About Soil

Definition of soil

The unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time.

Soil Science Glossary (Soil Science Society of America).

Why is soil important?

Soil is the earth’s natural foundation, filter, and seed bed. The failure or success of building foundations are a function of soil properties and the engineering needed to overcome these properties. Soil acts as a filter; it cleans as water percolates through it. Soil is the medium in which plants grow and seeds germinate. How well a plant grows is largely dependent upon soil properties, which control a soil’s water and nutrient holding capacities that are vital to the success of plant growth.

Managing for Soil Quality Each combination of soil type and land use calls for a different set of practices to enhance soil quality. Yet, several principles apply in most situations. Add organic matter. Regular additions of organic matter are linked to many aspects of soil quality. Organic matter may come from crop residues at the surface, roots of cover crops, animal manure, green manure, compost, and others. Organic matter, and the organisms that eat it, can improve water holding capacity, nutrient availability, and can help protect against erosion. Avoid excessive tillage. Tillage has positive effects, but it also triggers excessive organic matter degradation, disrupts soil structure, and can cause compaction. Carefully manage fertilizer and pesticide use. Pesticides and chemical fertilizers have revolutionized U.S. agriculture. In addition to their desired effects, they can harm non-target organisms and pollute water and air if they are mismanaged. Nutrients from organic sources also can become pollutants when misapplied or over-applied. On the positive side, fertilizer can increase plant growth and the amount of organic matter returned to the soil. Increase ground cover. Bare soil is susceptible to wind and water erosion, and to drying and crusting. Ground cover protects soil, provides habitats for larger soil organisms, such as insects and earthworms, and can improve water availability. Cover crops, perennials, and surface residue increase the amount of time that the soil surface is covered each year. Increase plant diversity. Diversity is beneficial for several reasons. Each crop contributes a unique root structure and type of residue to the soil. A diversity of soil organisms can help control pest populations, and a diversity of cultural practices can reduce weed and disease pressures. Diversity across the landscape and over time can be increased by using buffer strips, small fields, contour strip cropping, crop rotations, and by varying tillage practices. Changing vegetation across the landscape or over time increases plant diversity and the types of insects, microorganisms, and wildlife that live on your farm.

State Soil? Ohio's 'Unofficial' State Soil (.pdf) is an article published in 2003 in 'The Buckeye', the official publication of the Ohio Nursery and Landscape Association. Written by freelance author Becke Davis. It is an informative article with some humor mixed in and makes a good argument for the state soil to be officially recognized.

What Does Soil Do?

Healthy soil gives us clean air and water, bountiful crops and forests, productive rangeland, diverse wildlife, and beautiful landscapes. Soil does all this by performing five essential functions:

1. Regulating water. Soil helps control where rain, snowmelt, and irrigation water goes. Water and dissolved solutes flow over the land or into and through the soil.
2. Sustaining plant and animal life. The diversity and productivity of living things depends on soil.
3. Filtering potential pollutants. The minerals and microbes in soil are responsible for filtering, buffering, degrading, immobilizing, and detoxifying organic and inorganic materials, including industrial and municipal by-products and atmospheric deposits.
4. Cycling nutrients. Carbon, nitrogen, phosphorus, and many other nutrients are stored, transformed, and cycled through soil.
5. Supporting structures. Buildings need stable soil for support, and archeological treasures associated with human habitation are protected in soils.

Ideal topsoil

Ideal topsoil will have three components in the proportions shown on the chart.

The percentages in the chart are dependent on soil texture, soil structure and
amount of organic material in a soil.

Soil Texture

Soil texture is the proportion of sand, silt, and clay in a soil. A soil’s texture can range from a very sandy soil to a very clayey soil.

A very sandy soil allows water to move through it quickly which can result in groundwater contamination if the soil was to be used as a filter (septic leach field would be an example). Also a very sandy soil does not have much ability to hold water which can lead to plant stress during dry periods.

A very clayey soil allows water to move through it very slowly which allows water to pond and keeps the soil saturated for a long time during wet periods. In addition clayey soils have a very high ability to hold water which further impacts the wetness of the soil.

