Soil, Mulch, and Compost–The Role of Each in the Organic Garden

Spread mulch. Amend the soil with this or that. Add compost. Are we ever done? How do all these components help us grow healthy, vigorous plants? 

If you’re a bit confused about the purpose of all these materials in our organic gardens, you’re not alone. Understanding what each component does for our plants helps us make informed decisions and nudge the entire system in the right direction. 

Soil’s Role in Our Gardens

This one sounds straightforward–it’s the stuff plants grow in! Soil is the material, composed of weathered minerals (rocks), organic material, air, water, and soil organisms, which supports plants and therefore, us! It’s much more than just dirt. It is the stuff of life.

Soil serves several significant functions. It’s one of the few things gardeners have some control over, which is why we spend so much time and effort on it. In the garden, you can’t make the sun shine, or change the level of carbon dioxide at the leaf surface. But you can help the soil to be the best it can be for your plants.

Soil physically provides the support for roots to anchor your plants and keep them from flopping over, sliding down a hill, or rolling away. A tumbleweed won’t roll until it breaks off from the roots. Soil provides the structural anchor for plants’ above-ground vegetation. A tall tree has a massive support system in the soil. The tree tips over when the roots fail to hold, making a huge mound.

Water is stored in the soil, specifically in pore spaces. Our garden plants absorb most of their water through their roots. Sandy soils hold very little water, while clay soils drain slowly and sometimes hold too much. Soils high in organic matter hold more water. Improved soil moisture levels–less fluctuation–often means better plant growth.

Plants draw carbon dioxide from the air to get the carbon they need, and water provides them hydrogen and more oxygen. But where do they get magnesium, nitrogen, or phosphorus? Iron, calcium, potassium, or boron? That’s right, it comes from the soil. 

Soil is the storage site for nutrients in mineral form. These nutrients, often as cations or anions, are taken up by plant roots. In natural settings, when the plant dies, it falls back where it grew, and the nutrients it used are returned to the soil, broken down into their component parts by soil microorganisms. Some soils are lower in nutrients in the first place–sandy soils, for example. We test the soil to identify nutrient deficiencies. 

The Role of Compost

If you’ve read more than two articles about gardening, at least one of them probably mentioned compost. What’s the big deal? What does compost do for our garden plot? We add it to improve the soil. Compost helps with drainage problems, increases fertility and water-holding capacity, and feeds your soil life.

Compost is the broken-down leftovers from things that once were living, usually plants. It is dark, smells like we instinctively know good earth should (due to the microbial action), and plants love it. Composted organic materials break down at different rates–some stuff is quickly made into plant-usable forms, and others will take years or even decades to decompose. The slow-to-decay material is called humus. The humus in compost holds water, nutrients, and improves soil structure. 

Compost also is full, absolutely teeming, with soil microbes and other soil life. Beneficial bacteria, fungi, beetles, worms, nematodes (the good kind), and protozoa are all found in good compost. These plant-helping organisms are a massive part of the reason why compost is so beneficial to your gardening efforts. The plants and the soil life form mutually beneficial relationships, feeding each other. 

The Role of Mulch

You can be an organic gardener without mulch, but it’s not as much fun. If more frequent watering, crusty soil, and constantly pulling weeds are your thing, skip the mulch. For the rest of us, let’s spread it around!

Mulch is a natural form of weed barrier. Tiny weed seeds germinate and try to reach the sun, but your mulch blocks it, and they croak before they can get through. While it won’t stop every weed, it stops most. The few that make it through are much easier to deal with. 

Mulch also armors the soil, preventing the sun from baking the top into a hard crust that resists water infiltration. It shades the soil so the top few inches don’t get as hot (bad for soil microbial life), and reduces the impact of heavy rains that splash bare soil up on the leaves, causing issues like bacterial wilt of tomatoes.

Mulch from organic materials like straw, grass clippings, chopped-up leaves, or pine needles will also break down over the season (you may have to add more in late summer) and provide an additional composted layer of goodies for your soil. With all those benefits, it really doesn’t pay to skimp on the mulch.

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