Density of Topsoil

What Is Topsoil?

• In diagrams of soil layers, called soil horizons, topsoil is usually labeled as the A layer, occurring just below the O layer composed purely of decaying plant matter in various stages of decomposition. As the O layer decomposes, its nutrients sink down into the A layer, which accounts for the topsoil's characteristically dark rich color. What starts out as a pure mineral base composed of parent materials eventually evolves into denser soil whose particles are bound more tightly together due to the high content of organic materials and moisture. An ideal topsoil reflects a ratio of 1/2-pore space to 1/2-solid material. Water passes quickly through large pores that fill with air while smaller pores hold water for longer periods of time.

Soil Structure

• Density is controlled by many factors, including the size and shape of the individual soil particles, how close they are, how they fit together and the gaps, or pores, that exist between them. Rainfall amounts also impact soil density as the more water the soil contains, the denser it is. According to the University of Minnesota, organic matter plays a primary role in soil development and modification, and its stabilization is dependent upon the types of binding agents that hold the individual particles together. Binding forces include fungi, plant roots and the freezing and thawing that occur during the cold months.

Importance of Soil Density

• Water moves much more rapidly through sand than it does through clay, due entirely to the density levels reflected in each type of soil. Density affects topsoil porosity, which is the ability of the soil to absorb water, and permeability, which is its capacity to move water through the spaces between the individual particles. It's easy to see why this would be important to so many people, from gardeners to soil conservationists. Plants have a harder time growing in very dense soils while from a conservation point of view, erosion is helped along when water cannot move through the soil, leaving it no choice but to simply wash it away.

Considerations

• Very dense soil compacts easily as is seen in areas where it has a high clay content. Unless it is amended with lots of organic materials, most plants will grow poorly, if at all, in clay soil as there is virtually no pore space for roots to develop and grow. Conversely, very loose, dry sandy soils allow water to pass through too quickly, making them also unsuitable for crops. The density properties of each can be combined, and that, with the addition of organic matter, produces a good quality topsoil that will support all plant life.