Soil Basics Part I: Physical Properites of Soil

Soils are one of the most important resources a farmer has. Soil health is fundamental to profitable and sustainable production. How we manage nutrients, organic matter and microbial populations is key to building soil health. Following is a series of fact sheets on soil basics discussing physical and chemical properties of soils, soil organic matter and using soil tests as a tool in building and managing nutrients and soil quality.

Part 1: Physical Properties of Soil

This Fact Sheet is the first of four, about the most important resource in agriculture

Soils are the most basic and most important resource we use in agriculture. Proper management of the soil is key to plant health and crop productivity. This fact sheet on physical properties of soil is the first in a series of four about soils, their management and fertility. Up-coming fact sheets will address some of the most commonly asked questions about soils, organic matter, soil testing, fertility and management.

Soils are composed of solid particles which have spaces between them. The soil particles consist of tiny bits of minerals and organic matter. The spaces between them are called pore space and are filled with air and water. It is desirable for an agricultural soil to have about one-half soil particles and one-half pore space by volume. Ideally, organic matter will account for 5% or more of the weight of soil particles. Moisture content varies considerably with factors such as soil drainage and the amount and frequency of rain or irrigation. For most agricultural crops conditions are best when the pore space is filled about equally with water and air.

Figure 1.

Mineral soil particles are derived from rock which has been broken down into smaller and smaller pieces over millions of years. This process is referred to as "weathering" and is caused by physical and chemical factors. Physical weathering is a result of mechanical activity. Wind, running water, falling rain, glaciers, freezing and thawing and root growth are typical abrasive forces which cause physical weathering. Chemical weathering results from the billions of chemical reactions that occur continuously in our soils. As some of the mineral components of rock are dissolved by water or acids, small fragments break off. Over time rock material is converted to many tiny fragments. Microbes play an important role by creating many of the organic and inorganic acids which contribute to this process.

Mineral soil particles vary considerably in size. These particles are grouped according to size. Beginning with the smallest sized particles these groups are classified as clays, silts, sands and gravel (Table 1).

Table 1.

Texture is the proportional amount of each of these groups. Soils consist of mixtures of various size particles. A soil textural triangle (Figure 2) is used to determine the textural class of soil according to the percent sand, silt and clay. These percentages can be determined from a mechanical soil analysis which can be done by most soil testing laboratories. To use the textural triangle, draw a line parallel to the appropriate arrows through the per cent sand, silt and clay. These three lines will intersect at a point inside the triangle indicating the soil textural type. Note that the word "loam" does not refer to a specific group of particles, but is used to describe mixtures of sand, silt and clay.

Figure 2.

Soil texture is determined solely by the sizes of the mineral particles. Weathering can change the size of these particles, but only over thousands or millions of years. For all practical purposes, the texture of the soil does not change except for loss or deposition of soil due to erosion. We can also haul in new soil, but this is rarely practical in agriculture.

Soil texture has a major effect on the physical and chemical characteristics of soil. We'll discuss the physical effects now and talk about the chemical implications in an up-coming fact sheet. Sandy soils have rather large particles and large pore spaces (macro pores). Clay soils have very tiny particles with very small pore spaces (micro pores), but because there are many times more pore spaces, clay soils have greater total pore space than sandy soils. Capillary action is much greater in micro pores than in macro pores. Clay soils absorb and retain much more water than sandy soils, but are typically poorly drained and not well aerated. Loams combine some of the moisture retention characteristics of the clays with the aeration of the sands and are widely considered the best agricultural soils. Sandy soils are coarse-textured are often referred to as "light" because they are easy to work. Clay soils and fine-textured and their particles will bond tightly together when they dry out after being wet. These soils can become very hard and difficult to work and are often called "heavy". Keep in mind that the terms "heavy" and "light" refer to the ease with which the soil can be worked and not to its weight (a sandy soil actually weighs more than a clay soil).

We have said nothing about organic matter.. That's because a soil's texture is determined by the sizes of its mineral particles, not its organic particles. It sounds strange to most people that the word "loam" has nothing to do with organic matter. Although organic matter has nothing to do with soil texture, it is vitally important regarding soil structure.

While soil texture is of great importance, the grouping or aggregation of soil particles has a great deal to do with its productivity. Structure is the over-all arrangement or aggregation of soil particles. Terms such as loose, hard-packed, granular and cloddy are among those use to describe structure. Soil structure can be modified by activities such as tillage, moisture level, freezing and thawing, root growth, earth worms and other soil inhabiting animals and driving or walking on the surface.

Very sandy soils nearly always have a loose structure because they don't form aggregates or become hard-packed or cloddy. Fine-textured soils can become hard-packed. This condition interferes with root growth, inhibits movement of water into (infiltration) and through (percolation) the soil. The micro pores in fine textured soils can easily be filled with too much water to the exclusion of air and the exchange of gases (oxygen and carbon dioxide) is limited. The macro pores of coarse-textured soils facilitate infiltration and percolation of water and the exchange of gases, but they retain little water for crop use. By loosening and granulating a fine textured soil, we can improve water infiltration and percolation and gas exchange and still maintain the ability to retain water for plant growth. A granulated soil consists of granules that resemble crumbs. A granule consists of millions of clay or silt particles clumped together as aggregates. A well granulated soil has micro pores within the granules and macro pores between the granules.

Natural activities including freezing and thawing and the movement of roots contribute to granulation of soils. Tillage at proper levels of soil moisture is an effective way to cause granulation. Excessive tillage in an effort to prepare a fine seed bed, especially when soils are dry, will destroy soil aggregates. It is very easy to over work a soil with a roto tiller. Rain or irrigation can also destroy soil aggregates. We must therefore be aware of factors that influence the stability of soil aggregates.

It might seem as though granulation is only a physical process, but biological processes are just as important. Earth worms pass soil through their digestive systems, adding viscous juices which bind particles together. Snails and other organisms leave a trail of slime behind them which acts as a glue. Organic matter is an important factor in the formation of soil aggregates and it adds greatly to their stability. Soil organic matter, particularly humus, is a binding agent which holds clay particles together. Its often said that organic matter is the sticky stuff that holds soil particles together. There is a lot we don't know about these processes, but it appears that chemical unions occur between humus and clay particles. It seems clear that soil organic matter plays a major role in granulation. Increasing the stability of soil aggregates, makes the soil easier to work and more resistant to compaction.

Organic matter not only improves the structure of fine-textured soils; it is equally beneficial for coarse textured soils, but in a different way. These soils have a high proportion of macro pores, facilitating gas exchange and water movement. However, due to a low proportion of micro pores, these soils are not moisture retentive. This makes frequent irrigation a necessity during dry periods. Organic matter substantially increases the proportion of micro pores, greatly improving the water holding capacity of a coarse-textured soil.

In the next issue we will discuss soil fertility and how to interpret soil tests.

The information in this material is for educational purposes. The recommendations contained are based on the best available knowledge at the time of printing. There trade names or commercial products are used, no company or product endorsement is implied or intended. Always read the label before using any pesticide. The label is the legal document for product use. Disregard any information in this newsletter if it is in conflict with the label.

By John Howell, University of Massachusetts Extension
VegSF 1-98
Printed October 1997