Chapter 9 Topics: The relationship between soils and agriculture Major agricultural developments Fundamentals of soil science Causes/consequences of soil erosion/degradation Soil conservation: principles, solutions, and policies

Soil: foundation of agriculture 38% of land surface used for agriculture Agriculture = practice of raising crops and livestock for human use and consumption Cropland = land used to raise plants for human use Rangeland/pasture = land used for grazing Soil = a complex plant-supporting system Consists of disintegrated rock, organic matter, water, gases, nutrients, and microorganisms It is a renewable resource that can be depleted

The need to feed more of us Feeding a growing human population requires Increasing agricultural production or Changing our diets Land suitable for farming is running out Mismanaged agriculture degrades soils Allows fertile soil to be eroded Turns grasslands into deserts Removes forests

Agriculture arose 10,000 yrs ago Many cultures independently invented agriculture The earliest plant and animal domestication is from the “Fertile Crescent” of the Middle East

Traditional agriculture Agriculture allowed people to settle in one place Populations increased Leading to more intensive agriculture Traditional agriculture = biologically powered Uses human and animal muscle power Hand tools, simple machines Subsistence agriculture = families produce only enough food for themselves Polyculture = different crops are planted in one field

Industrialized agriculture Industrialized agriculture = uses large-scale mechanization and fossil fuels to boost yields Also uses pesticides, irrigation, and fertilizers Monoculture = uniform planting of a single crop Green revolution = new technology, crop varieties, and farming practices were introduced to developing countries Increased yields and decreased starvation Created new problems and worsened old ones

Soil as a system Soil consists of mineral and organic matter, air, and water Soil is a habitat for earthworms, insects, mammals, reptiles, and amphibians Soil is an ecosystem

Soil formation is a slow process Parent material = the initial rock or sediment Weathering = processes that form soil Physical (mechanical) = no chemical changes in the parent material (changes to size and shape) Chemical = parent material is chemically changed Organisms are important agents of both physical and chemical weathering Humus = spongy, fertile material formed by partial decomposition of organic matter

Processes/factors of soil formation The processes: Additions, accumulations Transformations, removals The factors: Climate: soils form faster in warm, wet climates Organisms: plants/decomposers add organic matter Topography: affect exposure to sun, water movement Parent material: influences properties of resulting soil Time: soil can take decades to millennia to form

A soil profile consists of horizons Horizon = a soil layer Soil profile = the cross- section of soil as a whole Topsoil = inorganic and organic material most nutritive for plants Leaching = dissolved particles move down through horizons

Characterizing soils Soil color = indicates its composition and fertility Black or dark brown = rich in organic matter Pale gray or white = indicates leaching Soil texture = determined by the size of particles From smallest to largest: clay, silt, sand Loam = soil with an even mixture of the three Affects how easily air and water travel through the soil Influences how easy soil is to cultivate

Characterizing soils Soil structure = a measure of soil’s “clumpiness” A medium amount of clumpiness is best for plants Repeated tilling compacts soil, decreasing its water- absorbing capabilities Soil pH = affects the ability to support plant growth Soils that are too acidic or basic can kill plants pH influences the availability of nutrients for plant

Characterizing soils Cation exchange = allows plants to gain nutrients Negatively charged soils hold cations (positively charged ions) of calcium, magnesium, and potassium Roots exchange hydrogen to soil for these nutrients Cation exchange capacity = ability to hold cations Cations that don’t leach are more available to plants A useful measure of soil fertility Greatest in fine or organic soils

Regional differences In rainforests, the nutrients are in plants, not the soil Rain leaches minerals and nutrients, reducing their accessibility Rapid decomposition of leaf litter results in a thin topsoil layer with little humus

Regional differences In temperate prairies, the nutrients can accumulate in the soil Lower rainfall leaches fewer nutrients Long root systems extend organic matter deep into the soil

Land degradation Human activities reduce and limit productivity by degrading soils in many areas Land degradation = a general deterioration of land, decreasing its productivity and biodiversity Erosion is the leading cause of land degradation Other effects include loss of organic matter, nutrient depletion, water issues (scarcity, salinization, water- logging), changes to structure, chemical pollution

Erosion The removal of material by water, wind, glaciers Occurs more easily on steeper slopes The uppermost material, the topsoil, is removed first Often occurs faster than soil is formed Land is made more vulnerable to erosion through Over-cultivating fields (over-use or excessive tilling) Overgrazing rangelands Clearing forests on steep slopes or with large clear-cuts

Erosion can be prevented Erosion can be hard to detect and measure Five tons/acre is lost per ton of grain This amounts to a layer only as thick as a penny Physical barriers to capture soil can prevent erosion Plants can reduce soil loss by slowing wind/water Roots hold soil in place No-till agriculture leaves plant residue on fields Cover crops protect soil between crop plantings

