Lecture 45

Soil Chemistry

Reading Assignment: Read the information below and the Brief History of Soil Chemistry by Sonon, Chappell and Evangelou at Iowa State University. Study the soil biogeochemical processes diagram from the Stanford Soil Chemistry web page and read the information about the basic 12 soil types at the University of Idaho Soil Taxonomy web page, linked below.

Homework: HW-14, Due Friday, May 2. WARNING: Final Exam is scheduled for 10:00-12:00 noon, Monday, May 5!!!!

Summary and Important Terms for Chapters 16

Links to Additional Resources

History of Soil Chemistry by Sonon, Chappell and Evangelou

Stanford soil biogeochemical processes

University of Idaho Soil Taxonomy

Desertification: Loss of fertility in soils associated with water loss and drought.

Essential Micronutrients: B, Cl, Cu, Fe, Mn, Mo, S & Zn.

Macronutrients: Ca, C, H, Mg, N, O, P, K & Na.

Non-essential Micronutrients: Al, Cd, Co, Pb, Hg, Ni, Se & Si.

Rhizosphere: The layer of the soil where plant roots are most active.

Soil Conservation: Reducing soil loss from cultivated land through the use of modern agricultural practices.

Soil is a precious resource that is every bit as essential to life as air and water--and just as susceptible to pollution! For most of us, the soil is the part of the lithosphere with which we are most familiar. Our agricultural system depends on healthy soils to grow food. The economies of many regions are totally dependent upon "cash crops" that provide income for many people, starting with the farmer who raises the crop and ending with the retailer who markets and sells the food we buy. No nation has ever been successful without first being able to provide food for its citizens.

Grains, vegetables and fruits come from plants growing in the soil. The chemical composition of these food sources is predominately carbon, hydrogen, oxygen, phosphorous, nitrogen, potassium, sodium and calcium. Plants extract these elements from the soil in which they grow and configure them into the products we recognize as food. Each plant has unique nutritional requirements that are obtained through the roots from the soil. A healthy soil has physical properties that allow roots to easily penetrate to the smallest particles, a high exchange capacity to allow nutrients to be used as needed, and the correct chemicals needed for optimum growth rates. The same principles that govern transport of pollutants also govern the way in which nutrients are moved into growing plants. Nutrients are stored in soil on "exchange sites" of the organic and clay components. Calcium, magnesium, ammonium, potassium and the vast majority of the micronutrients are present as cations under most soil pHs. The cation exchange capacity of soils is an important measure of its ability to store these nutrients and provide them to growing plants as needed. Organic substances (humic and fulvic acids, humus) contain exchange sights because of the presence of carboxylic acids. An organic component is an essential ingredient of all healthy soils.

The carboxylic acids exchange protons in soil in the same manner that protons are exchanged in aqueous solution. Soil pH is therefore an important parameter, which affects the ability of a soil sample to exchange cations and the ease with which nutrients move through the soil. Ideally, a soil sample should have a pH near 6.5 to provide for optimum nutrient storage capacity and ease of movement to the plant roots. Phosphorous is present in soil as orthophosphate (PO₄^3-). The two forms of phosphate that are present in soil under most conditions, HPO₄^2- and H₂PO₄^-, are anions. Phosphate complexes of most metals have very low solubilities, and phosphate is therefore relatively immobile in soil.

Nitrogen, phosphorous and potassium are used in large quantities by all growing organisms. Since they are typically present in soil at the percentage level, they are call macronutrients. Deficiency of any of these three elements reduces plant growth and lowers crop production. Fertilizers are substances used to replenish these essential nutrients. Synthetic chemicals are often used for this purpose, but manure and compost work just as well (some people think they are better), and permit organic wastes to be put to good use. The macronutrients are used by plants (and animals) to build amino acids and proteins. Phosphate forms the backbone of DNA and is used to store energy in chemical bonds.

Table 45.1 The Macronutrients

Micronutrients are classified as "essential" and "non-essential." An essential plant nutrient is one that is required for life, whereas a non-essential plant nutrient (present in soil a very low levels) will increase crop yield--but its absence will not cause the organism to die.

Table 45.2 The essential micronutrients

Table 45.3 The non-essential micronutrients

Essential micronutrients are elements used at trace levels in enzymes to assist with special body processes. Some examples where essential micronutrients are required include oxygen transport by hemoglobin and electron transport in cell metabolism. The role of non-essential micronutrients is not well understood and is an area of on-going research.

Pesticides are used in all forms of agriculture to control unwanted insects and plants. Paris green, a form of arsenic, was used to control the potato beetle until the advent of modern synthetic pesticides that have been used since the 1950s. Fields that were treated with Paris green fifty years ago still contain unacceptable levels of arsenic, since the arsenic cannot be degraded into another substance and the only natural removal mechanism is leaching. Chlorinated hydrocarbons became popular as broad-spectrum pesticides after World War II, the most widely used being DDT (dichlorodiphenyltrichloroethane). Other pesticides that are classified as chlorinated hydrocarbons include: Methoxychlor, Dieldrin, Endrin, Chlordane, Aldrin, Endrin, Heptachlor, Toxaphene and Lindane. These substances do not last as long as non-degradable substances like arsenic, but they do have unacceptably long half-lives in the soil. Table 45.4 lists the half-life in soil for several of these compounds. The very long time needed to degrade chlorinated hydrocarbons, and the fact that they are concentrated in the food chain, led to legislation banning their use in the United States and Europe. Unfortunately, these compounds are still in common use in underdeveloped countries. Modern pesticides have half-lives of weeks or days, and if used properly, do not build up in soil like their predecessors.

In years past it was common practice to deposit industrial wastes in landfills of simply bury containers of waste chemical in the soil. This material is not easily degraded and has the potential to pollute the groundwater. Once in the soil, these substances can enter the food chain by being incorporated into plant tissue, which is eaten by livestock. The most infamous example of such a case occurred in Michigan, where dairy cattle were exposed to PCBs. PCB contaminated milk was consumed by people who drank the milk and the cattle had to be destroyed.

Lead can be a problem when lead washes off buildings that have been painted with lead based paint (common with old buildings). Lead additives to gasoline caused significant increases to lead soil levels near major highways. Legislation to remove the lead additives stopped this source of lead, but the legacy of lead based gasoline will remain in the soils near major metropolitan thorofares for years to come. In years past, lead and copper smelters caused significant environmental degradation with fallout from the smelting process. Restrictions on environmental emissions have done much to correct this problem, but the soil near most smelters is still contaminated. In most cases, the only mechanism for cleanup of these areas is the natural leaching process that was discussed earlier in the course.

Soil is a resource that can be lost through erosion or degraded to the point where it will no longer support plant life. Loss of fertility is a major problem in arid regions of the world, and is responsible for significant decreases in the earth’s arable land. Desertification describes this loss of fertility when drought, or lack of water for irrigation, is the cause for loss of viability. Many countries in Africa and the Middle East must deal with desertification in an on-going basis. It is a problem in the southwestern U.S. and in South America and Australia. Regulating the types and amounts of crops that are planted and the number of livestock that can feed on grasslands where there is limited precipitation is an approach to this problem that seems to be working.

Soil loss through erosion is a serious problem in many locations, but can be controlled with modern tilling practices. Sediment traps can be built to recover soil that washes off steep terrain and till-less agricultural methods have been successfully used in China, Japan and Israel to minimize soil loss in locations that have excessively steep slopes.

ENV 440 - Course Topics

Environmental Chemistry -- ENV 440
Last Updated:  04/30/2008