The Amazing World Beneath Our Feet: An Introduction to Soil Health

Intro to Soil Health: The Amazing World Beneath Our Feet

A Hensley's Seed Co. guide

Good soil underpins productive growing, nutritious crops and, ultimately, our own health. The ideal soil for most vegetables, flowers and fruit is deep, crumbly and well drained, with enough available nutrients for strong growth. A lucky few start with soil like that, but most growers have to put in some work to improve their soil and keep it healthy. That effort pays off over time in better yields, fewer pest and disease problems and crops that hold up better in extreme weather.

What is Soil Health?

Soil health is the soil's continuing ability to function as a living ecosystem that supports plants, animals and people. Its main components are:

1. Physical structure (texture and structure, or tilth)
2. Chemical composition (pH and nutrients)
3. Biological life (microbes, fungi and small invertebrates)

Healthy soil does several essential jobs in the farm and garden. It:

• Supports plant growth and helps crops take up nutrients;
• Limits erosion, holds water and drains well;
• Helps plants withstand pests and diseases;
• Stores carbon, which can lessen agriculture's climate impact.

4 Soil Health Fundamentals

Four basic principles guide soil health management. They apply to every soil type and every scale, from garden beds to farm fields, and they work by imitating natural systems.

1. Disturb the soil as little as possible (through tillage, synthetic fertilizers and pesticides, for example). Less disturbance protects soil structure, supports healthy microbial populations and reduces the moisture loss that comes with tillage.
2. Keep living roots in the groundfor as much of the year as you can. Roots shelter and feed soil life and protect the soil from erosion and temperature swings.
3. Keep the soil covered. When living roots aren't an option, mulch, straw or terminated cover crops can shield the soil.
4. Maximize biodiversity. A wide mix of cover crops and cash crops fosters a more diverse soil microbial community, and encouraging biodiversity above ground can improve nutrient cycling and pollination.

Physical Properties of Soil (Tilth)

Tilth describes the soil's physical condition and how well it supports seed germination, root growth, water infiltration, drainage and root aeration. A soil's natural texture depends on the size of its particles, which come in three kinds: sand, silt and clay. Sand particles are the largest, silt particles are much smaller, and clay particles are smaller still.

Knowing your soil's makeup helps, because texture affects how soil responds to management. Sandy soils, with their big particles, let water and nutrients drain through quickly, while clay soils hold onto both much longer.

Most soils are a blend of all three in varying proportions. Loam, for instance, is a balanced mix with roughly similar amounts of sand and silt and less clay. Soils that lean toward one particle size are named accordingly, such as sandy loam, silt loam or clay loam.

Home “Jar Test” for Determining the Physical Composition of Your Soil

Here's how to check your soil texture at home.

1. Gather soil and remove bark, twigs and stones. Take soil from several spots in the garden or field and mix it for a representative sample.
2. Fill a jar about a third full with soil.
3. Top it up with water.
4. Cap it and shake hard until all the soil is suspended.
5. Set it on a level surface. After 1 minute, mark the top of the sand layer (sand is heaviest and settles first).
6. After about two hours, mark the top of the silt layer.
7. After 48 hours, mark the top of the clay layer.
8. Measure each layer's thickness and work out its percentage of the total.

Chemical Properties of Soil

We look at soil chemistry to get the biological engine going, so soil microbes thrive and nutrients cycle and become available to crops.

First Test, Then Amend

A soil test from your local extension service or a soil lab is well worth it. For a modest fee you'll learn a great deal: pH, major and minor nutrients, percent organic matter, cation exchange capacity (how well the soil holds nutrients) and basic amendment recommendations for the crops you plan to grow. In general, focus first on pH, organic matter and any nutrient shortfalls.

How to Take a Soil Sample to Test

1. Take core samples from around the garden or field with a soil probe. Without a probe, dig a hole 6–8" deep and cut a thin vertical slice from the side so your sample covers the whole 6–8" depth.
2. Mix the cores well for a representative sample of the whole area, and put about a cup of soil in a soil test box or plastic bag.
3. Send it to the lab, following its instructions for packing and labeling.

Soil pH (Soil Acidity)

Soil pH strongly affects both soil life and how well fertilizers and amendments work. pH measures acidity: 7 is neutral, lower is acidic and higher is alkaline. Nutrient availability changes considerably across the pH range. Most garden crops do best between about 6.0 and 7.0, where most key nutrients are near peak availability, and aiming for around 6.5 covers most macro- and micronutrients for most crops. There are exceptions, of course, such as acid-loving blueberries. Most soil microbes also do their best work in this range, breaking down and recycling organic matter and helping plants take up nutrients.

If pH is off, fix that first. Raise pH with agricultural lime or wood ash; lower it with elemental sulfur or, in smaller areas, sphagnum peat. Your soil test report will recommend application rates.

Biological Properties of Soil: The Soil Food Web

The soil food web includes bacteria and fungi along with protozoa, nematodes, insects and small mammals. It stores carbon, makes nutrients available to plants, and improves aeration and water infiltration. Soil biology is central to plant health and plant defenses, much as the gut microbiome supports our own digestion, health and immunity.

The food web starts in the rhizosphere, the zone right around plant roots. Bacteria and fungi, its smallest members, feed on carbon-rich root exudates, break down organic matter and tie up (store) nutrients in their cells. Protozoa and nematodes, a step up the chain, eat the bacteria and fungi and release (mineralize) those nutrients in forms plants can use.

