Soil & Agriculture

Texture• The percentages (by weight) of different

sized particles of sand, silt and clay that it contains.

Soil Properties:

Structure

• How soil particles are organized and clumped together. (Sand, silt, clay)

Friability

• How easily the soil can be crumbled.

Porosity

• A measure of the volume of soil and the average distances between the spaces.

Permeability• The rate at which water and air

moves from upper to lower soil layers. It is distances between those spaces.

Some Soil Properties

• Soils vary in the size of the particles they contain, the amount of space between these particles, and how rapidly water flows through them.

Figure 3-25

Shrink-Swell Potential

• Some soils, like clays, swell when H2O gets in them, then they dry and crack. This is bad for house foundations, etc.

pH

• The pH of most soils ranges from 4.0 to 8.0.

• But, the soil of the Pygmy Forest in California is extremely acidic (2.8-3.9) and in Death Valley, California, it is very basic (10.5).

• Plants are affected by pH because of the solubility of nutrient minerals.

Slope

• Steep slopes often have little or no soil on them because of gravity.

• Runoff from precipitation tends to erode the slope also. Moderate slopes and valleys may encourage the formation of deep soils.

Depth

• Some soils are very shallow (like in some places in San Antonio). It can be only two inches of soil and then you hit rock. Other areas can have soil 36 inches deep or more.

Color

• Dark soil is rich with lots of organic matter.

• Light soil (like sand) is not so rich with very little organic matter.

Fig. 3-23, p. 68

Fern

Mature soil

Honey fungus

Root system

Oak tree

Bacteria

Lords and ladies

FungusActinomycetes

Nematode

PseudoscorpionMite

RegolithYoung soil

Immature soilBedrock

Rockfragments

Moss and lichen

Organic debrisbuilds upGrasses and

small shrubs

Mole

Dog violet

Woodsorrel

Earthworm Millipede

O horizonLeaf litter

A horizonTopsoil

B horizonSubsoil

C horizon

Parent material

Springtail

Red Earth Mite

Organic Layer (O-horizon)

• The uppermost layer; it is rich in organic material.

• Plant litter accumulates in the O-horizon and gradually decays.

• In desert soils the O-horizon is completely absent, but in certain organically rich soils it may be the dominant layer.

Topsoil (A-horizon)

• It is dark and rich in accumulated organic matter and humus.

• It has a granular texture and is somewhat nutrient-poor due to the loss of many nutrient minerals to deeper layers and by leaching.

Subsoil (B-horizon)

• The light-colored subsoil beneath the A-horizon; it is often where nutrient minerals have leached out of the topsoil and litter accumulate.

• It is typically rich in iron and aluminum compounds and clay.

Parent Material (C-horizon)

• This contains weathered pieces of rock and borders the unweathered solid parent material. Most roots do not go down this deep and it is often saturated with groundwater.

Layers in Mature Soils

• Infiltration: the downward movement of water through soil.

• Leaching: dissolving of minerals and organic matter in upper layers carrying them to lower layers.

• The soil type determines the degree of infiltration and leaching.

• 6.4 billion tons of soils are eroded from the U.S. each year; this would fill 320 million average-sized dump trucks that, if parked end-to-end, would extend to the moon and ¾ of the way back!

Erosion

Definition

• Erosion is the movement of soil components, especially surface litter and topsoil, from one place to another.

Importance

• In undisturbed ecosystems, the roots of plants help anchor the soil, and usually soil is not lost faster then it forms.

• Erosion destroys in a few decades what nature took hundreds to thousands of years to produce.

Global Outlook: Soil Erosion

• Soil is eroding faster than it is forming on more than one-third of the world’s cropland.

Figure 13-10

SOIL EROSION AND DEGRADATION

• Soil erosion is the movement of soil components, especially surface litter and topsoil, by wind or water.

Soil erosion increases through activities such as farming, logging, construction, overgrazing, and off-road vehicles.

Figure 13-9

SOIL EROSION AND DEGRADATION

• Soil erosion lowers soil fertility and can overload nearby bodies of water with eroded sediment.– Sheet erosion: surface water or wind peel off thin

layers of soil.– Rill erosion: fast-flowing little rivulets of surface

water make small channels.– Gully erosion: fast-flowing water join together to

cut wider and deeper ditches or gullies.

