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 13-12Figure 13-12

Salinization and Waterlogging

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

Figure 13-13Figure 13-13

Fig. 13-13, p. 281

EvaporationTranspiration

EvaporationEvaporation

Waterlogging

Salinization Waterlogging1. Irrigation water contains small amounts of dissolved salts

2. Evaporation and transpiration leave salts behind.

3. Salt builds up in soil.

1. Precipitation and irrigation water percolate downward.

2. Water table rises.

Less permeable clay layer

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)

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 13-14Figure 13-14

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.

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.

SUSTAINABLE AGRICULTURE THROUGH SOIL CONSERVATION

Terracing, contour planting, strip cropping, alley cropping, and windbreaks can reduce soil erosion.

Figure 13-16Figure 13-16

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.

Macronutrients

Macronutrients are larger in atomic structure. Ex. Nitrogen, Phosphorus & Potassium.

Micronutrients

These are smaller in atomic structure. Plants need them in small amounts.

Ex. Selenium, Zinc & Iron.

Soil Nutrients

Fertilizers and LabelsOrganic Fertilizers – animal manure, crop residues, bone meal, and compost

Inorganic Fertilizers – man-made from chemical compounds

Benefits – exact compositions are known; they are soluble & thus immediately available to the plant

Costs – quickly leach away; this pollutes the water; doesn’t help the water holding capacity of the soil like organic fertilizers do.