Planting for soil health

By Lauren Lewis

With the arrival of the (hopefully) rainy season, our thoughts go directly to what the rain can do for our gardens. Besides the obvious benefit of free, un-transported, apolitical water to nourish the plants, the rain also benefits the soil, by catalyzing the decomposition of dead plant material that’s lying around. So much of the soil in a city is compacted beneath buildings and pavement, but the health of our remaining exposed urban soils has a real impact on how our city responds to the winter rains. So with soil health in mind, my next thought is how can our gardening behavior most benefit the soil?

The four elements of soil are minerals, organic matter, water and air. Those last two are counterintuitive because they really have to do more with the spaces in between the minerals and organic matter than can be filled with water or air. The spaces are created mostly by the movements of soil-dwelling insects, worms, microbial species, etc. Those animals move through the soil in search of food, so the presence of organic matter (their food) encourages their presence and facilitates their movements, which in turn creates space in the soil. The spaces created, and also the sponginess of the organic matter itself, makes that soil much more able to soak up rain and prevent it from running off into the streets and drains and into the bay.

Happily, our gardening practices have a big impact on soil health. To increase the amount of organic matter (dead plant and animal parts) in the soil we can choose to leave dried leaves and chopped up plant cuttings hidden around the garden rather than moving them to the green bin. When the rain comes, the moisture helps that organic matter soften and become easily accessible food for the soil animals.

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Research has shown that the diversity of our landscape plant choices can also affect soil health. In a study of prairie species, researchers created plots planted with between one and 16 species, and tracked various measures of soil health over many years. The study found that more diverse plots had greater overall plant production, meaning the diverse mix of plants facilitated each other’s growth, and as a result, the soil had greater microbial biomass and fungal presence. The researchers point out that this relationship is likely only relevant in a soil that’s lacking in organic matter to support the soil dwellers, which suggests that when a soil is low on organic matter, diverse plantings can help remedy the problem.

Diverse plantings and hidden piles of organic matter are two of the Small Spot Gardens calling cards. Soil health has always been our guiding goal, since healthy soil grows better plants and makes a small but real impact on our Bay Area environment. It works out so nicely that the soil-supportive practice of diverse plant choices also lets us design gardens with the dense, varied aesthetic we love.

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