Soil is a living ecosystem that includes minerals, air, water, habitat for creatures and the creatures themselves.
Why is soil important?
A typical soil in good condition is composed of approximately 45% mineral matter, 25% air, 25% water and 5% organic matter.
Soils vary across the landscape. The development of a soil reflects the weathering process associated with the dynamic environment in which it has formed. Five soil-forming factors influence the development of a specific soil:
Whenever these five factors are the same on the landscape, the soil will be the same. However, if one or more of the factors varies, the soils differ as well.
Approximately 45% of soil is made up of mineral matter, or tiny rock fragments. The size of the mineral soil particles varies substantially, from large bits of gravel to microscopic clay fragments. Soil mineral particles are classified based on their size.
Clay particles are so small that you would need a powerful microscope to see them.
Soil texture is determined by the ratio of sand, silt and clay particles, which gives soil its look and feel.
A simple test lets us measure the ratios of sand, silt and clay in a soil. Once these ratios are known, you can use the Soil Texture Triangle to identify the texture of the sample.
Soil structure describes the size, shape and friability (or crumbliness) of the aggregates that form a soil. The aggregates are formed from sand, silt, clay and organic material bound together with mineral and organic cements. The crevices and spaces in and between the soil aggregates are important habitat for soil biota. They also are critical for holding moisture. While soil texture cannot be easily changed, soil structure can be improved through good management practices.
Stable soil aggregates are the underpinnings of a healthy soil habitat. Here, fungal hyphae, organic matter and mineral and organic cements known as glomalin hold mineral particles together.
Soil aggregates that hold together, even when wet, are critical to good soil structure.
The Natural Resources Conservation Service of the U.S. Department of Agriculture has identified and mapped soils for most of the U.S. Each of these soils is defined by the texture and horizons that compose the soil and is referred to as a soil series. Many of these soil series are named for a town or feature near where the soil was first identified. Each state in the U.S. has a state soil series, just as states have official state birds and flowers.
Find maps and descriptions of local soil series in county soil surveys. The online tool Web Soil Survey is the most up-to-date place to find soil maps and soil survey information.
The soil survey includes valuable information about each soil series. This includes the potential uses of the soil (for agriculture, grazing, forestry, building foundations, septic drain fields); limitations, such as erosion potential or poor drainage; and productivity for agriculture or forestry uses. You can learn a lot about potential uses of your land by learning what soil types you have and their uses, limitations and productivity.
You can quickly generate a soil map of an area by entering the address or site coordinates on the Web Soil Survey page or on a mobile app. In addition to a map showing the names of the soils, you will see a description of soil textures and slope in a table. Clicking on the highlighted soil name in the table generates a more thorough description of each soil.
A soil texture jar test will tell you the proportions of sand, silt and clay in the soil on your property.
Follow instructions from a University of California Agriculture and Natural Resources handout.
A slake test, where soil clods are saturated in water, will tell you about your soil’s structure and aggregate stability. Stable aggregates, those which do not dissolve when wet, are key to healthy soil. Ray Archuleta, a conservation agronomist at the NRCS, describes this test in a USDA video.
While test kits can allow you to evaluate your soil’s chemical properties at home, send a soil sample off to a commercial laboratory for the most accurate results. Routine soil tests will evaluate the soil’s pH or acidity and measure the phosphorus, potassium, calcium, magnesium and organic matter content. These are key factors in growing healthy and productive plants in your garden, lawn or crop field.
Collect a representative soil sample from the areas of your property in which you’re interested. See A Guide to Collecting Soil Samples for Farms and Gardens.
Many commercial laboratories and universities perform routine soil fertility testing.
Testing labs can provide recommendations of fertilizers and amendments needed to grow various plants or crops. Your local Extension agent can help interpret results. Or, consult the Soil Test Interpretation Guide, listed in “Resources.”
Many testing laboratories offer soil health assessments and tests for soil contaminants like pesticides and heavy metals. If you think you’re a candidate for these tests, contact a testing laboratory or your local Extension office.
The Natural Resources Conservation Service defines soil health or soil quality “as the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals and humans.” What soil health strategies you use depend on the soil type and your management goals.
