Soil Structure

The more you know about the structure of your soil, the better you can help it support your plants.

Soil (Jing) Pixabay

Soil doesn’t exist in isolation. Except for sand, soil is a combination of minerals, air, water, microbes, earthworms, and more. Soil structure refers to the particular arrangement of these combined particles. These clumps of different particles are called soil aggregates. Aggregates contain solids and spaces and are held together by organic matter in various stages of decomposition, earthworm and insect poop, chemical excretions of nearby plant roots, and a fungal coating called glomalin.

​Soil structure is impacted by several factors:

• adsorbed cations (atoms of minerals that attach themselves to other things, such as roots)
• animal and insect activity
• climate
• freezing and thawing
• microbial activity
• root activity
• wetting and drying

Types of soil structure 

While soil types may not look terribly different to the naked eye, at the microscopic level, where root hairs, microorganisms, water and plant nutrients are moving around, soil structure is really important. Soil structure should not be confused with soil texture. ​

Compacted soil makes it difficult for roots to grow (J.G. Davis, Bugwood.org) CC BY 3.0 US

Soil texture describes the percentage of silt, clay, and sand present. Soil structure is about how everything is arranged into clumps, or aggregates. Soil structures are categorized by their size, shape, and stability:

• granular - rounded surfaces; sand
• blocky - cube shaped with sharp or rounded corners
• lenticular - shaped like a lentil
• platy - horizontally rectangular
• wedge - thick at one end and thin at the other, or thin at both ends
• prismatic - vertically rectangular with flat ends
• columnar - vertically rectangular with rounded ends
• crumbly - rounded, but larger than grains
• single grain/structureless - no aggregation of particles when dry

(USDA) Public Domain

Soil compaction

The large and small spaces between soil aggregates, called macropores and micropores, respectively, determine soil porosity, permeability, and water-holding capacity. Soil structure can be damaged by digging, rainfall, and heavy traffic, which all destroy those important spaces, causing compaction. Instead of loose collections of many different sized aggregates, crusts form along cut edges or on top of the soil, blocking the flow of water, air, and nutrients, and making it difficult for seedling roots to take hold. It’s not very fun to work with, either. Good soil structure can help move water away from roots when over-watering occurs and it allows plant roots to go where they need to go for food and water.
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How to build good soil structure

Nature has been making good soil for a really long time. Plants, insects, and animals die, decompose into nutrients, and water pulls them into the soil. Then microorganisms and soil-dwelling animals, insects and worms eat those nutrients and poop out stuff that holds soil particles into various sized aggregates. Because many of us live in homes where construction soil is common, we must recreate the natural cycles needed to maintain healthy soil structure. To maintain good spacing between aggregates, especially for clay soil, use these tips:

• apply top dressing
• avoid digging when soil is wet
• try your hand at vermiculture
• encourage earthworms
• add compost regularly
• aerate soil, as needed
• rake in vermiculite and/or coffee grounds
• protect bare areas by planting ground cover or adding mulch​

Construction often compacts soil (Peter Bedker, Bugwood.org) CC BY 3.0 US

Healthy soil produces healthy plants that are better able to fight off pests and diseases on their own.