Why Soil is Better Than Dirt


This compost bin is a great place to recycle garden clippings and kitchen scraps into food for the soil.

We’ve all heard the words “soil” and “dirt,” but what’s the difference? Both have the same basic components: particles of sand, silt, clay, and organic matter. The difference lies in how they function: dirt is where we stick our plants and dump water, fertilizers, and pesticides in the hopes of keeping plants alive; it loiters beneath our feet, supporting the weight of everything from humans to skyscrapers, all the while being completely ignored by the average citizen.

Soil, on the other hand, deserves a lot more respect. It contains a thriving community of organisms, many of which interact directly with plant roots to the benefit of all. This mutually beneficial relationship is crucial to plant health and can make gardening easier on humans if we allow the process to go along undisturbed.

One of the most important services soil organisms provide is making nutrients available to plants. The area immediately surrounding plant roots is covered with bacteria and fungi, which eat 50% of the sugars produced during plant photosynthesis. They also feed on nutrients from the surrounding soil, whose crystalline structure makes them unusable to plants. Bacteria and fungi have the enzymes and acids to break down these nutrients into forms plants can use, which is released when microscopic predators such as nematodes and protozoa come along and burst them open for a snack.

The living layer around plant roots, called a biofilm, also provides protection. A strong population of beneficial organisms crowds out invading pathogens and keeps existing bad guys in check. This biofilm is slightly alkaline, ensuring that the nutrients released by the bacteria and fungi will not be tied up by an overly acidic or alkaline environment. Remember that just because your soil may test as being extremely alkaline (rarely acidic in California), plants and organisms work as a team to change their lot in life.

The powers of living soil don’t end here; outside of the biofilm, organisms eat dead plant matter, releasing nutrients back into the soil. They secrete fluids (my apologies to squeamish readers) that bind together soil into little clumps called aggregates, which create air channels. These channels provide drainage and oxygen flow, the latter of which is essential in preventing anaerobic organisms and their toxic wastes from flourishing.

How do you know if your garden is full of dirt or soil? Think about how you treat your soil: dirt results from the over use of synthetic fertilizers, pesticides, inorganic amendments, and frequent tillage, leaving you with a heavier work load to make up for the lack or organisms. Another dirt-indicator is dead plant matter (“detritus”) that sits on the bare ground for months without decaying at all. Real soil contains the life to start decomposition quickly; in fact, keeping soil bare is a sure way to remove nutrients and starve soil organisms, leaving you with dirt.

Obtaining soil is not overly complex: apply good compost, which is organic matter that has been fully decomposed by bacteria and fungi. This acts as an inoculant to repopulate your soil with the organisms to build a healhty food web. Depending on your situation (and physical ability), compost may be difficult to make; compost can be purchased, but quality material is expensive, so apply where it is most needed, such as around vegetables or prized ornamentals.

Compost-making requires another article (and a certain amount of work), but the reduction in chemical use and increase in plant health is worth it. Help your soil organisms along by looking into composting at <sacmg.ucanr.edu/Composting>. If nothing else, switch to organic fertilizers and leave non-diseased detritus (or organic mulch such as bark)  on the soil surface to feed soil organisms and recycle nutrients whenever possible.

If you have a gardening related question you can contact the UC Master Gardeners at 209-953-6112. More information can be found on our website: sjmastergardeners.ucanr.edu.

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