Soil is more than something to scrape off your shoe – it’s the lifeline of the garden. There are those who say that soil is “alive!” While this is partially poetic license, it’s not completely inaccurate. Much of soil is inert mineral particles – sand, silt and clay – but they house an underground ecosystem. A healthy soil is full of living organisms including insects, fungi and bacteria, as well as vigorous plant roots.
Learning about the physical, chemical and biological properties of your particular soil helps you garden ecologically and economically. If you know what you have, you are less likely to waste money on excessive water, fertilizer, or the wrong plants. A soil test is needed to find out the chemical composition, but physical texture can be easily analyzed at home.
This year I am taking a 14-day Permaculture course at the Regenerative Design Institute in Bolinas. The 14 days are spread out over a year, one or two days per month. This weekend the topic was soil, and I had a fun opportunity to do some simple science and analyze the soil texture of 20th Street.
Soil texture is about the percentage of sand, silt and clay particles in a particular site and can vary greatly from place to place. Sand particles are large, easily seen, and feel gritty. Sandy soils have a lot of air pockets, which plant roots like, but sand does not hold onto nutrients and water very well. Clay particles are microscopic with few air spaces between them. Clay soil will feel sticky. Although clay soil gets a bad rap, clay particles hold water and nutrients for plants to use. Silt particles are somewhere in between and have some of the advantages and disadvantages of both sand and clay. Silty soils feel smooth and silky.
The Soil Texture Triangle shown above illustrates the various kinds of soil textures. We can easily figure out the soil texture in a particular place by collecting a sample and doing this fun analysis. Try it in your garden, or with kids!
First I collected a soil sample from the block of 20th Street between Florida and Alabama. I scraped away the top layer of leaves and organic matter and got into the soil itself. The top 6-8 inches are where most roots grow, so that’s where it’s important to analyze. I took a little bit from each tree well to create a blended sample, so the results better reflect the soil of the block itself. I took out roots and rocks from the sample as much as possible and collected enough to fill a glass quart jar halfway.
Then I added water and a squirt of soap. The soap breaks up any clumps in the soil and separates the individual particles. I shook the jar vigorously and then let it sit for at least 24 hours.
The sand and gravel particles settle first, in just a few minutes. Silt particles settle in the next few hours. Clay particles stay in the water solution for a while. Very fine clay takes a few days to a week to completely settle. Various bits of organic matter stay floating on the surface or eventually settle on top of the clay – don’t pay attention to those.
Now here’s the science part. I measured the total height of the soil sample and compared it to the total height of each of the layers. The results were:
• Sand: 80%
• Silt: 15%
• Clay: 5%
According to the Soil Texture Triangle, the 20th Street Garden has a Loamy Sand texture. To verify this, I visited the USDA online Soil Texture Calculator and entered the percentages, getting a similar result of Loamy Coarse Sand (the USDA recognizes different grades of sand from coarse to fine).
What does this mean? The coarse sandy texture on 20th Street means a well-drained soil with a lot of pore spaces for air. Roots do need air as well as water so this is good – soil does not become waterlogged. However, this soil does not have a lot of water and nutrient-holding capacity. It’s important to water a little in summer to keep plants looking good. Drought-tolerant plants that can live in nutrient-poor soils are a must for this site. Fortunately, those are the kind of plants growing here.
Soil texture cannot be radically altered. Even if it were practical to rip out all the plants, take away the top 2 feet of soil, and import a new texture of topsoil, this would cause problems with drainage and would be unlikely to change enough of the soil to make a difference. Instead, we can improve pretty much all soil textures by the regular addition of organic matter in the form of compost and mulch. Compost contains nutrients that plants can use, feeds beneficial soil organisms, and over time helps improve drainage for clay soils and water retention for sandy ones. Mulch shades and cools soil, slows water evaporation and eventually breaks down to provide nutrients. Sweeping fallen leaves and twigs onto the garden beds provides a natural mulch, imitating natural ecosystems. Save your money on expensive chemical fertilizers and treat your soil to some good old-fashioned organic matter.
Ellyn Shea is a gardener and garden consultant in San Francisco. Visit her at http://www.garden-guidance.com