In Crowded Places you found population density: people divided by land area. That single number can mislead, because not all land can feed people. Geographers use three densities, and the difference between them is which people and which land you count.
| Density | Divide | By |
|---|---|---|
| Arithmetic | all the people | all the land |
| Physiological | all the people | arable (farmable) land |
| Agricultural | farmers | arable land |
- Arithmetic tells how crowded the country is overall.
- Physiological tells how much pressure people put on farmland.
- Agricultural tells how many farmers work each piece of farmland.
Key idea
Physiological density = people ÷ arable land. When it is much higher than arithmetic density, most of the country's land cannot be farmed. Agricultural density = farmers ÷ arable land. High agricultural density usually means farming uses many hands and few machines.
Why it works
People eat food grown on farmland, not on deserts or ice. Dividing by arable land measures how many mouths each square kilometer of farmland must support. Rich countries often have low agricultural density even when physiological density is high, because a few farmers with machines work large areas.
Worked examples
Worked example: All three densities
A country has 200,000 km² of land, of which 10% is arable. It has 30 million people, and 9 million of them farm. Find the three densities.
- Arable land = 10% of 200,000 = 20,000 km².
- Arithmetic: 30,000,000 ÷ 200,000 = 150 people per km².
- Physiological: 30,000,000 ÷ 20,000 = 1,500 people per km² of farmland.
- Agricultural: 9,000,000 ÷ 20,000 = 450 farmers per km² of farmland.
Answer: 150, 1,500 and 450 per km².
Worked example: Reading the gap
Country X: arithmetic density 50, physiological density 1,000. Country Y: arithmetic 50, physiological 120. What does the gap tell you?
- Same people per km² of all land.
- X has far more people per km² of farmland, so little of its land is arable (50 ÷ 1,000 = 5%).
- Y has much more farmland (50 ÷ 120 ≈ 42%).
Answer: X has little arable land and much more pressure on it.
Common mistake
The usual error is dividing by the wrong land area. Physiological and agricultural density divide by arable land only, never by total land. And agricultural density counts farmers only, not everyone. Check: physiological density must always be at least as large as arithmetic density, because arable land is part of all land.