurseries. Once the bed is well tilled, I water and then let the bed settle for one or two days before planting.
Soil improvement is not a one time event – if you continue to work with the soil, adding organic matter each season and working it gently into the earth, your soil structure will continue to improve and the land will reward you with a bountiful garden.