Organic products produced on American farms more than doubled between 2012 and 2017, according to the USDA’s latest agricultural census, released this week.

Organic product sales totaled $7.3 billion in 2017, a 133% increase from 2012. That food was produced on 18,166 farms, up from 14,326 in 2012. Of those farms, 17,741 (98%) were USDA-certified as organic, up from 12,771 in 2012.

The increase in farms producing organic food comes in the face of a 3.3% decline in the overall number of farms, to just over 2 million.

Organic crops can be significantly more lucrative on a pound-for-pound basis than conventional ones. For instance, Bloomberg quoted an Indiana farmer as saying that food-grade organic corn sells for about $10.50 a bushel, compared with $3.50 for conventional corn.

But organic farming poses major challenges. In many cases, fields can only be certified for organic growth after several years of being free from pesticides and artificial fertilizers. This means that the field either has to lie fallow for that time, or the farmer has to grow crops on it with all the trouble and expense of organic farming, such as clearing weeds by hand, only to have to sell the crops at the lower conventional rate.