Building Super Soil
Soil is the crucial building block for a city farm. Learn how to build healthy soil to nourish your farm garden.
By Jessica Walliser
Have your soil tested to make sure it's free from contamination and contains the nutrients needed for proper growth.
When it comes to urban gardening, your soil will either make you or break you. To nurture healthy soil that will provide nutrients to your garden’s crop, understand what your soil is made of and how you can improve its structure.
In order to understand how best to care for the soil, it’s important to know what’s in it. Every sample of soil contains the same basic ingredients in relatively the same proportions.
About 50 percent of a soil sample is mineral fragments resulting from millions of years of erosion and sedimentation. These fragments come in three basic sizes: sands (visible to the naked eye), silts and clays (individual clay particles are flat and can only be seen with an electron microscope). The USDA uses the percentage of soil that falls into each one of these size categories to describes soil texture.
Soil texture plays a significant role in determining other soil qualities, so find out what’s in your backyard. Soils with a higher percentage of clay often drain poorly, while soils containing a larger percentage of sand have trouble holding nutrients.
A soil testing lab can conduct a texture test on any given soil sample, but this isn’t always necessary. Often a simple “feel” test will suffice. Use these clues:
- Clay soil: forms a dense clump when damp
- Sand soil: feels gritty; won’t form a clump
- Silt soil: powdery when dry; forms a loose clump when wet
About 35 to 45 percent of a soil sample is pore space, which contains water (with water-soluble nutrients) and air. The amount of pore space and the size of the pores is determined in part by the soil’s texture. Sandy soils have larger pores while clay-based soils have smaller pores but many more of them. Just as a colander with a few large holes drains much faster than one with many small holes, a sandy soil drains more quickly than a clay-based soil.
About 1 to 6 percent of the soil sample is organic matter. This tiny percentage is the most important part of the soil. It’s a nutrient source for plants, food for beneficial soil microbes and the main ingredient for developing good soil structure — the most important measure of soil quality.
“Soil structure” is the way mineral fragments group together, or aggregate. It’s an essential factor in soil and plant health. Organic matter creates good soil structure by binding soil particles together and changing the way they aggregate. It separates clay particles. It binds sand particles to decrease pore spaces and increase water retention. It eventually leads to loose, crumbly, nutrient-dense soil.
Living organisms make up the final soil component. Ranging in size from microscopic bacteria and fungi to mice, moles and earthworms (vermacompost), soil life is essential for plant growth. They process the soil, convert certain nutrients into a useable form and many of them (think earthworms) have nutrient-rich castings.
Soil life is also partially responsible for aerating soils and helping build good soil structure. Fungal networks beneath the soil surface form symbiotic relationships with plant roots, enabling the plant to gather more nutrients from a larger area.
The measurement of soil acidity — soil pH — is an important quality. Soil pH is represented on a scale of zero to 14, (seven being neutral). Any measurement below seven is considered acidic soil, while above seven is alkaline soil. At certain soil pH values, nutrients are more readily available to plants while at other levels they are unusable. In general, the broadest range of nutrients are available at soil pH 6.5. The exceptions to this rule are blueberries and evergreens, which use copious amounts of iron, a nutrient primarily available at more acidic soil pH values.
The only way to determine soil pH is to test the soil. If the test results determine that the soil is too acidic, add lime to raise the soil pH. If the results are alkaline, add sulfur to drop the soil pH. Most soil-test results will advise on the amounts of these products needed to adjust the soil pH appropriately.
Keep in mind that plants will eventually use the calcium and magnesium provided by lime for growth, and that these nutrients, along with any added sulfur, may also leach from the soil. Because of this, the soil-pH change they affect is temporary, lasting three to four years at most. It’s important to retest soil pH regularly because it can change easily.
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