California is a big diverse state. According to stereotypes, it is composed of Northern Californian hippies, Southern Californian beach bums, and Central Valley farmers, but these stereotypes are a gross over-simplification. People living in each region may have some cultural similarities, but visit any town and you'll meet a variety of people.  There are even beach bums in the Central Valley.

Is the same thing true of California agriculture? California grows a much wider variety of crops than any other state. Does each region focus on one crop, or is each crop spread throughout the state?

In California as a whole, the top ten crops make up 75% of the land planted to crops.  In each of the top agricultural states other than California, the top ten crops make up more than 98% of acreage.

Some California counties have more diverse agriculture than others. Let's first look the 34 counties that typically harvest more than 25,000 acres.  In 2019, the top 10 crops comprised over 98% of acreage in only six of these counties: Shasta, Lassen, Modoc, Siskiyou, Napa, and Sonoma. Thus, 28 of the 34 largest California counties have more diverse agriculture than any other state.   

Counties in the south tend to have more crop diversity than counties in the north. Fresno, which is California and the nation's largest agricultural county, devotes a quarter of acreage to its largest crop, whereas Napa county devotes 98% of its acres to its largest crop.

Counties with the least diverse crop mix tend to fall into one of three categories. Counties with relatively low-value land focus on hay (e.g., Siskiyou, Modoc, Shasta, and Lassen). The premier wine regions focus on wine grapes (Napa, Sonoma, and Mendocino), whereas rice dominates in the northern Central Valley counties (Butte and its neighbors). 

The set of top crops has changed in the last 40 years as farmers have moved away from the big field crops.  Cotton has been replaced by almonds as the number one crop in the southern part of the valley, wheat is no longer number one in Yolo and Solano Counties, and corn has been displaced in Sacramento and San Joaquin Counties.

Although the dominant crops have changed, crop diversity has not changed much in the state. Almonds and grapes have increased in acreage, but the top crops don't systematically use a higher proportion of acres than they did in 1980.  The picture below looks similar whether I plot the change in acreage devoted to the top 1, 2, or 5 crops.  

Most California counties grow a wider variety of crops than their peers in other states, but some counties specialize. Climate, soils, and water availability suit some crops in some regions better than others. These factors likely explain the dominance of wine grapes in Napa and Sonoma and hay in the far north of the state. On the other hand, diversity reduces the profit risk faced by farmers, and it can help preserve soils and the ecosystem. 

As I noted in this article on grass seed, agglomeration allows farmers to benefit from networks of suppliers and processors and from each other, which increases profitability. That's why technology firms cluster in Silicon Valley. Agglomeration is likely a major factor in rice continuing to be an important crop in the northern Central Valley. In contrast, almonds and pistachios have spread throughout the valley rather than concentrating in some areas. Why it happened that way is a good topic for further study.

I made the graphs in this article using this R code using data from county Ag Commissioners reports. The data-quality caveats I outlined here apply. In particular, these data likely overstate almond acreage and therefore understate the diversity of crops in almond-dominated counties.