Soil: Texture, Horizons, and Why It Erodes
- Read a soil texture triangle and connect texture to water and nutrient behavior
- Identify soil horizons and explain what forms each
- Explain the causes of erosion and evaluate conservation practices
Texture governs everything else
Soil texture is the proportion of sand, silt and clay, and it determines the two properties that matter agriculturally. Sand has large particles and large pore spaces, so it has high permeability — water drains through quickly — and low water-holding capacity and low nutrient retention. Clay has tiny particles and tiny pores, so it holds water and nutrients well but drains poorly and can become waterlogged and anaerobic. Silt is intermediate. The best agricultural soil is loam, a roughly balanced mixture, because it holds enough water and nutrients to feed a crop while still draining enough for roots to get oxygen. On a soil texture triangle, read each side's percentage and find the intersection — the naming is what the diagram is for, and the exam asks you to connect the name to a drainage or fertility prediction.
Horizons, top to bottom
A soil profile has layers formed by different processes. O is the surface organic layer of leaf litter and decomposing material. A, the topsoil, mixes organic matter with mineral particles and is where most roots and biological activity are — this is the layer erosion removes and the layer that takes centuries to rebuild. E is the zone of leaching, where percolating water strips out clays and minerals. B, the subsoil, is where those leached materials accumulate. C is weathered parent rock, and R is bedrock. Two exam points: leaching moves material from E to B, so a question about nutrient loss concerns the upper layers; and topsoil formation runs at roughly a centimeter per century, which is why erosion is treated as effectively irreversible on human timescales.
Erosion, and the practices that reduce it
Erosion accelerates when soil is bare, sloped, dry and unstructured — so the causes to name are removal of vegetation, tillage that breaks up aggregates, overgrazing, and cultivation on slopes. The conservation practices each attack one of those, and the exam expects the mechanism rather than the name. Contour plowing: furrows follow the slope contour so water cannot run straight downhill. Terracing: steps flatten a slope into level surfaces. No-till agriculture: crop residue is left in place, protecting the surface and preserving structure. Windbreaks or shelterbelts: rows of trees reduce wind speed at ground level. Cover crops: keep roots in the soil between cash crops. Crop rotation: varies nutrient demand and breaks pest cycles, and rotating in a legume adds nitrogen through fixation. Naming the practice earns little; naming what it physically prevents earns the point.
A field on a 12% slope has been plowed up and down the hill and left bare over winter. Identify two practices that would reduce erosion and state the mechanism of each.
- 1.Identify the causes present: bare surface, significant slope, and tillage furrows running downhill.
- 2.Contour plowing addresses the furrow direction — furrows across the slope act as small barriers so water cannot run straight downhill and gain speed.
- 3.A winter cover crop addresses the bare surface — living roots hold soil in place and the canopy intercepts raindrop impact, which is what dislodges particles.
- 4.Note that terracing would also work but is far more expensive, which is a legitimate point of comparison on a free-response question.
For any conservation-practice question, give the physical mechanism. "Contour plowing reduces erosion" restates the question; "furrows across the slope prevent water from running straight downhill and gaining speed" is the answer. Every practice on the list has a one-clause mechanism.
A soil that drains too quickly and holds few nutrients is most likely dominated by
In a soil profile, leaching moves dissolved minerals
Answer the 2 checkpoints as you read.
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