The Nature of Soil Soil Conservation Sustainable Ag.

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1 Chapter 6 & 7 The Nature of Soil Soil Conservation Sustainable Ag. Climate Soil Forming Factors Parent material Organisms Topography Time Value of Soil Soil is under appreciated Condition of soil affects future of food supply Good condition soil prevents erosion Processes wastes and pollution Allows water to infiltrate and recharge aquifers Soil Formation Parent Material > subsoil > topsoil Parent material goes through process of weathering. Two main types of weathering mechanical and chemical. Mechanical Weathering Freeze-thaw Roots Friction (glaciers, water) Wind and water move small particles and expose new materials.

2 Chemical Weathering Lichen Close-up Oxidation Hydrolysis Acid content of rain Primary Succession Lichens - Increase humus content and lower ph Earthworms - average 500, per hectare process 9 M tons per yr. Fungi and bacteria - decompose organic matter and reduce the size of organic particles

3 Soils These physical, chemical, and biological processes work together to allow soil formation Very slow process 1 centimeter per 15 years under ideal conditions. Can take MUCH longer. 45% minerals 25% air 25% water 5% organics Soil Composition 5% organisms 10% roots 85% humus Topography In flat areas, as soil is built it stays in place. Steeper slopes, soil is transported until it reaches a flat area usually a floodplain As a result, floodplains typically have deep soil horizons and generally good soils. Composition of healthy loam soil

4 Soil Properties - Texture Gravel >2 mm Sand 0.5 to 2 mm Silt (0.002 to 0.05 mm) Clay <0.002 mm Texture (combination of sand, silt, and clay) determines how much air and water the soil can contain. Loam = good drainage but enough retention of nutrients Importance of clay via Ion exchange

5 Structure of soils Clumpiness and friability Sands don t clump clays do Soil texture and moisture content determine the friability of the soil. Good Soils Moderately friable Good drainage enough air after drainage enough water retained for plants too much water - plants die from no O 2 too much air - plants may die from lack of water Soil ph How lime neutralizes soil acidity

6 Nutrient sources of crops Soil Profile - Horizons A horizon - thickness varies, most life forms, nutrients, and organic matter in upper A zones - less in lower A zones B horizon - less organic material, fewer organisms, and nutrients leached out of A horizon. Nutrients required by plants Soil Horizons C horizon - weathered parent material, little or no nutrients. Influences soil ph and texture others as well O horizon : organic material / leaf litter on topsoil usually found in forests E horizon : light layer under A horizon formed by leaching usually found in areas of high rains

7 Two basic soil types Grassland soils deep A horizons low rainfall, little leaching thin B horizon, little organics and minerals Forest soils thinner A horizons More rain, leaches more material to B Much leaching of clay can lead to hardpan Other Soil Issues - Deserts Very little rain - poorly developed horizons Little plant growth - little organic matter if climate is cold, organics accumulate and acidity increases. Other Soil Issues - Tropical High temperature and humidity leads to rapid decomposition of organics Very large amounts of leaching Soils Categorizing Soil Map Can categorize many soils within the two basic types, based on numerous factors However, there are 15,000 distinct soil types just in North America.

8 Erosion Movement of soils by wind or water In the north central Texas area, more than 15 tons per acre per year is considered critical Most of these soils end up in streams and rivers to be deposited in lakes and oceans Mississippi River - moves 325,000,000 mtons per yr. Erosion Issues In the U.S., about 50% of lands are capable of raising crops (21% currently, 26% in pastures) Only about 2% of this land is not susceptible to excessive erosion. Worldwide, only about 35% of lands are capable of raising crops (11% currently, 24% in pastures)

9 Water erosion

10 Number of Dust storms March 1936 Plant cover influence on soil erosion and water runoff Costs Most productive layers (A) lost first Farmers must add fertilizers Stream bottoms become covered with silt, destroying much habitat sediments must be dredged if in shipping lanes - paid for by taxes. Soil Conservation Efforts Contour Farming: tilling at right angles to the slope of the land. Can reduce erosion by 50% and adds more water to soils for crops. Strip Farming : Intertwining plots of contour tilled row crops with sown grain crops. Under the right conditions, can be even more effective than contour farming alone.

11 Soil Conservation (cont) Terracing: level areas carved out of steeply sloped lands. Flat areas used for crops, while walls of terrace are protected. Quite costly, high maintenance, not feasible for highly mechanized farms Waterways : channels designed to move water with little or no gully erosion Windbreaks Layers of vegetation left on the soil when not cultivated Trees, shrubs, or fences placed perpendicular to the prevailing wind direction breaks up energy, prevents movement of soils, good in flat areas. Normal Tillage 1. Soil is plowed: buries weeds, adds nutrients, lifts deep nutrients, exposes dark soils which warm fast 2. Field is disked or harrowed: breaks up clods, kills remaining weeds, prepares soils for seed. 3. Temporary crop is often grown to prevent erosion Each pass costs money and exposes soils Variations Reduce secondary tillage (plant seeds after first plowing) Strip tillage (only till narrow strips where seeds will be planted) no-till (special planters that inject seeds into soils) Requires less time and fuel, but more herbicides

12 Benefits of Reduced Tillage Less erosion - saves valuable soil, clearer streams, less dredging Winter food and cover for wildlife Allows row crops to be grown on hillsides where previously impossible Fuel is saved, soil less compacted Can often plant a second crop immediately after the first harvest Disadvantages Plant residues may delay soil warming, which is bad for seeds Evaporation may be lessened by plant residues, which reduces nutrient flow from deeper layers of soil. Requires more herbicides!! (insects and disease in residue, more weed control

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