Animal, vegetable, or mineral?” is a question that kicks off every game of “20 questions.” In the garden however, animal, vegetable, and mineral mean something totally different. These are the main building blocks of soil – along with air, water, and microbes.
Soil’s mineral components come from different kinds of rock. The particles that make up soil are clay, sand, and silt. The spaces between particles fill with water when it rains and when we irrigate. Air replaces the water as it is depleted or drained.
Clay particles are very small and shaped regular, packing together tightly. Sand particles are large and shaped irregular, leaving lots of empty spaces between particles. Silt particles are in-between sizes.
The larger spaces between sand particles allow water to move through quickly, while the tiny spaces make it difficult for water to penetrate clay soils. Once there, clay holds moisture for a very long time.
Oxygen (from the air) is critical for healthy plant roots. Water from the soil moves into the roots and up through the plant, where it is used in photosynthesis — the plant process of making energy from water, sunlight, and carbon dioxide.
Mineral, water, and air comprise 95 percent of soil. Five percent of soil is “organic matter” that comes from decomposed animals and plants (the “vegetables”). As animals and plants die and decompose, they release nitrogen, phosphorus, and other critical elements into the soil.
The last component of soil makes up less than half percent by volume but is critical. The “soil food web” is the living component of soil. It is made up of beneficial bacteria and other microscopic organisms such as fungi and protozoa. Earthworms, sowbugs, millipedes, and other tiny critters are also part of the soil food web. They comprise an entire universe of creation, consumption, and decomposition happening underground. Soil food web organisms digest organic matter and release nutrients into the soil,
where roots recycle them back into plants.
Organic matter also absorbs and holds water, which helps plants survive drought. In fact, if you can increase the organic matter in an acre of soil by just one percent, that soil can hold 25,000 more gallons of water. That is a pretty good reason to mulch!
Most soil food web organisms live in the top few inches, where they feed on organic matter. They are not all mixed up together, however. Each component has its sweet spot, with the perfect complement of food, temperature, moisture, and so on. Mixing those very distinct layers by tilling or turning the soil destroys the soil food web. Tilling compacts the soil too, so if you learned to garden with a rototiller or a spading fork, it is time to retire them.
Best Soil for Your Garden or Best Plants for Garden’s Soil
Drought-tolerant plants – whether trees, shrubs, succulents, etc. – tend to come from regions where soils are much like our local soils. That is one of the reasons these plants grow so well in local gardens. Grow them in native soil; do not amend planting holes. Use organic (plant-based) mulch for non-succulent plants. Use rock mulch for succulent plants.
Vegetables – Most vegetables require soils that are high in organic matter, so grow them in raised beds filled with custom soil mixes, typically 60 percent mineral and 40 percent organic matter. Layer on lots of compost and add worm castings before you plant. Add vegetable fertilizer and mulch the beds with a thick layer of straw (not hay). The straw eventually breaks down into organic matter, too.
Plants native to high rainfall, acid soil, and soils high in organic matter struggle when planted in local soils. If you absolutely cannot live without hydrangea, gardenia, azalea, etc., grow these plants in pots. You can simulate their native growing conditions with acidic potting soil that is high in organic matter, acidic fertilizer, and extra water – all more doable in a container.