Wind Erosion

• Saltation – one particle hitting another and being blown across the surface of the soil.

Suspension – airborne soil. Ex. soil from Lubbock is found in Temple, Texas.

Desertification: Degrading Drylands

• About one-third of the world’s land has lost some of its productivity because of drought and human activities that reduce or degrade topsoil.

Figure 13-12

Salinization and Waterlogging of Soils: A Downside of Irrigation

• Example of high evaporation, poor drainage, and severe salinization.

• White alkaline salts have displaced cops.

Figure 13-14

Salinization and Waterlogging

• Repeated irrigation can reduce crop yields by causing salt buildup in the soil and waterlogging of crop plants.

Figure 13-13

Solutions

• More efficient irrigation• Better planning of farmlands• Genetic Engineering

Fig. 13-15, p. 281

CleanupPrevention

Soil Salinization

Solutions

Reduce irrigation

Switch to salt-tolerant crops (such as barley, cotton, sugarbeet)

Flush soil (expensive and wastes water)

Stop growing crops for 2–5 years

Install underground drainage systems (expensive)

Erosion Control (see Miller pg. 282)

• Shelterbelts – can reduce wind erosion. Long rows of trees are planted to partially block the wind. They can also help retain soil moisture, supply some wood for fuel, and provide habitats for birds.

Minimum Tillage – (conservation tillage) to disturb the soil as little as possible while planting crops.

Special tillers break up and loosen the subsurface soil without turning over the topsoil, previous crop residues, and any cover vegetation.

Traditional Agriculture: Low Input Polyculture

• Many farmers in developing countries use low-input agriculture to grow a variety of crops on each plot of land (interplanting) through:– Polyvarietal cultivation: planting several genetic

varieties.– Intercropping: two or more different crops grown at

the same time in a plot.– Agroforestry: crops and trees are grown together.– Polyculture: different plants are planted together.

SUSTAINABLE AGRICULTURE THROUGH SOIL CONSERVATION

• Modern farm machinery can plant crops without disturbing soil (no-till and minimum tillage.– Conservation-tillage farming:

• Increases crop yield.• Raises soil carbon content.• Lowers water use.• Lowers pesticides.• Uses less tractor fuel.

• Contour Farming –sloping your growing crops, etc.

• You run terraces parallel to the ground to stop soil from running down a steep slope. Plowing and planting crops in rows across, rather than up and down, the sloped contour of the land.

Terracing – (what you use for contour farming.) Dirt goes up to hold the dirt in place. Broad, nearly level terraces that run across the land contour. Helps to retain water for crops at each level and reduce soil erosion by controlling runoff.

• Strip Cropping – a row crop such as corn alternates in strips with another crop that completely covers the soil, reducing erosion. It catches and reduces water runoff and helps prevent the spread of pests and plant diseases.

Cover Cropping (alley cropping) – several crops are planted together in strips or alleys between trees and shrubs that can provide shade (which reduces water loss by evaporation) and helps to retain and slowly release soil moisture.

Irrigation Techniques

• Conventional center-pivot irrigation- allows 80% of the water input to reach crops

• Gravity-flow irrigation- Valves that send water down irrigation ditches.

• Drip irrigation- Can raise water efficiency to 90-95% and reduce water use by 37-70%.

• Floodplain irrigation- allowing the natural floods to irrigate the crops. Soils in flood zones tend to be nutrient rich and fertile.

Definition

• Hydroponics are growing plants in fertilized water.

Hydroponics:

Method of suspending plants in water and the solutions involved. Ex. cranberries are grown this way.

Costs of Hydroponics:

• It is labor-intensive and expensive.

You can control the environment & grow plants where there is no soil; NASA is looking into this.

Benefits:

THE GREEN REVOLUTION

THE GENE REVOLUTION• To increase crop yields, we can mix the genes of

similar types of organisms and mix the genes of different organisms.– Artificial selection has been used for centuries to

develop genetically improved varieties of crops.– Genetic engineering develops improved strains at

an exponential pace compared to artificial selection.• Controversy has arisen over the use of

genetically modified food (GMF).

Mixing Genes

• Genetic engineering involves splicing a gene from one species and transplanting the DNA into another species.

Figure 13-19