What is Soil?

There is archaeological evidence of early farming practices that date back 12,000 years ago. These early farmers laid the foundation for modern agriculture. Like today's farmers their goal was to maximize plant production. The foundation of this production is the field, which at its most fundamental level consists of a variety of soils distributed across a landscape. Two basic questions are: (1) What information can a practicing agronomist provide a producer about soils that will benefit crop growth? (2) What are the consequences of these management decisions to resources outside of the field and crop growth?

The first step toward answering these questions is to define soil:

soil: The superficial, unconsolidated, and usually weathered part of the mantle of a planet and especially of the earth.
soil: The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants.
soil: The unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and micro-organisms, conditioned by relief, acting on parent material over a period of time. A product-soil differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics.
soil: a dynamic body composed of mineral and organic solids, liquid, and living organisms which serve as a medium for plant growth.
soil: the collection of natural bodies occupying parts of the earth surface that is capable of supporting plant growth and that has properties resulting from the integrated effects of climate and living organisms acting upon parent material as conditioned by topography over a period of time.

As you can see the word soil may be defined in a number of ways. For the purpose of this course we will define soil as: a terrestrial medium capable of supporting plant life. The role of soil in supporting plants includes:

  • anchoring and providing nutrients to roots
  • maintaining nutrient cycling via decomposition of animal and plant residues
  • storing and transporting water
  • exchanging gases, especially carbon dioxide (CO2) and oxygen (O2), between the ground and the atmosphere
  • buffering environmental change

If a field is defined as a variety of soils distributed across a landscape, would you expect the field to be uniform? See Fig.2 below. How many types of variability could we infer examining a photograph of this field? Some are easy to see, crops being grown and position on the landscape varies as we move across the field. There are also a myriad of invisible variables that we cannot see in this particular photograph. The soil profile figure below is a better indicator of variability across the landscape but it is sometimes difficult to understand without this visual level of detail. The primary goal of this course is to understand the nuances of soil variability, holistically and within three major areas of soil science: soil physics, soil chemistry and soil biology. We will also infer how these characteristics contribute to the variability of the landscape, farm fields, and ultimately crop production.


Fig. 2 Strip cropping in Southcentral Iowa.

 So, how do these properties of soils relate to management decisions made by farmers every year? Soil is an integral part of the agroecosystem and in large part responsible for plant nutrition (total nutrients, available nutrients, available water). Soil sampling and soil testing for nitrogen (N), phosphorus (P), potassium (K), sulfur (S), pH, and cation exchange capacity (CEC) allow farmers to evaluate the fertility status of the soil. Nutrient availability can be manipulated by adjusting pH with soil amendments (lime and sulfur are commonly used). CEC is an indicator of the amount of nutrients (cations) that will adsorb onto negatively charged soil particles that subsequently will be accessible to growing plants. Soil water content is dependent on climate and the characteristics of the soil. Compaction results in increased runoff, increased surface erosion, decreased available water capacity, and decreased gas exchange. Drainage can also affect gas exchange and tile (artificial drainage) is integral to agricultural production when utilizing poorly drained soils. Conversely, excessively drained soil may require irrigation to be productive and with irrigation there is always the potential for soil salinity problems. Water, fertilizer, and amendments are routinely added to soil. Pesticides (herbicides and insecticides) are also applied directly to soil and foliar applications ultimately end up in soil also. Anything residing in soil may be lost through erosion (wind or water) or leaching or volatilization. The repercussions of these inadvertent losses have ecological impacts locally, regionally and globally. Also, any inputs lost from the system before they can be effective or used by plants is a direct financial loss to the producer.

Whew! Who knew there were so many considerations and consequences related to soils? The listing of considerations and interactions could go on and on, but the main point is that soils are complex (Fig. 3). Their properties are dependent on all of the factors that affect them and their interactions, whether they are chemical, physical, or biological or combinations (most likely). However, we are going to try to present the course content broken down into these areas and hopefully connect them together as we go along.


Fig. 3 Soil profiles and the landscape. Notice the variability of soils that can occur over short distances (Adapted from Nature and Properties of Soils, 2002).