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Key Ingredients of Garden Soil

Radish seedling in humus rich garden soil

What makes good garden soil? Here are some key ingredients:

Air and water in the soil

Half the volume of soil is air and water which fill the spaces or pores between crumbs of soil. Plant roots and soil microorganisms breathe in oxygen from the soil air. Plant roots absorb minerals dissolved in soil water. Soil air and water are essential for plant root, stem, and leaf growth. When soil does not drain well and the air is forced out, water occupies soil pore space and plants suffocate or drown.

Soil tilth and texture

Soil texture is important because it determines a soil’s ability to act as a growing medium for plants—that ability is referred to as “tilth.” Tilth describes not only a soil’s texture but its capacity to hold nutrients and water. A sandy, coarse-textured soil is called “light” soil. A clay or fine-textured soil is called “heavy” soil. A soil that combines sand, silt, clay, and humus is called “loam”; loam is a “medium” soil. The subjective “feel”—how the soil feels between your fingers—determines its texture and tilth. With experience, a gardener can name the soil texture in the garden.

Trace elements in the soil make for soil fertility.

Soil fertility

The fertility of soil depends upon the availability of essential elements needed for plant growth. Those elements are largely nitrogen, phosphorus, and potassium (called macronutrients), and to lesser degree sulfur, calcium, and magnesium (called micronutrients),  and an even lesser amount of other elements—iron, manganese, boron, copper, zinc, and molybdenum (called trace elements). From the atmosphere, plants draw upon the macronutrients carbon, hydrogen, and oxygen.

Types of soil

Scientists have described many different soil textures, all a combination of differing proportions of sand, silt, clay, and humus. Several kinds of soil can exist in one backyard. Here are basic soil types and soil-related descriptions:

  • Clay soil is made up mostly of very small finely textured flat particles.
  • Sandy soil is made up mostly of large, coarse particles.
  • Silty soil is minute, smooth, and fine particles.
  • Humus rich soil is not a soil per se but organic matter such as leaves and other plant material that is in a state of advanced decay
  • Loam and sandy loam is the combination of a moderate amount of sand, some clay and silt, and humus. This is the best soil for growing vegetables.
Handful of soil
A simple handful of soil will tell you how your garden will grow.

How to know your soil

Here are three simple tests to determine what kind of soil is in your garden.

Soil Test 1. Place a small clump of soil in your hand and squeeze. If the soil forms a shiny, wet ball, when you open your hand, you have clay soil. If the soil disintegrates in your palm and holds no form, you have sandy soil. If the soil holds together in a light, slightly loose clump until you drop it to the ground, you have loam soil. Loam is friable soil.

Soil Test 2. Place a small handful of moist soil between your thumb and the first knuckle of your forefinger. Squeeze the soil out between your thumb and your finger to form a ribbon. If the ribbon holds together for more than an inch, the soil is heavy clay. If a ribbon forms for ¾ of an inch, the soil is silty-clay loam. If the ribbon breaks shorter than ¾ of an inch, the soil is silty. If no ribbon forms, the soil is sandy.

Soil Test 3. Dig a small hole in the garden about 12 inches (30 cm) deep. Fill the hole with water. Measure the time it takes for the hole to naturally drain. If the hole is empty in less than five minutes, the soil is sandy or sandy loam. If the hole takes 15 minutes or longer to drain, the soil is clay-like.

Related articles of interest:

Your Vegetable Garden Soil

Soil Making the Kitchen Garden

How to Improve Clay Soil

Vegetable Plant Nutrients: Sources and Deficiencies

Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. has more than 10 million visitors each year.

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