Topsoil

Surface runoff, a type of nonpoint source pollution, from a farm field in Iowa during a rain storm.
Topsoil as well as farm fertilizers and other potential pollutants run off unprotected farm fields when heavy rains occur.

Topsoil is the upper, outermost layer of soil, usually the top 2 inches (5.1 cm) to 8 inches (20 cm). It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs. It also houses many organisms.

Importance

Plants generally concentrate their roots in and obtain most of their vital nutrients from this layer. The actual depth of the topsoil layer can be measured as the depth from the surface to the first densely packed soil layer known as subsoil.

Classification

In soil classification systems, topsoil is known as the "O Horizon or A Horizon," therefore, it is the very top layer.

Evaluation

When starting a gardening project, it is crucial to check whether or not the soil is satisfactory. Different types of plants vary in their nutrient needs and preferred soil conditions, many are strongly adapted to particular conditions. However, some general guidelines for "desired levels of Topsoil nutrients" have been made, broadly suitable for many plants.

Category	Desired Results
pH Level	5.0 to 6.2
Phosphorus (P-I)	Index of 50
Potassium (K-I)	Index of 50
Calcium (Ca%)	40-60% of Cation Exchange Capacity (CEC)
Magnesium (Mg%)	8-10% of CEC
Base saturation (BS%)	35-80% of CEC
Manganese (Mn-I)	Index > 25
Zinc (Zn-I)	Index > 25
Copper (Cu-I)	Index > 25

The two common types of Topsoil are Bulk and Bagged Topsoil. The following table illustrates major differences between the two.

Topsoil Type	HM%	BS%	pH	P-I	K-I	Ca%	Mg%
Bulk	0.3	69	5.2	009	026	45	10
Bagged	0.7	78	5.8	166+	178	56	12.3

Commercial application

A variety of soil mixtures are sold commercially as topsoil, usually for use in improving gardens and lawns, e.g. container gardens, potting soil and peat. Another important yet not commonly known use for topsoil is for proper surface grading near residential buildings such as homes. "The ground around the home should slope down six inches for the first ten feet away from the home. This can often be done by adding topsoil (not sand or gravel)."

Erosion

A major environmental concern known as topsoil erosion occurs when the topsoil layer is blown or washed away. Without topsoil, little plant life is possible. The estimated annual costs of public and environmental health losses related to soil erosion exceed $45 billion. Conventional agriculture encourages the depletion of topsoil because the soil must be plowed and replanted each year. Sustainable techniques attempt to slow erosion through the use of cover crops in order to build organic matter in the soil. The United States alone loses almost 3 tons of topsoil per acre per year. This is of great ecological concern as one inch of topsoil can take between 500 and 1,000 years to form naturally. On current trends, the world has about 60 years of topsoil left.

See also

• Agricultural wastewater
• Desertfication
• Dust bowl
• Erosion control
• No-till farming
• Nonpoint source pollution
• Subsoil
• Sustainable agriculture
• Waterway degradation

References

• The lowdown on topsoil: It's disappearing

External links

• USDA Electronic Field Office Technical Guide - Detailed soil conservation guides tailored to individual states/counties