Soil Basics

Why is soil important for plant growth and health?

Soils provide water, air, nutrients, and mechanical support for plants. Soils also tie up, filter, and break down natural and man-made toxins. Soils sustain all life on Earth and might be the most important, neglected, and least understood resource in the landscape. 

Soil problems such as compaction, low fertility, poor drainage, and thin topsoil, can cause stress, poor growth, and decline in our garden and landscape plants.

The native topsoil has been removed from many urban and suburban soils. These soils are often compacted and low in organic matter. Poor soil management and misuse of fertilizers contribute to surface and groundwater pollution.

It’s our job to protect and improve our soils so they can nourish future generations of plants and animals - including humans!

What you should know about soil

• Soil is a natural resource and a living ecosystem (the "living skin of the earth").
• Soils sustain all life on earth and filter and break down natural and man-made toxins.
• Soils provide water, nutrients, and support, along with oxygen for the plant's root growth.
• Soils have four main components: mineral particles (sand, silt, and clay), organic matter, water, and air.
• There are many different types of soil in Maryland. You may have several types even in a small yard!
• Healthy soils grow healthy plants that keep people healthy.
• Information found on this page.

Soil texture

• Nearly all soils in Maryland contain a combination of sand, silt, and clay particles. Soil texture is determined by the relative amounts of these three types of particles and doesn't change over time. Soil texture is determined by the parent material (rocks from which the soil formed), climate, slope, and other factors.
• Texture determines, in large part, how the soil feels and behaves. Sand particles are the largest (0.05-2 mm) and can be seen without a magnifier. Silt particles (0.002-0.05 mm) are next in size, followed by clay particles that are 1,000 times smaller than sand particles (< 0.002 mm) and can only be seen with an electron microscope.
• Too much clay in the soil can make it difficult to manage. But clay plays a positive role in holding and protecting organic matter particles. Also, clay's enormous surface area carries negative charges that hold positively charged nutrients (e.g., calcium, magnesium, potassium, ammonium) used by plants and soil microbes.
• You can roughly determine your soil's texture by the "feel method" or the "jar test." Some soil testing labs will conduct a "mechanical analysis" (for an additional fee) to identify soil texture.
• You can determine your specific soil type through the U.S.D.A.'s Web Soil Survey.

 

The soil triangle shows the 12 textural classes of soils. In the example below, the soil has 25% sand, 10% clay, and 65% silt, giving it a silt loam texture.

Pore space

• One half of the volume of most soils is composed of solids (sand, silt, clay, and organic matter) and the other one half is composed of pores--spaces between solid particles, that are filled with air, water, roots, microorganisms, earthworms, and other soil animals.
• Healthy soils have a large number of pore spaces of varying sizes that allow for the free movement of water, air, and growth of plant roots, soil microbes, and animals.

In the diagram below we can imagine that rain fell two or three days ago. Most of the pore spaces would have been filled with water. Gravity carries excess water down through the soil profile. The water in the diagram is held on, and between, soil and organic matter particles and is available for uptake by plant roots.

• You may have seen soils with a high percentage of clay draining very slowly after a thunderstorm. The tiny clay particles are packed closely together which slows the movement of water. Soils with a lot of clay are also more susceptible to compaction from foot traffic, vehicles, and machinery, further slowing the movement of water and air into (infiltration) and through (percolation) the soil.
• Soils with a high percentage of sand drain quickly after a thunderstorm, carrying nutrients from fertilizers and organic matter down through the soil and out of the root zone of plants. As a result, plants growing in sandy soils often require more frequent watering and fertilizing.

This chart shows how soil texture affects water, air, and nutrients in the soil

	Sands, sandy loams and loamy sands(coarse-textured soils	Loams, silt loams, silts and all the clay loams(medium-textured soils)	Clay, silty clay, sandy clay, clay loam and silty clay loam(fine-textured soils)
Water and nutrient holding capacity	low	moderate	high
Infiltration and drainage	fast	moderate	slow
Leaching potential	high	moderate	low
Aeration	good	moderate	poor

Credit: Melissa L. Wilson, Ph.D., and Patricia Steinhilber, Ph.D. 

Soil structure

• Soil structure describes the way that particles fit together and form small clumps, called aggregates.
• Roots, fungal hyphae (thread-like growths), and sticky substances produced by soil microbes and plant roots hold and "glue" clay and silt particles into aggregates. Very small microaggregates combine to form macroaggregates.
• Earthworms and small soil animals also contribute to aggregation by burrowing and producing fecal pellets.
• The topsoil of a healthy soil should have a crumb-like structure with ½ of the volume made up of different size pores.
• Soil structure can be improved over time by increasing the amount of organic matter in the soil.

Organic matter

 

• Soil organic matter is made up of living, dead, and decomposing organisms within and on top of the soil. These include plants (leaves, stems, roots), earthworms and other soil animals, and microorganisms. Organic matter accounts for a relatively small part of soil (1%-5% by weight) but is critically important to soil health because it supports the soil food web that drives the biochemical action in the soil.
• Organic matter exists on a continuum from living organisms to "protected organic matter" (formerly described as humus) which is made up primarily of dead microbial cells found inside small soil crumbs (microaggregates) or attached to clay particles. The "protected organic matter" in the graphic accounts for about 75% of total soil organic matter. It's made up mostly of dead soil microbes that are protected from further decomposition because of their location and attachment to clay particles.
• Once plants and animals die, their tissues ("residues and by-products" in the graphic above) are shredded, chewed, and digested by huge populations of soil animals and microbes (fungi and bacteria). This leads to the release of nutrients, like potassium and nitrogen, that plants can use. In the process nutrients, once part of organic compounds in the living organisms are converted through biochemical processes into inorganic forms used by plants and microbes.
• Organic Matter and Soil Amendments 

 The soil food web

• On first glance, soil may appear dense, dead, and deserted. But it's actually teeming with life. Soils are home to thousands of different species of soil -animals, including tiny mites, nematodes, springtails, and earthworms, as well as bacteria and fungi. A teaspoon of soil can contain several billion microorganisms!
• All of these life forms occupy particular positions (trophic levels) in the soil ecosystem. Organic matter feeds the soil food web with larger animals consuming smaller ones.
• Living plant roots leak sugars and other compounds that support huge populations of bacteria and fungi in the root zone.
• Roots and microbes release sticky substances that help form stable aggregates, giving the soil a crumb-like structure.

Additional resources

Soil Texture "Feel Method"

Washington State University video - Determining Soil Texture by Hand
University of California-Davis video - Soil Texture by Feel
University of Kentucky - (PDF) Determining Soil Texture by Feel

Soil Texture "Jar Test" Method

Clemson University - Soil Texture Analysis "The Jar Test"
University of California - (PDF) Sedimentation Test of Soil Texture
 

Author, Jon Traunfeld, HGIC Director, and Extension Specialist, Fruits and Vegetables.

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