INTRODUCTION TO ENVIRONMENTAL SCIENCES

LECTURE NOTES

Soil Resources and Conservation

Soil is a fundamental resource required for all agriculture. It is nonrenewable on a human time scale. There is a limited amount of it and if not used properly it can be degraded and/or eroded away. We have to use it in a sustainable fashion if we are not to negatively impact our ability to grow food.

Inappropriate use of soil also leads to other environmental problems, including air pollution, water pollution, and flooding. Controlling soil erosion helps control many of these other problems.

I. Soils: Structure and Types.

Soil is a complex mixture of regolith (weathered rock), humus (organic material), air, water, and living organisms. It takes natural processes thousands of years to make and is, therefore a nonrenewable resource.

Soil composition and characteristics change with depth. Divided into layers or horizons. A typical or average soil profile contains (from the top down):

O Horizon - Leaf litter; decomposing organic material.

A Horizon - Topsoil; organic rich; controls fertility. It is usually full of small, burrowing organisms. They help break down complex organic molecules to simpler forms, which are then taken up by plants through their roots. This horizon also needs to hold water effectively. Color depends on the exact mix of humus (dark) and regolith (light). This horizon controls the flow of nutrients to plants and is, therefore, critical in determining the fertility (ability to support plant growth) of the soil

B Horizon - Less organics, more regolith. Compounds leached from A Horizon (zone of leaching) often deposited here (zone of accumulation) to form a hard caliche deposit.

C Horizon ≠ Broken-up rock or regolith with little organic material.

D (or R) Horizon ≠ Parent rock or protolith.

Not all horizons are always there or fully developed. Many different types of soils are recognized, depending on local climatic environments and parent rock type.

Most agriculturally productive types are grassland and deciduous forest soils. Best type is a loam, a mix of clay, sand, silt, and humus. Loam has a good combination of nutrients, water holding and infiltration capacity, oxygen, and workability.

II. Soil Erosion and Loss.

Soil erosion is the loss of soil, particularly the O and A horizons, due to the action of the wind or water. May be caused or increased by various human activities. Most natural systems have a balance between rate of soil loss and rate of production.

Many human activities enhance or cause soil erosion, including farming, logging, construction, overgrazing, off-road vehicles, and slash-and-burn agriculture. Leave soil bare and unprotected by vegetation and its roots.

Soil particles removed by running water (most effective) and wind. Water erosion occurs as sheet erosion (wide surface flow), rill erosion (narrow rivulets), and gully erosion (wider and deeper ditches). Process tends to progress from former to latter over time with increasing amounts of soil being removed.

Loss of eroded particles makes remaining soil less fertile, clogs lakes and streams, pollutes these bodies of water, causes eutrophication, and increases flooding.

A. Global and U.S. Soil Loss

Globally, losing soil in agricultural areas at 7-100 times the rate it is being made. Affecting 1/3 of the world's cropland. 15% too degraded to support agriculture. Each year, we have to feed approximately 90 million more people with about 30 billion tons less soil. Cannot continue this way indefinitely.

As FDR said, "The nation that destroys it soil destroys itself."

We have already lost about 1/3 of U.S. best topsoil (some places about half). Average rate of erosion is 16 times average rate of renewal. In some locations this rate of loss is much higher (i.e. Great Plains and California). These soil loses will lower crop yields by as much as 10% this century.

B. Soil Salinization and Waterlogging

These are two extremes conditions. Salinization is caused by excessive evaporation of irrigation water in arid climates. Salts precipitate and stunt plant growth and lower crop yields. If not corrected, salinization may eventually kill plants. Has severely affected 20% of the world's irrigated cropland, particularly in Asia.

Can reverse the damage over a number of years by flushing away salts with fresh water, if available, and if it does not waterlog the soil (see below). Cost of doing this may be quite high.

Waterlogging soil (to rid soil of salts) causes water table to rise and envelope deep roots, reducing productivity and eventually killing plants. Damned if you do and damned if you don't.

