Comprehensive Study Guide on Soil Science, the Dust Bowl, and Soil Conservation

Plant Nutrition and Soil Composition

• Plant Growth and Nutrient Acquisition:

  • Photosynthesis occurs when plants gather energy by absorbing sunlight.

  • Photosynthesis transforms carbon dioxide ($CO_2$) from the air into the carbohydrate matter of the plant.

  • Roots absorb water ($H_2O$), nitrogen, phosphorus, and potassium from the soil into plant tissue.

  • Soil serves as the primary reservoir for plant water and mineral nutrition.

• Components of Soil:

  • Humus: Partially decomposed plant and animal matter.

  • Rock particles: Contain essential minerals including iron, potassium, sulfur, and phosphates.

  • Water.

  • Air.

  • Living organisms.

• Soil Formation and Weathering Processes:

  • Soil forms as parent rock undergoes weathering processes.

  • Mechanical weathering: The physical breakdown of rock driven by wind, water, cold, and heat.

  • Chemical weathering: Alteration of the molecular nature of rock via chemical reactions with water, oxygen, and other reactive agents.

  • Weathering gradient: Highly weathered material concentrates near the surface, while unweathered rock remains deeper in the earth.

Soil Horizons and Profile Structure

• Soil Profile Overview:

  • A soil profile displays the sequential vertical layers beneath the ground surface.

  • Each individual layer is defined as a horizon characterized by a distinct composition.

• Horizon Designations:

  • O Horizon: Located directly at the surface; consists of humus and nondecomposed plant matter.

  • A Horizon: Positioned beneath the O horizon; comprises topsoil, consisting of organic matter blended with weathered rock particles.

  • B Horizon: The subsoil layer; composed of tiny rock particles that leach downward from the topsoil.

  • C Horizon: Parent material; consists of rock that has experienced minimal weathering due to its subterranean depth.

  • R Horizon: Bedrock; unweathered, solid rock underlying all other soil strata.

Soil Particle Sizes and Textural Classification

• Topsoil Particle Categories:

  • Sand:

  • Largest soil particle size, measuring from $0.05\,mm$ to $2\,mm$ under $10\times$ magnification.

  • Features large pore spaces between particles, permitting rapid water percolation.

  • Silt:

  • Intermediate particle size, measuring from $0.002\,mm$ to $0.5\,mm$ under $10\times$ magnification.

  • Clay:

  • Smallest particle size, measuring less than $0.002\,mm$.

  • Features minimal pore spaces between particles, preventing water from percolating easily.

• Soil Texture Triangle Classification:

  • Soil texture triangles classify soil types according to the relative percentages of sand, silt, and clay.

  • Example 1: Soil containing $50\%$ sand, $30\%$ clay, and $20\%$ silt is categorized as sandy clay loam.

  • Example 2: Soil containing $55\%$ sand, $40\%$ silt (sill), and $5\%$ clay is categorized as sandy loam.

Ecological History and the Dust Bowl Case Study

• The Great Plains Biome:

  • The Great Plains region of the United States belongs to the temperate grasslands biome, divided into three distinct prairie types:

  • Tall grass prairie: Receives sufficient precipitation to support plant growth up to $6\,feet$ in height.

  • Mixed grass prairie: Intermediate grassland ecosystem.

  • Short grass prairie: Semi-arid grasslands located in the rain shadow of the Rocky Mountains.

• Native Prairie Soil Dynamics and Cultivation:

  • Prairie grasses possess deep, complex root systems that historically impeded mechanical tilling.

  • Tilling is the agricultural process of loosening and mixing the soil, promoting water percolation, and integrating the O horizon humus into the topsoil.

  • Breaking up the native sod revealed the deepest and richest A horizon biome on the continent, yielding exceptionally productive farmland.

  • High agricultural value in the Western Plains triggered events such as the Oklahoma land run of $1889$.

• Economic and Climatic Triggers of the Dust Bowl:

  • The stock market crash of $1929$ initiated the Great Depression, driving wheat prices down to historic lows.

  • Farmers overplanted wheat acreage to compensate for plummeting prices; the resulting surge in supply reduced market values further.

  • A catastrophic drought commenced in the winter of $1931$ to $1932$, persisting for $7\text{ years}$ across portions of the Great Plains.

  • Dry, unanchored soils exposed to severe winds resulted in unprecedented, severe soil erosion, marking one of the largest ecological disasters in United States history.

  • Native plant communities had been completely removed in favor of cultivated wheat.

  • Cultivated wheat is harvested during summer, leaving fields completely devoid of vegetative cover during the hottest months of the year.

  • Root architecture disparities:

  • Native prairie plants possess dense root networks extending nearly $6\,feet$ deep, anchoring soil against erosive forces.

  • Wheat develops a considerably shallower and narrower root system, rendering the soil susceptible to mass displacement.

Mechanisms of Soil Erosion

• Desertification:

  • The Dust Bowl represents a large-scale occurrence of desertification, defined as the degradation and conversion of fertile land into desert biomes.

  • Desertification is driven by drought, deforestation, and unsustainable farming practices that strip soil nutrients and accelerate erosion rates.

  • Uncultivated, preserved forests and grasslands mitigate erosion through dense root networks that stabilize soil, as well as forest canopies that intercept wind and decrease the impact velocity of rainfall.

• Erosion by Wind and Water:

  • Erosion is defined as the physical detachment and transportation of soil particles by wind or water.

  • Splash erosion: Occurs when falling raindrops strike bare soil, dislodging and loosening individual soil particles; this effect is intensified on sloped terrain.

  • Sheet erosion: The uniform detachment and removal of thin surface soil layers by overland water runoff.

  • Rill and gully erosion: Occurs when concentrated surface runoff carves visible, expanding channels and gullies into the landscape, transporting vast quantities of soil away.

Soil Conservation and Agricultural Practices

• Windbreaks:

  • Consist of linear plantings of trees and shrubs along field boundaries.

  • Reduce wind velocity entering cultivated land to prevent wind erosion.

• Crop Management and Ground Cover:

  • Row crops:

  • Crops such as corn must be spaced out in discrete rows, leaving wide strips of bare soil exposed to wind and splash erosion.

  • Cover crops:

  • Vegetation grown to form a complete, continuous blanket over the soil surface, protecting it from kinetic droplet impact and wind detachment.

  • Strip cropping:

  • The practice of alternating rows of exposed crops with bands of cover crops.

  • Generates protective buffer strips that trap loosened soil particles within the boundaries of the farm (e.g., alternating rows of corn, oat, and hay).

• Topographic and Mechanical Soil Conservation:

  • Contour plowing:

  • Tilling and planting along the natural elevation contours of sloped land.

  • Decelerates the velocity of surface runoff, preventing the formation of rill and gully erosion.

  • Terracing:

  • Carving level, stair-like platforms into steep hillsides to catch water and sediment.

  • Inhibits channelization, effectively mitigating rill and gully erosion.

  • No-till agriculture:

  • A farming practice where seeds are drilled directly into the ground without turning or mechanically disrupting the soil.

  • Leaves the soil consolidated and compact, increasing structural resistance against both wind and water erosion.