Loam: About 40% sand, 40% silt, 20% clay. Mixture of pore sizes to balance water retention and aeration. Considered the best soil for growing crops.

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1 Soil Characteristics Soil is formed from a combination of two Primary sources: Weathering of Parent Rock ( ) = material introduced by plants, animals, and decomposers Minor sources: Nutrients incorporated by microbes from the (i.e. nitrogen) Material transported in by rivers during and ice ( ) from atmosphere (dust, etc). Chemicals from precipitation (i.e. )

2 Organic Soil Components: Humus - organic matter in the Top (O) layer (a carbon sink) Dark brown or black can indicate high content. Inorganic Soil Components: Mostly from bedrock Classification based on mixture of Grain ( ) Sizes: Clay- Very fine (individual particles invisible to naked eye, and has no feeling Silt- Fine Sand- Medium Gravel- Large Grain size largely determines and

3 Each corner represents 100% of that grain size) What type of soil is 20% Clay, 70% silt, And 10% Sand? Loam: About 40% sand, 40% silt, 20% clay. Mixture of pore sizes to balance water retention and aeration. Considered the best soil for growing crops.

4 Importance of Grain Size How would the pore spaces in clay soil compare to the pore spaces in sandy soil? How would the different pore spaces affect each of these soil types ability to: Allow water to infiltrate Hold water Allow air to infiltrate Hold nutrients Why is aeration one of the important characteristics of soil? The cells in plant roots need to perform to obtain energy.

5 Soil Porosity Micropores : allow for forces that retain water. and silty soils form more micropores. Macropores: allow for better water and air spaces. soils form more macropores module06/permeability.htm

6 Soil Texture Water- Infiltrating Capacity Water/ Nutrient- Holding Capacity Aeration/ Permeability Clay Poor Good Poor Silt Medium/ Poor Medium Medium/ Poor Sand Good Poor Good Loam (even mix of particle sizes) Medium Medium Medium

7 Discuss with your table partner: What is the relationship between with water (and air) infiltrating capacity of a soil and its water-holding capacity? The ability of a soil to allow water to infiltrate is related to it ability to retain water. Soils that can best balance these competing demands of the plants are generally best suited for plant growth.

8 Soil Nutrients and ph Depending on the needs of the crop, soils will need a blend of macro and micronutrients in the soil If the soil is, fertilizer or organic amendments are needed.

9 Soil Nutrients and ph The acceptable range of ph for a soil will depend on the crop planted soils can lose nutrients through leaching (dissolving and leaving via groundwater flow) (CaCO 3 ) is a buffer for acids and can be added to make soils more alkaline if they are too acidic. Sulfur can be added if soils are too (S reacts with water to form sulfuric acid)

10 Soil Structure Soil has layers called Horizons material is concentrated near the top (O and A horizons) E horizon = nutrients material more predominant in lower layers (B and C horizons) R horizon = (bedrock)

11 Typical Topsoil in Different Biomes Temperate Deciduous forest: Mixture of humus and minerals Coniferous (evergreen) Forest: Thick, organic debris Grassland:, alkaline, dark, very rich in Tropical Rain forest:, because organic matter is decomposed and recycled back into biomass (i.e. trees) quickly Desert:, humus-mineral mixture

12 Soil Erosion & Degradation While soil is a potentially renewable resource, it takes long periods of time to form topsoil, typically years to form centimeters ( inch) of topsoil; Topsoil is eroding faster than it forms in about of the world's cropland. Losing topsoil makes soil less & less able to hold

13 Soil Erosion & Degradation Conventional Tillage: plowing and tilling can leave soil and and vulnerable to erosion. Overgrazing: destroys, leaving soil exposed Deforestation: Erosion will be even more extreme in areas where runoff will be higher. Wind and Water Tilling Disk Harrow

14 Soil Erosion & Degradation Dust Bowl: Combination of drought & poor soil conservation in the 1930 s led to severe wind erosion of topsoil Contributing factor to during the depression

15 Soil Erosion & Degradation Runoff and Types of runoff: Sheet Erosion: Surface water runs down a slope and erodes layers of off; Rill Erosion: fast-flowing water cuts small rivulets or in soil; Gully Erosion: rivulets become bigger and join to form gullies

16 Soil Degradation: Irrigation Salinization: Salt build-up in irrigated soils Irrigation water (unlike rain) contains some salts which increases in concentration as evaporation of water leaves behind salt. Irrigation is more common in arid areas, where evaporation rate will also be high

17 Soil Degradation: Irrigation Waterlogging Results when excess irrigation water raises the water table (level of water below the surface & lowers crop productivity. Why would water logging reduce productivity? Fig.14 20

18 Soil Erosion & Degradation Desertification the process by which fertile land becomes desert, typically as a result of drought, deforestation, or inappropriate agriculture. Land productivity drops markedly. Especially common in lands. Causes: Conventional tillage Overgrazing Deforestation & Devegetation Surface Poor techniques Salt buildup Farming on unsuitable terrain Soil by farm machinery Fig.14 19

19 Population, Arable Land, and Agriculture As Population increases, the amount of per person decreases. The amount of irrigated agricultural land in the world has been, leading to groundwater and soil depletion. Fertilizer Use As soils become depleted, fertilizers must be utilized Fertilizers can replace nutrients, but not Other soil damage (i.e. compaction, salinization, etc.) is also not remedied by fertilizers Fertilizer use by type Fertilizer use by three largest countries

20 Fertilizer Use N fertilizer production and use: -intensive Excess runoff of fertilizer causes Contributes to.

21 Soil Conservation Soil conservation reduces soil erosion & restores soil fertility. Farming Practices 1. : minimizes soil disturbance and reduces water erosion Use of special tillers which loosen subsurface soil without turning over top soil No till methods that inject seeds, fertilizers, & herbicides in soil Maintaining a layer of crop from the past year s crop on 30 percent or more of the soil surface Crop residue

22 Soil Conservation 2. Terracing protects steep slopes; retains water for crops at each level and reduces erosion by controlling ; 3. Contour farming follows natural land contours; each row of crops acts as a to slow water runoff 4. Strip cropping uses strips of different vegetation between crops; keeps soil covered, slows erosion, and some plants can increase fertility 5. Alley cropping grows crops between rows of : The result is better erosion control, improved crop production, supplemental income, and more habitat and travel corridors for

23 Soil Conservation 6. breaks uses trees between fields to slow wind erosion 7. Organic fertilizer: manure, "green manure" = plant matter, &, reduces use of synthetic fertilizers 8. Crop rotation: Restore nutrients: if that host N-fixing bacteria are in the rotation Reduce pests: May reduce the number of pests that on a particular crop such as the corn borer. 9. Good techniques: slows erosion and conserves water

24 History Progress on Soil Erosion & Degradation in US Source: USDA