Desertification Desertification = a loss of more than 10% productivity due to Erosion, soil compaction Deforestation, overgrazing Drought, salinization, water depletion Climate change Areas most prone are arid and semiarid lands

Impacts of desertification Affects one-third of the planet’s land area In over 100 countries Endangering food supplies of 1 billion people It costs tens of billions of dollars each year China loses over $6.5 billion/year from overgrazing 80% of land in Kenya is vulnerable Desertification can be a positive feedback cycle Degradation forces farmers onto poorer land Farmers reduce fallow periods, so land loses nutrients

The Dust Bowl Late 1800 – early 1900, growing of wheat, grazing cattle removed vegetation Dust Bowl = 1930s drought + erosion caused “black blizzards” of sand and silt Thousands of farmers left their land and were forced to rely on governmental help

Soil Conservation Service (SCS) Started in 1935, the Service works with farmers to: Assess the land’s resources, problems, opportunities Develop conservation plans for farms Work with landowners to implement conservation plans Conservation districts = organized by states, operate with federal direction and funding SCS is now called Natural Resources Conservation Service (NRCS); added responsibility for water quality protection and pollution control

Techniques for protecting soil Crop rotation = different crops from year to year Reduces pests Legumes fix N Contour farming = plowing level furrows Prevents rills/gullies Enhances infiltration

Techniques for protecting soil Terracing = level platforms cut into steep hillsides Intercropping = planting different crops in alternating bands Increases ground cover Replenishes soil Decreases pests and disease

Techniques for protecting soil Shelterbelts (windbreaks) trees planted along the edges of fields slow the wind Conservation tillage reduces amount of tilling Leaves at least 30% of crop residues in the field No-till farming disturbs the soil even less

Conservation tillage Increases organic matter and soil biota Reduces erosion and improves soil quality Prevents carbon from returning to the atmosphere Reduces fossil fuel use May increase use of herbicides and fertilizers 40% of U.S. farmland uses conservation tillage

Plant cover reduces erosion Plants anchor soil Techniques that increase plant cover Cover crops that become green manure Move livestock to prevent overgrazing Cut fewer trees Plant vegetation along riverbanks & roadsides

Irrigation issues - quantity Irrigation = providing water to support agriculture Unproductive regions become productive farmland Waterlogging = when soils are over-irrigated, the water suffocates roots Inefficiencies in application Large volumes of water evaporate during spraying Watering needs may vary within a field

Irrigation issues – quality Salinization = when soils are “under-irrigated”, salts accumulate in surface soil layers Water volume insufficient to drain out of root zone Inhibits production of 20% of irrigated cropland, costing over $11 billion/year Easier and cheaper to prevent than to fix Avoid water-guzzling crops in sensitive areas Irrigate with low-salt water

Fertilizers Fertilizers = substances containing essential nutrients Inorganic fertilizers = mined or synthetically manufactured Inorganic fertilizer use has increased significantly Inorganic fertilizers are chemically simpler and more mobile in the environment

Fertilizers Organic fertilizers = remains/wastes of organisms Manure, crops/residues Compost = produced when decomposers break down organic matter Organic fertilizers improve: Soil structure Nutrient retention Water-retaining capacity

Overgrazing Overgrazing = over-use of rangeland or pasture 70% of the world’s rangeland is degraded, costing $23.3 billion/year U.S. government subsidies include few incentives to protect rangeland

Impacts of overgrazing

Agricultural policy Industrial agriculture puts huge demands on the land Farming can be a year-to-year gamble, encouraging a short-term perspective Degradation occurs slowly and can be hard to see Subsidies encourage cultivation on fragile land Ranchers graze cattle extremely cheaply on BLM (Bureau of Land Management) land Agriculture can be sustainable, but there is a lot of “policy inertia” in favor of the status quo

Wetlands then and now Wetlands = swamps, marshes, bogs, river floodplains Government policy encouraged draining Swamp Lands Acts (1849, 1950, 1860) = drained and converted wetlands to control floods and malaria Over 50% have been drained for agriculture Wetlands are now seen as vital ecosystems Provide habitat, flood control, recharged water supplies Wetlands Reserve Program = landowners are paid to protect, restore, and enhance wetlands

A direction for the future? Conservation Reserve Program (1985): farmers are paid to put highly erodible land in conservation reserves Trees and grasses are planted instead of crops Each dollar spent saves 1 ton of topsoil Generates income for farmers Improves water quality Provides habitat for native wildlife The 2008 farm bill limited reserve lands to 32 million acres but funded 14 similar land conservation programs