Physical Properties of Soil

• color
• texture
• tilth
• water-holding capacity
• drainage
• depth
• slope

   Color 

The Color of a soil can give clues to its health, origin, and long-term changes. It can also indicate the color of the parent material. Subsoil color can be a valuable indicator of how well the soil drains, which can be influence by the Topography. We discuss this more in Drainage.

Dark colors in the topsoil usually indicate the that soil has a high organic matter content. You can often see this in prairie grassland soils of the Great Plains in the United States. The more humus the blacker the soil. It could mean that the parent material from which the soil developed was also black. Remember the soils that derived from coral on the Pacific? They are really black, but that is because the parent material, the coral, was black.

Distinct red and yellow colors usually indicate older, more weathered soils.

   Texture

Texture refers to the relative proportions of sand, silt, and clay in a soil. Soils get their textural names, 12 in all, and some specific physical properties from the proportions of these three particle sizes. Note that humus content technically has nothing to do with texture.

   Tilth

Tilth refers to a soil's physical condition. A soil in good tilth

• is easily worked,
• is crumbly, and
• readily takes in water when dry.

Factors that influence a soil's tilth include:

• texture
• organic matter
• moisture content

Therefore, tilth can vary markedly with changes in moisture content, the amount of humus present, and compaction, especially if it is clayey. Tilth can be improved, which is why farmers plow their fields to break up the clods and add manure to their fields when they can.

Soil with poor tilth	Soil with good tilth

   Water-holding Capacity

About half of a soil's volume is pore space that is occupied by varying amounts of air and water, depending on how wet the soil is. Water is held in the pore spaces in the form of films adhering to the soil particles. Small pores are called micropores, while large pores are called macropores.

Macropores

Macropores do not hold water well because the water films become too thick to adhere well to the surrounding soil particles. This water is lost downward as it drains below the root zone by gravity. So, macropores allow a soil to retain enough air for root growth, as long as drainage is not impeded.

Micropores

The films of micropore water, however, resist being drained away by gravity and are responsible for the water-holding capacity of soils. This water is what the roots can tap into and extract for plant use.

As you may guess, sands have a lot of macropores due to their large grain size but few micropores. Thus, their water-holding capacity is low although their drainage is good.

Heavy clay soils that have a lot of micropores may have a higher water-holding capacity, but because they have fewer macropores, their drainage is poor. Plants really like a soil that lies in between these two extremes so that their roots have both the air and water that they need.

 

   Drainage

Drainage refers to a soil's ability to get rid of excess water, or water in the macropores, through downward movement by gravity. It is affected by topography, texture, tilth, depth, and the presence of compacted layers in the subsoil.

With few exceptions, one would be crops like rice, most plants need fairly good drainage. Without good drainage, plant roots would lack oxygen, nitrogen would be lost, and certain elements like iron and manganese may become soluble enough to injure plant roots. Although clayey soils are more likely to have drainage problems, drainage problems also occur on sandy soils where the watertable is close to the surface. The water table is the upper surface of the ground water below which the soil is completely saturated with water.

Soil color can be affected by drainage. Soil color can be a tool to check if your soil is having drainage problems.

Red, reddish-brown, or yellow subsoil colors generally indicate good drainage. The presence of sufficient air allows the soil's iron and manganese to oxidize or "rust" and give these brighter colors.	Dull grays and blues indicate a reduced state, with little oxygen, which means poor drainage.

   Depth

Soil depth is important for plants because deeper rooting means more soil to explore for nutrients and water. Greater soil depth can also mean better drainage, as long as there are no restrictive layers in the subsoil.
 
 

   Slope

A field's slope has a marked influence on the amount of water runoff and soil erosion caused by flowing water. Slope is usually measured in terms of percentages. A ten percent slope has ten feet of vertical drop per 100 feet horizontal distance. Soil conservation measures become necessary on land with as little of a slope as 1-2% to avoid erosion problems.

Flat slope	Gently rolling slopes	Steep slope

   Questions:

1. Which of the following pictures depicts a shallow soil?
2. Which of the following pictures depicts a deep soil?
3. If you were a plant, in which soil would you rather be? Why?

Soil A	Soil B