Food Production and Land Use Change

Why do we have so much urban land?

First, nitrogen availability limits plant growth, as shown in this plot by Vogel etal (2002) for switchgrass biomass production:


Getting high yields means adding nitrogen fertilizers. The same thing happens with food crops. (Switchgrass is just one of the cellulosic crops touted for biofuels….hopefully fossil fuels won’t be used to produce them.)

Second,  in the early 1900s synthetic nitrogen fixation arose using the Haber-Bosch process (see Wikipedia), a process driven by large amounts of fossil fuel energy. This plot from Galloway etal (2003) shows nitrogen fixation from various routes:

gallowaySince 1950, our nitrogen production from fossil fuels exploded.

Third, fertilizers and farming technology greatly increased our crop yields. Here’s a plot showing just one important crop, corn, using data from the USDA:

cornSince about 1940, our crop yields increased six-fold.

Fourth, one might think this situation helped farmers, but, in reality, increased agricultural productivity caused small farmers to become economic disasters. This plot, using data from the USDA, shows the distribution of farm sizes changing over the last four decades:

farmdistNotice how small family farms between 50 and 1000 acres have been lost to large farms (>2000 acres) and small hobby farms (<50 acres). The underlying reason is their economic viability:

farmprofitThis data shows that a farm with a quarter million dollars in sales just breaks-even. What does that figure mean? Above corn yields show 150 bushels on one acre, bringing in $3/bushel, or $450/acre. Call that $500. If you have 1000 acres of corn, your sales total $500,000. So, the $250,000 in sales means you have 500 acres being farmed. Let’s say one acre of land has a value of $1000. That means a farmer’s land value totals $500,000, but the return on that investment — the operating profit — is 0%. Not terribly economically viable. Thus, the wise economic decision is sell the land and retire, or double the size of the farm to increase economic efficiency.

Fifth, these inputs produce the following land use change pattern in the U.S.:

landuseSo, fossil fuels make fertilizers that grow crops that increases per-acre agricultural efficiency and human populations that increase urban sprawl.

A question I now have: We had lots of small farms, obliterated by agricultural efficiency. The local and/or organic foods movement seemingly adds value back to older, inefficient ways, but those small plots no longer exist, having been suburbanized. Is it realistic to hope we can feed the present population without the fossil-fuel dependent fertilizers and technologies that today’s farms rely on for economic viability? I don’t see how we can handle cutting back our crop yields while simultaneously maintaining a high human population.


Galloway, J.N. et al. 2003. The nitrogen cascade. Bioscience 53:341-356.

Lubowski, R.N. etal. 2006. Major Uses of Land in The United States, 2002. USDA Economic Research Service Bulletin 14.

MacDonald, J., R. Hoppe, and D. Banker. 2006. Growing Farm Size and the Distribution of Farm Payments. USDA Economic Brief #6.

Vogel, K.P. etal. 2002. Switchgrass Biomass Production in the Midwest USA: Harvest and Nitrogen Management. Agron. J. 94: 413-420.

Leave a Reply