AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/5/5-15)
Unit 5 · Topic 5.15
5.15 Sustainable Agriculture
Sustainable agriculture keeps soil in place and fertile so land can be farmed for generations. Practices like contour plowing, windbreaks, terracing, strip cropping and no-till farming reduce erosion, while crop rotation, green manure and adding limestone to acidic soil maintain fertility.
Key terms
- contour plowing
- windbreak
- terracing
- no-till agriculture
- strip cropping
- green manure
Reducing erosion
No-till farming has added benefits: it keeps carbon in the soil, saves fuel because tractors make fewer passes, and helps soil hold water. A drawback is that farmers may rely more on herbicides to control weeds instead of plowing them under.
| Practice | What it is | How it helps |
|---|---|---|
| Contour plowing | Plowing across a slope, following its contour lines | Furrows catch water and soil instead of channeling them downhill |
| Terracing | Cutting steep hillsides into flat, level steps | Slows runoff and lets water soak in on each step |
| Strip cropping | Alternating strips of a row crop (like corn) with a close-growing crop (like grass or alfalfa) | The dense strips catch soil washing off the row-crop strips |
| Windbreaks | Rows of trees or shrubs planted along field edges | Block wind to reduce wind erosion; also give habitat and can be harvested |
| No-till agriculture | Planting seeds directly into last year's crop residue without plowing | Residue protects soil from rain and wind; soil keeps its structure and organic matter |
| Perennial crops | Crops that live for several years, like fruit trees or perennial grasses | Roots hold soil year-round, and the ground isn't plowed each year |
Keeping soil fertile
- Crop rotation: changing crops from season to season. Rotating in legumes like soybeans, clover or alfalfa adds nitrogen to the soil through nitrogen-fixing bacteria in their roots. Different crops also use different nutrients and break pest cycles.
- Green manure: plants grown specifically to be plowed back into the soil while still green, adding organic matter and nutrients. Cover crops, which are planted between main growing seasons, protect bare soil and can be used as green manure.
- Limestone: crushed limestone (calcium carbonate) is added to acidic soil to raise its pH, which makes nutrients more available and reduces toxic aluminum. Soils can become acidic from acid rain, heavy fertilizer use and natural processes.
- Compost and manure: organic fertilizers add nutrients and organic matter that improve soil structure and water holding.
Grazing and the bigger picture
Rotational grazing, which moves livestock between pastures so plants can recover, keeps plant cover on grazing land and prevents the erosion that comes with overgrazing.
Sustainable agriculture often combines these practices with efficient irrigation, integrated pest management and careful fertilizer use. Agroforestry, which mixes trees with crops or livestock, is another approach that holds soil, stores carbon and diversifies a farm's income. The common goal is to produce food while protecting soil, water and biodiversity, so future farmers can keep using the same land.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Matching practices to problems
Recommend one sustainable practice for each farm and explain how it helps: (a) a farm on steep hillsides where heavy rains wash soil downhill; (b) a flat farm on the Great Plains where strong winds blow soil off bare fields; (c) a farm whose soil test shows pH 5.0, too acidic for its crops.
Show the solutionHide the solution
- Step 1: (a) Terracing turns the slope into level steps, so water slows and soaks in on each step instead of rushing downhill carrying soil. (Contour plowing also works on moderate slopes.)
- Step 2: (b) Windbreaks, rows of trees along field edges, slow the wind before it reaches the soil. (No-till farming, which leaves residue covering the soil, also works.)
- Step 3: (c) Adding limestone neutralizes acidity and raises the pH toward the 6–7 range most crops prefer.
Answer: (a) Terracing (or contour plowing); (b) windbreaks (or no-till); (c) adding limestone.
- Example 2
Evaluating no-till farming
Describe one environmental benefit and one environmental drawback of switching a cornfield from conventional plowing to no-till farming.
Show the solutionHide the solution
- Step 1: Benefit: crop residue stays on the surface, protecting soil from rain and wind, so erosion drops sharply. Soil also keeps more organic matter and carbon.
- Step 2: Drawback: without plowing to bury weeds, farmers often use more herbicide, which can run off into streams and select for herbicide-resistant weeds.
- Step 3: Each point should include both the change and its consequence.
Answer: Benefit: less erosion and more soil carbon. Drawback: possible increase in herbicide use and runoff.
Common mistakes
- Describing contour plowing as plowing up and down the slope. It runs across the slope, along contour lines.
- Saying limestone adds nitrogen. Limestone raises soil pH; legumes and fertilizers add nitrogen.
- Forgetting that no-till has a tradeoff in herbicide use.
On the exam
- Questions often describe a farm problem and ask for a sustainable solution. Name the practice and explain the mechanism.
- Know which practices reduce erosion and which maintain fertility, since questions may ask specifically for one or the other.
Connected topics
Videos
Check yourself
4 questions on 5.15 Sustainable Agriculture. Pick an answer to see if you got it, and why.
A farmer must grow crops on a steep hillside where heavy rains wash soil away. Which practice would best reduce soil erosion?
A soil test shows that a field's pH is 4.8, which is too acidic for the farmer's crops. Which amendment would best raise the soil pH?
On the flat, windy Great Plains, many farmers plant rows of trees along the edges of their fields. What is the main purpose of these windbreaks?
After harvesting a crop, a farmer plants clover, a legume, and then plows it into the soil before the next planting. How does this green manure help the field?
0 of 4 answered