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Soil Profile

in Issue 29 by

The soil profile is an important tool in farm management. A soil profile, can give valuable insight into soil texture and fertility which is very important in growing crops. This simple overlooked procedure can save a lot of problems by taking a peek on what is laying beneath the surface of the land.

A soil profile is made up of distinct layers, known as horizons. The five most common horizons are collectively known as the master horizons. Scientists have developed methods to describe the various components and characteristics of the soil profile. By using common terminology, soil profile descriptions are valuable for deciding how the soil might be used and/or predicting how the soil might react to its intended use. Technical descriptions of the soil are not only useful for farmers, but for scientists, ecologists, soil engineers, hydrologists and land use planners.

A soil horizon makes up a distinct layer of soil. The horizon runs roughly parallel to the soil surface and has different properties and characteristics than the adjacent layers above and below. The soil profile is a vertical section of the soil that depicts all of its horizons. The soil profile extends from the soil surface to the parent rock material.

The regolith includes all of the weathered material within the profile. The regolith has two components: the solum and the saprolite. The solum includes the upper horizons with the most weathered portion of the profile. The saprolite is the least weathered portion that lies directly above the solid, consolidated bedrock but beneath the regolith.

Master Horizons

There are 5 master horizons in the soil profile. Not all soil profiles contain all 5 horizons; and so, soil profiles differ from one location to another. The 5 master horizons are represented by the letters: O, A, E, B, and C.

O: The O horizon is a surface horizon that is comprised of organic material at various stages of decomposition. It is most prominent in forested areas where there is the accumulation of debris fallen from trees.

A: The A horizon is a surface horizon that largely consists of minerals (sand, silt, and clay) and with appreciable amounts of organic matter. This horizon is predominantly the surface layer of many soils in grasslands and agricultural lands.

E: The E horizon is a subsurface horizon that has been heavily leached. Leaching is the process in which soluble nutrients are lost from the soil due to precipitation or irrigation. The horizon is typically light in color. It is generally found beneath the O horizon.

B: The B horizon is a subsurface horizon that has accumulated from the layer(s) above. It is a site of deposition of certain minerals that have leached from the layer(s) above.

C: The C horizon is a subsurface horizon. It is the least weathered horizon. Also known as the saprolite, it is unconsolidated, loose parent material.

Knowledge of the soil profile is very important when establishing a new orchard or crops. It reveals the properties of the soil which are not visible at the surface. Now that the different components of a soil profile have been explained above, an example in the field will be described. The following soil profile was done on in a field in Stann Creek Valley. The objective of the exercise was to see how adequate the soil was for establishing a new citrus grove and what impediments exists that would affect the life of the new orchard. The soil profile is adequate for citrus if the following conditions are met:

  • A horizon A of more than 30 cm in depth and with good texture

  • A horizon B with more than 75 cm in depth

  • A stable water table below 75 cm

  • Absence of compacted layers.

  • A soil pH of 5.5 to 6.5

  • Slope of land is less than 3 %

  • Two bores were made on the site to study the underlying properties of the soil.

The first profile was made about 650 feet from the southern end of the project site. The first three inches was a dark brown clay loam with plenty of roots and organic matter. The second layer consisted of a light brown clay loam with crumbly structure with the presence of plant roots in this layer. The third layer was a brick red, gritty clay, course and easy to break up. This layer continued up to 23 inches in depth. Under the red clay layer the soil color changes to a light red, brown and yellow mottle dense fine clay layer. This type of soil continued up to thirty inches where the soil profile ended. (See Figure 1)

The second soil profile was done on the northern end of the property. The first two inches of topsoil is a light brown clay loam with crumbly structure and with plenty of roots and decomposing organic matter. The second layer is also a brown clay loam with crumbly structure and has the presence of roots. This layer has good drainage. The third layer is a dense fine clay with red, brown and yellow color. This layer is hard and compacted. It was very difficult to dig and drainage was poor. There is very little presence of roots in this layer.

The slope of the land was less than 3% and appears to be from the western end to the eastern end. The higher land was located on the western border with dense forest canopy and the presence of cohune trees. As the slope of the land decreased the forest cover changed and on the eastern portion there was a mixture of pine trees with sedges and cutting grass typical of low lying areas.

In order for citrus to grow properly the land will need to be drained by installing at least two main drains to channel off the excess water. There will be the need to make cambered beds to raise the level to at least 18 inches to allow roots to penetrate up to three feet in depth.

The soil profiles showed that there is a dense clay layer at 24 to 30 inches. If the required root depth for citrus of four feet is needed then cambering has to be done to raise the soil level above the natural level of the land. This will allow for drainage of the soil and provide ample soil depth needed for root development. The compacted subsoil should be ripped to loosen this layer and permit better drainage. The land has very little gradient and deep drainage canals are needed to channel excess water. These modifications are necessary to make the land suitable for citrus production.

Figure 1. Soil profile for citrus orchard.

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