Soil Properties and Characteristics

1Soil: Foundation for Land Ecosystems

Soil profiles - refer to the horizontal layers of soil with distinct characteristics at each layer. O horizon A horizon E horizon B horizon C horizon

Soil structure refers to the arrangement of soil particles. Ex. A somewhat loose soil is ideal for infiltration, aeration, and workability. Soil consisting of any or all of the following will increase water-holding capacity – clay, loam, humus, organic fertilizers. Why don’t inorganic fertilizers increase the soil’s ability to hold water?

Soil texture - refers to the proportions of each of the three particle types in soil. The different particles, classified by size, are

* Sand - 2.0 to 0.02 mm * Silt - 0.02 to 0.002 mm * Clay - 0.002 mm

Touching is a good way to tell the difference between these three particle types. By touch, sand will feel gritty, silt will feel soft like flour, and clay will feel sticky and clumpy. A soil texture triangle can help determine the type of soil. Procedure: 1) Put soil in a test tube with water. The sand will sink to the bottom first (heaviest). The silt will settle out later, and the clay will be suspended in the water the longest. 2) Next, calculate what percentage of the soil in your test tube is each type. 3) Then match up the percentages along each side of the soil texture triangle to determine your soil texture.

Farmers use this technique to decide what crops will grow best and what they might need to add to the soil to maximize crop yields.

A good soil for crops has * a good supply of nutrients; * ample infiltration and good water-holding capacity; * good aeration * a pH near neutral * a low salt content

Loam is a mixture of all three soil types. It is approximately 40% sand, 40% silt, and 20% clay. Loam is a good soil type for plant production.

Soil Classification Soils are classified by their structures and textures. The following four soil types are important to agriculture, animal husbandry, and forestry.

1) Mollisols - fertile, dark soils found in temperate grassland biomes. - considered the best agricultural soils.

2) Oxisols - soils found in tropical and subtropical rainforests. - is infertile because most of the organic material is held in living plants. 3) Alfisols - moderately weathered forest soils found in moist, temperate forest biomes. - are adequate for agriculture if supplemented with fertilizers or organic material.

4) Aridisols - found in drylands and deserts. - are thin and light colored; usually contain a lot of sand.  are susceptible to salinization.

Fertilizers Fertilizers can be either organic or inorganic. Organic fertilizers contain plant and animal waste, such as manure and compost. While they have high water-holding capacities, organic fertilizers do not have the exact amounts of the necessary nutrients farmers may want to add to their crops.

Inorganic fertilizers are man-made chemical formulas. While they have the exact amounts of nutrients (like phosphates and nitrates that are limiting factors for plants) farmers need, they have poor water-holding capacities and leach from the soil very quickly.

ADVANTAGES DISADVANTAGES Easily obtained, transported, stored and Adds no humus or organic matter to applied soil/decreases water-holding capacity.

Lowers oxygen content of soil and If both have to be purchased, inorganic keeps nutrients from being taken up as fertilizers are more economical. efficiently. Nutrients are concentrated and only Does not completely supply all small amounts must be applied. micronutrients.

Computerized applications can release specific minerals needed by plants. Requires large amounts of energy for production, transportation, and Nutrients are immediately available to application. the crop.

Releases nitrous oxide (N2O), a Increase soil fertility. greenhouse gas.

Increases crop yield. Over-application may harm plant.

Speeds up growing process. Aquatic pollution resulting from runoff in surface or groundwater is detrimental to humans and ecosystems.

Can be very expensive to obtain and apply compared to organic fertilizer that is readily available (does not have to be purchased).

One effect of increased fertilizer use is eutrophication. And the consequent drainage of nitrogen-based fertilizers in the Midwestern United States is a major contributing factor to the hypoxia zone (high levels of chemicals, low levels of oxygen resulting in dead region) in the Gulf of Mexico. Soil Degradation Erosion – process of soil and humus particles being picked up and carried away by water or wind. In natural ecosystems , plant cover and roots protect soil from erosion. However, when forests are cleared or land is overgrazed, erosion can result.

Examples of water erosion: 1) Splash erosion – when raindrops hit soil and remove it. 2) Sheet erosion – when a small layer of soil is removed from an entire area. - often not noticed until much soil has been eroded. 3) Rill erosion – when water moves through small channels; intermediate between sheet and gully erosion. 4) Gully erosion – when water converges into small rivulets and streams and takes with it large amounts of soil.

Sediments occur when eroded soil is carried into streams and rivers, causing flooding, killing fish, destroying wetlands, killing coral reefs, and carrying pollutants into the rivers.

There are two major causes of erosion other than deforestation: over- cultivation and overgrazing.

1) Over-cultivation occurs when many farmers till and plow their farms to control weeds and increase crop yields This practice exposes the soil to the wind and rain, which causes erosion.

2) Overgrazing occurs when ranchers put large numbers of livestock such as cattle out to graze on arid grasslands; has led to decreased biodiversity and other ecological impacts. Desertification – results from soil degradation and the subsequent absence of vegetation in arid and semiarid areas; due to both climate change and human activities.

Salinization – the accumulation of salts in soil; is harmful to plants.

Subsidies are government tax breaks or financial assistance. These are sometimes made to farmers and are designed to offset cost and ensure a stable food supply. Subsidies encourage farmers to produce more crops, which has led to over-cultivation and soil erosion.

Conserving the Soil Sustainable agriculture refers to practices designed to maintain topsoil and reduce the use of fertilizers and pesticides while keeping farms economically viable and food safe.

There are many techniques farmers can use to lessen the effect of over- cultivation.

• Terracing – when crops are planted in “stairs” to stop water erosion; used on steep slopes.

• Contour farming – when crops are planted across slopes at right angles to slow down water erosion.

• Strip cropping – a technique of planting alternating rows of different crop types, for example, a row of tall crops next to a row of shorter crops or a row of crops with fibrous roots next to a row of crops with taproots.  Alley cropping – when trees are planted in strips and crops are grown between these strips of trees. The trees help shelter the crops from the wind, and the roots of the of trees help hold the soil in place. The trees are often fruit trees or trees to be cut and used as fuelwood.

• Shelterbelts -when trees are planted around crops to help shelter them from the wind.

 no-till or low-till farming – when the farmer uses special equipment to turn over the soil while covering it back up and planting new crops at the same time. Waste crops also are used as mulch during the next planting.

 Intercropping – growing several different crops on a plot of land.

 Interplanting – agro-forestry, polyvarietal cultivation, polyculture.

 Using organic fertilizers such as green manure, compost, and animal manure.

The Conservation Reserve Program pays farmers to put their highly erodible cropland in reserve as forests or grasses.

Farm bills refer to various laws that support or regulate agriculture.