What is the difference between physiological density and agricultural density?

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More farms, more food? Not necessarily. Fewer farmers, less food? It depends. Bigger farms, less hunger? Maybe, maybe not. Are you noticing a trend? Welcome to the world of agricultural statistics!

In this explanation, we look at agricultural population density, which is one way to understand the above questions.

Agricultural Population Density Definition

First, let's make sure we know what we are talking about:

Agricultural Population Density: The ratio of farmers (or farms) to arable land. "Agriculture" here refers solely to crops and not to domestic animals, thus in this definition arable land does not include rangeland for animal grazing.

Agricultural Density Formula

To calculate agricultural density, you need to know the number of farmers or farms in a given amount of arable land. Then, divide the number of farms by the arable land area.

Country A has 4,354,287 people (2022 figure) and 26,341 square miles. 32% of its land is arable. Its recent agricultural census measured 82,988 farms of all different sizes. Country A's arable land is 8,429 square miles (26,341 * 0.32) so its agricultural density is 9.85 farms per square mile. The average farm size is thus 0.1 square mile. This is often expressed in hectares or acres: 65 acres or 26 hectares per farm in this case (a square mile has 640 acres and there are 0.4 hectares in an acre).

Using this formula, we can see that Singapore has the highest agricultural density of any country in the world.

Agricultural Density and Physiological Density

It is useful to compare agricultural density and physiological density, as both are related to the amount of arable land available.

Physiological vs Agricultural Density

Let's continue with the example of Country A, above, where the average farm is 65 acres. Let's say the farm is owned by a family of three.

Meanwhile, the physiological population density of Country A, the total population divided by the amount of arable land, is 516 people per square mile of arable land. That's the minimum number of people who need to be fed by a square mile of land if the country is to be self-sufficient in food.

Now, let's assume that around half an acre is necessary to feed a single person per year. A 65-acre farm can feed 130 people, and a square mile, or around ten farms in Country A, can feed almost 1,300 people.

Everything is fine so far! With the farm only needing to feed three people (the farming family), the rest can be sold and go to feed 127 more people. It looks like Country A is not only self-sufficient in food but can be a net food exporter.

Confused about when to use physiological population density, agricultural population density, and arithmetic population density? You will need to know the differences for the AP Human Geography exam. StudySmarter has explanations on all three that include a variety of useful comparisons to help you keep them straight.

Arable Land, Farm Size, and Density

Here are some factors we need to know before we make assumptions about the relationships between arable land, farm size, and physiological density:

• Farmers are concerned about the prices they receive for their crops, and governments are concerned about crop prices and food prices for consumers. Higher prices may mean a farm sells its products on the international market rather than for domestic consumption.

• If farmers don't earn enough, they may choose not to sell or not to grow. Even if they sell it, the food may be destroyed down the line rather than sold if it is not making a profit (restriction of supply can raise profits).

• The amount of land needed to feed a person varies based on the quality of the land (e.g., soil), type of crops grown, access to nutrients, access to fertilizers, and other factors. Productivity can change from place to place and year to year for the same crop.

• A lot of food is raised not to feed people but rather to feed domestic animals.

• Farms may grow food exclusively for export earnings. Laborers on these farms, and other local people, thus may have little to no access to the food produced. This is why even places that COULD be food self-sufficient might not be, instead depending on food imports. When this food becomes too expensive, and such places can't fall back on domestic production, people may go hungry as a result.

With so many factors, it should be clear that we need to be very careful in making assumptions about relationships between farm size, arable land, and overall population. A higher physiological density or agricultural density doesn't necessarily make it more difficult or less difficult for a country to feed itself.

What Happens When Population Increases?

The overall population of a country is often rising. To feed more mouths, it is possible to bring new, non-arable land into production and make it arable (irrigating the desert or cutting down forest land to turn it into cropland, for example). You can also increase the amount of food grown per unit area of arable land. In general, the physiological density increases when the overall population goes up, while the relationship to agricultural density may be unchanged.

One factor seen as a result of rapid population growth is that farm household size may outstrip the capacity of the farm to feed the people who live on it. This has typically been a problem in countries where most farms turn little or no profit, or where the introduction of mechanization means farms may get bigger but fewer people are needed to work on them. In these conditions, the "excess" children in a household may then migrate to urban areas and enter other economic sectors.

Let's look at the example of Bangladesh.

Agricultural Population Density Example

Bangladesh, a country in South Asia, has the world's highest percentage of arable land, (59%) but was long associated with hunger and famine.