Soils hold thousands of species of bacteria and fungi, and they play a major part in how plants get their nutrients. Here are two groups that matter a great deal for garden plants; their partnerships with plants evolved over millions of years.

1. Rhizobium bacteria team up with legumes (peas, beans, alfalfa, clover, vetch and others) to turn nitrogen from the air into forms plants can use. They live in nodules on legume roots, where the plant supplies them with carbohydrates in exchange for this fixed nitrogen. Very few natural processes can break nitrogen's strong triple bond, and these bacteria are one of them. With air about 78% nitrogen, legumes are an excellent way to bring this free fertilizer into your soil.
2. Mycorrhizal fungi are another hardworking part of the soil food web. They form partnerships with the roots of most land plants and act as extensions of the root system. Their thread-like hyphae are extremely fine and abundant, and they reach well beyond the root zone to bring back water and nutrients the roots couldn't otherwise get. Phosphorus is a good example: it's often locked up in forms plants can't use, and mycorrhizal fungi help free it and move it to the plant. Plants need plenty of phosphorus for growth, so this role is important. In return, plants feed the fungi carbon-rich exudates. Because the fungi depend on their host, the partnership benefits both: the fungi help nourish the plant, can strengthen its defenses and help protect roots from pathogens, and make plants more resilient under stress.
   Many garden crops partner with mycorrhizal fungi, including beans, peas, sunflowers, carrots, onions and sweet potatoes. Brassicas, buckwheat, spinach and beets don't, and conveniently for gardeners, neither do many common weeds such as lambsquarters, pigweed and other amaranths, rushes and sedges.

Feed Organic Matter to Your Soil Food Web

Soil organic matter (SOM) makes up only a small share of most soils, from under 1% to around 10% in the richest, but it matters far out of proportion to its size. It holds most of the soil's stable nutrient reserves, which are released slowly in usable forms as soil organisms break it down.

Think of organic matter as your soil's savings account. Soil microbes use it as food, shelter and a medium of exchange with plants. It also improves tilth by boosting water-holding capacity, drainage and structure, and every increase in organic matter helps the soil store more water. Higher organic matter means a more resilient garden and healthier, more productive crops.

Ordinary farming and gardening practices deplete organic matter through:

• Tillage
• Harvesting
• Harvesting crops and removing residue
• Relying heavily on synthetic fertilizers

To rebuild organic matter, return carbon-rich materials to the soil, such as:

• Aged manure
• Compost
• Cover crop residues

Cover crops add organic matter several ways. While growing, their roots release exudates. After termination, both the roots and the aboveground growth break down and add organic matter over time.

You may also want amendments for specific nutrient needs. Soluble synthetic fertilizers can leach quickly and bypass the soil food web, so many growers prefer natural amendments such as gypsum, wood ash, blood meal or kelp meal. Remember, the aim is to feed the soil food web, which in turn feeds your plants.

Signs of healthy soil:

• Visible soil life, which points to an active microbial community
• Rhizosheaths: sleeves of soil held to the roots by exudates and other biological glues.
• Dark color and a crumbly feel.

Root Exudates

In photosynthesis, plants use sunlight, carbon dioxide from the air and water from the soil to make glucose. From glucose they build amino acids, proteins, starches and cellulose, everything they need to grow, along with root exudates that feed the soil food web. A meaningful share of a plant's photosynthetic output goes into those exudates, though estimates vary widely by plant and conditions.

Root exudates are carbon-rich compounds released by roots. Their makeup varies by plant species and by the microbes the plant is trying to attract, and can include organic acids, sugars, amino acids and enzymes. Through them, plants strongly shape the soil around their roots.

The soil food web consumes these exudates, and part of the carbon ends up as humus, the stable, well-decomposed fraction of soil organic matter. Fresh residue on the surface breaks down and oxidizes fairly quickly, but humus is far more stable and can persist in soil for a very long time.

Soil Armor—How to Keep the Soil Covered

Because root exudates are so important for building soil, it pays to keep living roots in the soil for as much of the year as possible. There are many ways to keep soil covered; here are two approaches used on farms and research plots.

Reduced Till System

1. Begin with a fall sowing of winter rye (an annual).
2. In spring, drill or broadcast white clover into the rye as an underseeding. The clover pushes up through the rye residue and grows fast, smothering weeds, covering the soil, fixing nitrogen, feeding soil life and offering nectar to beneficial insects.
3. Next, prepare the beds, usually with one pass of a stone burier followed by a plastic mulch layer.
4. Plant crops into the plastic-mulched beds, leaving the clover growing in the paths between them.
5. Mow the clover now and then to keep it in check and stop weeds from moving in.
6. After roughly 3 years, grasses and weeds start invading the clover and it's time to start over by "flipping the beds": the old clover paths become crop beds, the old beds go into cover crop, and the cycle starts again at step 1.

This reduced-till system isn't perfect, but it's a big improvement over conventional tillage. You get the advantages of plastic mulch (faster soil warming, less soil splash, efficient irrigation and weed control) while living plants cover most of the field and build soil health.

No Till System

A simple sequence for a fully no-till planting:

1. In early May, sow a spring green manure mix as a cover crop.
2. After about 2 months, crimp the cover crop. Timing matters, so you get plenty of mulch and a complete kill. With a spring green manure mix, follow crimping with 1–4 weeks under a tarp: longer in cool weather, shorter when it's warm.
3. Transplant seedlings directly into the terminated cover crop.

No-till plantings like this tend to have fewer weeds and need less irrigation. The method works best for transplanted crops such as tomatoes, peppers, squash and brassicas.