Having a very clayey or sandy soil is not the ideal situation.

For most land uses, a loam texture soil is ideal.

A loam texture is approximately 40% sand, 40% silt, and 20% clay.

The textural triangle shows different soil textures and relative percentages of sand, silt, and clay.

Soil Texture can be quantified in two ways:

1. Field estimation by a trained professional
2. Lab testing

Particle-Size Distribution Relative Size Comparison of Soil Particles Sand Silt Clay beachball frisbee dime

Making Sense of the Particle-Size Distribution Measurements

The amount of each size particle (sand, silt, or clay) in the soil is called the particle-size distribution.

Knowing the particle-size distribution of a soil sample helps to understand many soil properties such as how much water, heat, and nutrients the soil will hold, how fast water and heat will move through the soil, and what kind of structure, bulk density and consistence the soil will have.

Sand, silt, and clay are the three particle sizes of mineral material found in soils. The amount of each of these is called the "particle-size distribution" and the way it feels is called the "soil texture." Sand is the largest sized particle, silt is medium sized, and clay is the smallest.

Soil Structure

Soil structure is how soil particles are arranged.

Like soil texture, soil structure also plays an important role in the movement of water. For example, water will move faster through a very clayey soil with good structure than a very clayey soil with poor structure.

The chart illustrates examples of different types of structure.

Soil structure can be determined by a trained soils professional.

Soil Quality

Soil quality is based on how well the soil does what we want it to do. More specifically, soil quality is the capacity of a specific kind of soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation.

People have different ideas of what a quality soil is.

• For people active in production agriculture - highly productive land, sustaining or enhancing productivity, maximizing profits, or maintaining the soil resource for future generations.
• For consumers - plentiful, healthful, and inexpensive food for present and future generations.
• For naturalists - soil in harmony with the landscape and its surroundings.
• For the environmentalist - soil functioning at its potential in an ecosystem with respect to maintenance or enhancement of biodiversity, water quality, nutrient cycling, and biomass production.

Soil Has Both Inherent and Dynamic Quality

Inherent soil quality is a soil’s natural ability to function. For example, sandy soils drain faster than clayey soils. Deep soils will have more room for roots than soils with bedrock near the surface. These characteristics do not change easily.

Dynamic soil quality is how soil changes depending on how it is managed. Management choices affect the amount of soil organic matter, soil structure, soil depth, water and nutrient holding capacity. One goal of soil quality research is to learn how to manage soil in a way that improves soil function. Soils respond differently to management depending on the inherent properties of the soil and the surrounding landscape.

Soil Quality is Linked to Sustainability

Understanding soil quality means assessing and managing soil so that it functions optimally now and is not degraded for future use. By monitoring changes in soil quality, a land manager can determine if a set of practices are sustainable.

Assessing Soil Quality

Soil quality is an assessment of how well soil performs all of its functions. It cannot be determined by measuring only crop yield, water quality, or any other single outcome. The quality of a soil is an assessment of how it performs all of its functions now and how those functions are being preserved for future use.

Soil quality cannot be measured directly, so we evaluate indicators. Indicators are measurable properties of soil or plants that provide clues about how well the soil can function. Indicators can be physical, chemical, and biological characteristics.

Useful indicators:

• are easy to measure
• measure changes in soil functions
• encompass chemical, biological, and physical properties
• are accessible to many users and applicable to field conditions
• are sensitive to variations in climate and management.

Indicators can be assessed by qualitative or quantitative techniques. After measurements are collected, they can be evaluated by looking for patterns and comparing results to measurements taken at a different time or field.

Soil Quality is Not an End in Itself

The ultimate purpose of researching and assessing soil quality is not to achieve high aggregate stability, biological activity, or some other soil property. The purpose is to protect and improve long-term agricultural productivity, water quality, and habitats of all organisms including people. We use soil characteristics as indicators of soil quality, but in the end, soil quality must be identified by how it performs its functions.

For more information: A Compendium of On-Line Soil Survey Information
For more soils education information: http://www.soils.org/lessons