AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/5/5-4)
Unit 5 · Topic 5.4
5.4 Impacts of Agricultural Practices
Some common farming practices harm the environment: tilling loosens soil so it erodes, slash-and-burn clears forests and releases carbon, and fertilizer that crops don't absorb runs off into water. Synthetic fertilizers act fast but don't build soil, while organic fertilizers release nutrients slowly and improve soil.
Key terms
- tilling
- slash-and-burn
- synthetic fertilizer
- organic fertilizer
- runoff
Tilling
Tilling means plowing or turning over the soil to prepare a field for planting and bury weeds. It helps seeds get started, but it has costs.
Tilling breaks up soil structure and removes the layer of crop leftovers that protects the soil. Bare, loose soil is easily carried away by wind and water, so erosion increases. Tilling also exposes organic matter to air, so it decomposes faster, releasing CO₂ and lowering soil fertility over time. Dust from tilled fields adds particulate matter to the air. Over many years, repeated tilling can leave a compacted layer just below the plowed zone that roots and water have trouble passing through.
Slash-and-burn
Slash-and-burn farming means cutting down a patch of forest, letting the plants dry and then burning them. The ash adds nutrients to the soil for a short time, so crops grow well at first. In tropical rainforests, where soils are naturally thin and nutrient-poor, those nutrients are used up or washed away within a few years. Farmers then move on and clear a new patch.
When few people use this method and fields are left for many years to regrow, the forest can recover. But with more people and shorter rest periods, slash-and-burn leads to deforestation. It releases CO₂ from burning (and from the loss of a carbon sink), adds smoke and particulates to the air, destroys habitat and increases erosion.
Synthetic versus organic fertilizers
| Feature | Synthetic (inorganic) fertilizer | Organic fertilizer |
|---|---|---|
| Examples | Manufactured nitrogen, phosphorus and potassium compounds | Manure, compost, crop residues, bone meal |
| Nutrient release | Fast; nutrients are immediately available | Slow; released as microbes decompose it |
| Precision | Exact nutrient amounts and ratios | Nutrient content varies |
| Effect on soil | Doesn't add organic matter; overuse can acidify soil | Adds organic matter, improving structure, water holding and soil life |
| Downsides | Easily runs off or leaches; made with fossil fuels | Bulky and harder to apply; can still run off if overused; manure may carry pathogens |
Fertilizer runoff
Crops don't absorb all the fertilizer applied, especially when too much is used or it's applied right before heavy rain. Extra nitrate and phosphate run off into streams and lakes, causing algal blooms and dead zones (eutrophication). Nitrate also leaches through soil into groundwater, where high levels in drinking water are dangerous for infants. Applying the right amount at the right time, and keeping plant cover on fields, reduces these losses.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
How much nitrogen runs off?
A farmer applies 150 kg of nitrogen fertilizer per hectare to a 200-hectare cornfield. The corn takes up 90 kg/ha. (a) How much nitrogen per hectare is not taken up? (b) What percent of the applied nitrogen is that? (c) How many kilograms total are left over on the farm?
Show the solutionHide the solution
- Step 1: (a) Excess = 150 − 90 = 60 kg/ha.
- Step 2: (b) Percent = 60 ÷ 150 × 100 = 40%.
- Step 3: (c) Total = 60 kg/ha × 200 ha = 12,000 kg.
- Step 4: Much of that excess can run off into streams or leach into groundwater.
Answer: (a) 60 kg/ha; (b) 40%; (c) 12,000 kg of nitrogen.
- Example 2
Choosing a fertilizer
A farmer's sandy soil drains quickly, and nearby streams have algal blooms. Would you recommend synthetic or organic fertilizer? Explain using two properties.
Show the solutionHide the solution
- Step 1: Sandy soil holds water and nutrients poorly, so quick-release synthetic fertilizer would easily wash out.
- Step 2: Organic fertilizer releases nutrients slowly, so less is lost at once to runoff and leaching, which helps the streams.
- Step 3: Organic fertilizer also adds organic matter, which improves the sandy soil's ability to hold water and nutrients.
Answer: Organic fertilizer, because it releases nutrients slowly (less runoff into streams) and adds organic matter that helps sandy soil hold water and nutrients.
Common mistakes
- Saying organic fertilizer can't cause pollution. Overused manure can also run off and cause eutrophication.
- Thinking slash-and-burn permanently fertilizes the soil. The ash's nutrients last only a few years.
- Forgetting that tilling releases soil carbon, not just causing erosion.
On the exam
- You may be asked to compare synthetic and organic fertilizers. Give one advantage and one disadvantage of each.
- When asked about effects of a farming practice, give a specific environmental consequence, such as ‘nitrate runoff causes algal blooms’.
Connected topics
Videos
Check yourself
4 questions on 5.4 Impacts of Agricultural Practices. Pick an answer to see if you got it, and why.
A family clears a patch of tropical rainforest by cutting and burning the trees, grows crops for three years, then moves on to clear a new patch. Which statement best explains why they move on?
Compared with spreading composted manure, applying synthetic fertilizer to a field is more likely to
Repeatedly plowing (tilling) a field over many years tends to lower the amount of organic matter in its soil. Which statement best explains this?
Each summer, a large zone of very low oxygen forms in the Gulf of Mexico near the mouth of the Mississippi River, which drains much of the U.S. farm belt. Which sequence best explains how farming contributes to it?
0 of 4 answered