Soil 101

By Joe Lamp’l - Gardening Expert and Host of Growing a Greener World®
March 17, 2022

Understanding Soil Health and How to Improve It

Recognizing that soil is more than just “dirt” is a big step toward becoming a better gardener. The soil beneath our feet when we walk in our gardens is an ecosystem that feeds the roots of our plants. The healthier that ecosystem — known as the soil food web — the more vigorous our plants will be.

The ideal soil composition for growing vegetables or maintaining a lawn is 45% minerals (sand, silt, and clay), 50% air and water, and 5% organic matter. Though 5% doesn’t sound like a lot, that organic matter component makes a world of difference. Organic matter helps soil particles bind together while leaving room for air and water to flow through. Soil that has sufficient organic matter will resist compaction and will drain better while holding just the right amount of moisture for most plants. 

Organic matter also sustains the microbes (fungi, bacteria, algae, etc.) that live in healthy soil. Those microbes break down minerals and organic matter into plant-available nutrients that roots can readily absorb.

hands in soil

Types of Soil

The three major types of soil are sandy, clay, and loamy. Sandy soil is fast draining, clay soil is slow draining, and loamy soil is in the Goldilocks zone in between. 

Because sandy soil loses moisture quickly, it is not ideal for a vegetable garden or a lawn. Frequent watering is needed to support crops in sandy soil, which is also low in nutrients. All that frequent watering causes nutrients to leach away.

Because clay soil is slow to drain, roots sit in water and are deprived of the oxygen they need. Clay soil is also hard to dig in and compacts easily.

Loamy soil is a good mix of sand and clay and abundant in humus, the dark organic matter that is left over when dead plants fully decompose. Loamy soil is called “well-drained soil” because water passes through it at a moderate pace while retaining enough moisture to keep plants happy. It is friable (it crumbles easily) and fertile (high in nutrients).

Whether you struggle with clay soil or sandy soil, the solution is the same: add organic matter. Organic matter helps clay soil to drain faster and helps sandy soil to retain moisture. It does this by helping soil particles to clump together into aggregates, which also has the benefit of slowing soil erosion.

The Soil Food Web

The organisms that live in soil and their interactions with the roots in the ground form the soil food web. A healthy soil ecosystem will provide nutrients to plants when they need them. That’s why those of us who practice organic gardening like to say: “Feed the soil — let the soil feed the plants.” 

Feeding soil means adding organic matter such as compost, shredded leaves, rotted manure, grass clippings, plant debris, cover crops, wood fines, and ground-up bark. No matter your organic matter of choice, it becomes food for microbes and larger organisms such as worms and springtails. As these creatures digest organic matter, they are releasing nutrients in a form that roots can absorb. Microbes also free inorganic nutrients in sand, silt and clay and make them available to plants. 

One teaspoon of good soil contains between one hundred million and one billion bacteria, not to mention all the beneficial fungi and protozoa. If you keep feeding these microbes, they will reward you with healthier, more vigorous grass and plants.

How to Add Organic Matter to Soil

Now that you know the benefits of organic matter in the soil, it’s time to pick your approach to amend your soil. There are a few different ways to go about it, and your choice may depend on what organic materials are readily available to you. Another consideration is whether you are working on a new garden, an established garden, or a lawn.

Work it in – Soil microbes are most active in the first few inches of soil, so there is no need to deep till. Using a shovel or steel rake, scratch finished compost or well-rotted manure into the soil.

Apply mulch – I just love organic mulch. It suppresses weeds, holds moisture, regulates soil temperature, and resists soil compaction and erosion. In time, it decomposes, providing organic matter and nutrients to your soil. Shredded leaves, arborist wood chips, straw, and bark are a few of the most common organic mulch options. 

Turn in a cover crop – Cover crops are living mulch. Rye, clover, buckwheat and vetch are just a few examples of common cover crops used by farmers and gardeners. They will protect your garden soil between growing seasons, and then they can be cut down and turned into the soil, including their roots. 

Top dress – To add organic matter to a lawn, you can’t go wrong with finished compost. Apply a quarter-inch to half-inch of compost to your lawn and use a rake to scratch it into the grass. 

Use a mulching mower – Instead of bagging your grass clippings, use a mulching mower that returns the grass to your lawn. The clippings will suppress weeds and help the lawn retain moisture, and as the clippings quickly decompose, they will return organic matter to the soil. You can also use a mulching mower on fall leaves to mulch them directly into the lawn or to set aside for later.

Apply Milorganite® – Milorganite® is a slow-release, non-burning organic-based fertilizer. Unlike synthetic fertilizers, which leach away quickly and can cause nitrogen burn, Milorganite is a source of organic matter. Milorganite works in concert with soil microbes to feed grass and plants.

Get a Soil Test

Once you have added organic matter to your soil, it’s time to get a soil test. A soil test will tell you if your soil is still lacking any specific nutrients, but more importantly, the test will reveal the soil’s pH level. The pH scale runs from zero to 14, with low pH indicating acidic soil and high pH indicating alkaline soil. A pH of 7 means the soil is neutral. 

Whether growing a vegetable garden or maintaining a lawn, the ideal pH is between 6 and 7. When soil pH is too high or too low, roots struggle to obtain nutrients that are already present in soil. The soil test results will let you know what amendments to add to your soil to get the pH in line.