Bangladesh's Green Revolution struggle to feed itself has been one of the most important and instructive dramas in the relationship between population and food production. The main factors have been the weather and the changing climate, the struggle to reduce population growth in a socially conservative country, exposure to toxic agricultural chemicals, and a range of political and economic issues.

Bangladesh's 33,818 square miles of arable land have to feed 167 million people. Its physiological density is 4 938 people for every square mile of cropland. There are currently 16.5 million farming households in the country, so Bangladesh's agricultural population density is 487 per square mile. Each farm household farms on average of 1.3 acres.

Surviving in Bangladesh

We said above a person can survive on 0.4 acres per year. The average household size in rural Bangladesh is just over four people, so 1.6 acres would be needed for a farm to be self-sufficient.

Let's focus on rice, Bangladesh's staple crop, planted on 3/4 of the country's arable land.

In 1971, Bangladeshi farms on average produced around 90 pounds of rice per acre. Today, after decades of two percent or more increases in productivity per year, they average 275 pounds an acre! Productivity has increased with better control of water (including floods and irrigation), access to high-producing seeds, access to pest control, and many other factors.

In terms of household size, farm families topped eight in the early 1970s, and are now half that. Mothers averaged over six children in 1971 (fertility rate), and now are having only 2.3. Government policies and education that have given women more say in family planning are a large factor in this change.

What does all this mean? Well, a single adult needs at least 300 pounds of food per year (children need less, with the amount varying by age), much of which can be provided by a staple, carbohydrate-rich crop like rice. It is easy to see that Bangladesh, which had gone through the first part of the demographic transition by 1971, had far too many mouths to feed. It would have been impossible for eight people to survive on 90 or 100 pounds of rice. Now, enough rice is produced in Bangladesh to keep people fed and to export, along with other crops that help make Bangladeshis healthier every year.

Agricultural Density of USA

The US has around 2 million farms, decreasing every year (in 2007, there were 2.7 million farms).

The US has around 609,000 mi2of arable land (you may see figures ranging from 300,000 to 1,400,000, which reflects different definitions of "arable land" to include grazing land, and whether only land productive in a given year is measured). Thus, its agricultural density is around three farms per square mile, with an average size of 214 acres (some figures give an average of over 400 acres).

Fig. 3 - Cornfields in Iowa. The US is the world's leading corn producer and exporter

With 350 million residents, the US has a physiological density of around 575/mi2. With some of the highest yields in the world, far more than 350 million can be fed. The US does not have a problem with having too many mouths to feed. It is at the opposite end of the spectrum from Bangladesh.

In such a massive country, farm size varies radically depending on what is grown, where it is grown, and what type of farm it is. Nevertheless, it is easy to see that the US produces a massive food surplus, and why it is the largest food exporter in the world (and the second largest producer, after India).

However, the US also has malnutrition and hunger. How can this be? Food costs money. Even if there is enough food available in the supermarket (and in the US, there always is), people might not be able to afford it, or they might not be able to get to the supermarket, or they might be only able to afford food with insufficient nutritional value, or any combination of these.

Why are there fewer farms every year? To a small extent, this is because farmland in some areas is taken over by suburban development and other uses, or farms are being abandoned where farmers can't turn a profit. But the biggest factor is economies of scale: it is getting harder and harder for smaller farms to compete with larger farms, as costs of machinery, fuel, and other inputs go up. Large farms can better survive long term.

The trend is that small farms must get bigger, or be bought out. This isn't the case everywhere, but it explains why the US's agricultural density is shrinking yearly.

Agricultural Population Density - Key takeaways

• Agricultural population density is the ratio of farms (or farming population) to arable land.
• Agricultural population density tells us the average farm size and whether there are enough farms to feed the population.
• Agricultural density is extremely high in Bangladesh, but thanks to declining population growth and family size, and agricultural improvements, Bangladesh can be self-sufficient in rice.
• Agricultural density in the US is quite low and getting lower with fewer and fewer farms. Mechanization and economies of scale have made it difficult for small farms to survive.

References

1. Fig. 1 (https://commons.wikimedia.org/wiki/File:Unload_wheat_by_the_combine_Claas_Lexion_584.jpg) by Michael Gäbler (https://commons.wikimedia.org/wiki/User:Michael_G%C3%A4bler) is licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
2. Fig. 2 (https://commons.wikimedia.org/wiki/File:Map_of_Bangladesh-en.svg) by Oona Räisänen (https://en.wikipedia.org/wiki/User:Mysid) is licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
3. Fig. 3 (https://commons.wikimedia.org/wiki/File:Corn_fields_Iowa.JPG) by Wuerzele is licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en)

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