C. Desertification

By definition, desertification occurs when productive potential of arid and semi-arid lands falls by more that 10%. It is due largely to human activities (overgrazing, deforestation, surface mining, poor irrigation techniques, salt buildup, waterlogging, and soil compaction)). Classified as moderate (10-25%), severe (25-50%) and very severe (50%+). Can result in complete vegetation loss and true desert conditions.

Desertification often leads to drought, famine, declining living standards, and environmental refugees, as has been the case in Africa for decades.

Has occurred in over 3 million square miles worldwide in past 50 years, affecting almost 1 billion people. About 100,000 square miles added every year.

Occurred in the U.S. in the 1930s; known as the Dust Bowl. Was caused by a combination of overgrazing, poor agricultural agriculture activities, loss of native vegetation, and drought. Dust blown as far as the east coast. Resulted in 1935 in the Soil Erosion Act (established the Soil Conservation Service) Can happen again, particularly if global warming occurs and the central U.S. becomes drier. Is also becoming a problem in eastern Asia (Mongolia and China).

III. Soil Conservation

There are many potential ways to reduce the erosion, loss, and destruction of the world's soils. Method used depends on particular problem, local conditions, and cost.

A. Conservation-Tillage Farming

Often referred to as either minimum-tillage or no-till farming. The idea is to reduce the amount of soil disturbance so it is not broken up as often and exposed to wind and water. A bit more expensive than conventional farming methods, but yields are nearly the same and the soil is preserved. Now done on about 1/3 of U.S. cropland and projected to be about 1/2 by 2025. If done on a widespread basis will decrease soil loss by more than 50%.

B. Other Methods of Soil Preservation

1) Terracing: Steep slopes broken up into individual broad, flat terraces. Controls water runoff.

2) Contour farming: Plow across moderate slopes rather than up and down them. Each row acts as a small barrier to water runoff.

3) Strip cropping: Alternate strips of different crops, one of which acts as a ground cover to trap soil particles. One also can help restore soil fertility (nitrogen-fixing legumes such as soybeans).

4) Alley cropping: Various crops planted in strips between trees and shrubs. Trees and shrubs can provide fruits, wood for fuel, and shade (helps reduce moisture loss).

5) Gully reclamation: Fill in gullies and replant to reduce severe gully erosion.

6) Windbreaks: Rows and/or stands of trees around fields slow down the wind and provide shade. They also are used as habitats for pest-eating and pollinating birds and insects.

7) Land classification: Identify land where the soil is too marginal or degraded to be used for agriculture.

8) Polyacrylamide (PAM): Chemical applied in irrigation water. Helps bind soil particles together by attracting positively charged particles to negatively charged PAM crystals.

Thirty-seven million acres are now off-limits to cultivation (result of the 1985 Farm Act). All of the strategies have cut soil erosion by about 60% since 1985.

C. Restoring Soil Fertility

Nutrients lost by erosion, harvesting, and leaching can be replaced by applying fertilizers, either organic or chemical (commercial inorganic).

Organic Fertilizers: These can be animal waste or manure. It improves soil structure, adds nitrogen, and stimulates beneficial bacteria and fungi. Its use in the U.S has been declining substantially until recently.

Material can also be green manure, fresh or growing plant material, including compost, plowed into field. Improves water retention, aeration, and nutrients.

Can also use crop rotation, where nutrient-depleting crops are alternated with nutrient-enriching crops to restore original soil fertility.

Inorganic fertilizers are mixes of compounds rich in nitrogen, phosphorous, and potassium. Use has increased worldwide about 10X since 1950, although it has declined slightly recently. Without them world food production would drop by approximately 40%.

However, they do not improve soil structure or humus level. Instead, soils tend to become compacted, reducing water retention and aeration. Also tends to increase nonpoint-source pollution and eutrophication of streams and lakes and contamination of groundwater. In addition, not all nutrients are replenished (usually only 3 out of about 20). Many view its use as a partial solution.