Soil

Chemistry Review

• Cation: atom that forms a positive charge (example: Ca+2)

• anion: atom that forms a negative charge (example: S-2)

pH Scale

Soil (non organic)

• Clay• Silt• Sand

Particle Size

Name Particle diameter

Sand 0.05-2.0 mmSilt 0.002- 0.05

mmclay Less than

0.002

Soil (organic)

• a) bacteria—base of the soil food web

• b) humus—decomposed organic material; dark and rich

composting

• producing humus in controlled conditions

What particles make up the BEST soil

Clay, Silt, or Sand?

Water Holding Capacity

• Should retain water for plant growth• During heavy rains, if water is not

absorbed water can pool and take a long time to drain or run off resulting in flooding

• Most important factor: Spaces of air between the soil particles

Infiltration Rate

• Gravitational flow through soil• Soils with high infiltration rate are

easily leached (removing nutrients)

Capillary Action

• Strong attraction between water molecules and soil causes water to be drawn upward from the water table.

• Capillary action increases when soil is packed close enough to provide a continuous film of surface

• Soils with high rate of capillary action lose water more quickly through evaporation than soil where capillary action is slower.

Ion Exchange Capacity

• Plants need ions to use for nutrients. • Soil has +/- sites where ions can bind• The higher the capacity to exchange

ions, the more nitrate, phosphate, and potassium ions the soil can hold, and the less likely water will wash them out

• Soil has sites that are either positively or negatively charged. These sites can bind with ions.

• If the soil can’t hold onto ions, the plants will not be able to absorb nutrients

Free Ions

• Macronutrients that plants get from soil are nitrogen N-3, phosphorus P-3, potassium K+1, calcium Ca+2, magnesium Mg+2, and sulfur S-2.

Soil pH

• soil pH close to neutral is optimal for most plants and animals

Soil pH

• Mineral cations help keep soil pH higher. When these cations are depleted, the soil pH decreases.

• What causes nutrient depletion in soil?

1.Plant material being removed2.Water drains too quickly from soil it

can strip soil of nutrients

Why is soil pH important

• Phosphorus soluble between 6.8 and 9

• Nitrogen fixed bacteria function at specific pH

• If pH gets too low, aluminum becomes soluble and aluminum is toxic to plants

Soil Permeability

• If water passes too quickly through soil, the plants can not absorb the water or the nutrients dissolved in the water.

How is soil degraded?

1. overcultivation—frequent plowinga) crop rotation—alternating the type of crops grownb) no-till agriculture—weed control, planting, and covering up all at once (low-till farming—decrease the amount of tilling)c) contour-strip cropping—cultivation along sloping groundd) shelter belts—“belts” of trees planted around farmland to reduce wind erosion

Overcultivation

Crop Rotation

No-Til agriculture

Contour Strip

Shelter Belt Farming

2. Frequent Plantinga) problems with fertilizers• inorganic chemical fertilizer does not replenish organic material• mineral content remains high but soil degrades anyway

3. overgrazing—animals stripping vegetation at a rate too fast for natural growth patterns to counteract it

4. deforestation—depletion of forest landa) causes increased leaching and erosionb) worst-case scenario—layers above the subsoil are gonec) the other end of the erosion problem: Where do the particles go?•displaced sediments can wash into rivers and streams

5. Irrigation and Salinizationa) irrigation—controlled introduction of water to an areab) salinization—salts accumulating in and on the soil, hindering plant growth

From the Environmental Literacy Council:Salt buildup is an existing or potential hazard on almost all of the 42 million acres of irrigated farmland in the United States. Much of the world‘s unused land is in arid and semiarid regions where irrigation will be necessary. Water contains a small amount of salt and over time this salt accumulates in the soil. A small amount of salt in the soil will not affect the germination and growth of crops. However, as salt concentrations increase, negative impacts occur. Eventually salt concentrations will affect the germination of seeds. Excessive salinity is presently costing the U.S. billions of dollars in lost food crops.‖

Wind Erosion

Water Erosion

Irrigation

Irrigation

• 2/3 of fresh water is used for irrigation

• Excessive irrigation = waterlogged plants

Farming strips nutrients in soil

• Farmers add fertilizers to soil to put back nutrients

• Nitrogen, potassium, phosphorus

Fertilizer Runoff