Limiting physical disturbance and enhancing organic matter in the soil are key to soil health. NRCS has developed four principles of soil health to implement on your property:
Tillage, the mixing or cultivation of soil to control weeds or prepare a seedbed, is detrimental to healthy soils. The physical disturbance caused by tillage breaks up soil aggregates, which can reduce water infiltration, limit root penetration and cause the soil to crust. Tillage can kill soil macroinvertebrates and reduce the presence of beneficial fungi such as mycorrhizae. Tillage breaks open soil aggregates, exposing organic matter to oxygen and microorganisms, which hastens decomposition and loss of carbon from soil. While tillage improves the tilth or workability of soil in the short term, tillage degrades the soil structure over the long term. Protect the soil ecosystem by developing a management plan that minimizes soil disturbance.
Limiting compaction is vital to maintaining soil pore space for gas exchange and water infiltration. Working soil while it is wet is a major cause of compaction because of the pressure exerted by tractor and truck tires. Livestock on wet soil also cause compaction, sometimes called “pugging.”
Limit compaction by:
Remember that deep-ripping or tillage usually provides a short-term fix to compaction problems. Don’t plan to rely on tillage to fix compaction caused by poor management practices.
Prevent erosion and build soil health by maintaining a layer of physical protection, either as a living plant or mulch. Most erosion is initiated by the impact of raindrops onto bare soil. The splash caused by raindrops or irrigation detaches soil particles from aggregates and allows them to be moved by water.
Some soils have the propensity to form crusts. When raindrops impact the soil and break down aggregates, the clay and silt particles wash into soil pores and create a crust upon drying. These crusts can prevent seedlings from emerging, decrease water infiltration, and increase erosion.
Living plants are the major food source for soil organisms. Many people don’t appreciate how “leaky” roots are. Roots can exude 10%–40% of the sugars made during photosynthesis, providing an important source of food for soil microbes. This process is called “rhizodeposition.” Roots also continually slough off cells, which microorganisms decompose.
There are also examples of direct partnership or symbiosis between plants and soil microbes. Two examples include the Rhizobium bacteria, which fix nitrogen symbiotically with legumes, and the mycorrhizal fungi, which help to transport phosphorus and water to plants. Maintain growing plants and roots for as much of the year as possible to ensure an active and functioning soil food web.
Biodiversity is the range of living organisms on your property and in your soil. It includes plants, animals (including livestock), insects and microorganisms. Increased diversity has been shown to make natural systems more resilient to change. Crop rotations reduce disease and insect pressure compared with monocropping. Diverse species mixtures in pastures and hayfields improve the stability of yield from year to year, and alfalfa, clover and other legumes contribute nitrogen to grass species. Studies have shown that increased plant diversity can increase the diversity and function of soil fauna and microbes.
Diverse cover crops in farm fields and home gardens often include different species of grasses, legumes and brassicas.
The health and quality of your soil determine the potential of everything growing on your property. Your goals may vary. You may want to protect and improve soil health in your forest, pasture, farm and garden. You may want to prevent soil from eroding in all of these situations and keep it from contaminating your stream and waterways. Each of these situations calls for different strategies. You may have some soil goals under other categories. Keep soil goals in mind as you formulate overall goals for your property. Here are a few examples of agriculturally oriented soil goals to spark your thinking:
Agroforestry for Ecosystem Services and Environmental Benefits, The Center for Agroforestry
Analytical Laboratories Serving Oregon, EM 8677
Building Soils for Better Crops: Sustainable Soil Management, Sustainable Agriculture Research and Education
Conservation Buffers: Design Guidelines for Buffers, Corridors, and Greenways, General Technical Report SRS-109 U.S. Department of Agriculture, U.S. Forest Service
Cover Crop Innovators Video Series, Sustainable Agriculture Research and Education
Cover Crops for Soil Health Workshop, SARE
Crop Rotation on Organic Farms, SARE
Does Glomulin Hold Your Farm Together?, USDA Agricultural Research Service
The Five Factors of Soil Formation, Virtual Soil Science Learning Resources
A Guide to Collecting Soil Samples for Farms and Gardens, EC 628
Integrating Livestock and Crops: Improving Soil, Solving Problems, Increasing Income, ATTRA — Sustainable Agriculture Program
Living Mulch Builds Profits, Soil, American Society of Agronomy
Managing Cover Crops Profitably, SARE
No-Till Cover Crop Roller, Rodale Institute
Rototill Sparingly, University of Maryland Extension
Sheet mulching — aka lasagna composting — builds soil, saves time, OSU Extension Service
Silvopasture: An Agroforestry Practice, EM 8989,
Soil Food Web, Natural Resources Conservation Service
Soil Health, NRCS