Was there an increment of the percent of people working in the agricultural sector during the economical crises of the last couple of centuries?
What are the factors influencing the ratio of farmers?
The chief impact on the ratio of agricultural workers to workers in all other industries has been the dramatic growth of workers in other sectors.
Agricultural workers have increased in number at times, particularly with the proletarianisation of peasants, petits-bourgeois, and lumpen-proletarians in the development of modernity. The "total workforce" has also changed. Small scale industry has reduced, but large scale mobile regional workforces have increased. Labour has flowed from capital intensive sectors towards labour intensive sectors. Workers are generally where the profit isn't.
Moreover, the "farmer" the owner operator of a family concern, even if they exploited itinerant workers, has been reduced in power and status to a locked-in-contractor.
Noel Butlin's data on Australian agriculture is good here, as is the account in Connell and Irving, Class structure in Australian history.
Generally there is a large decrease in the number of people working in agriculture in the last couple of centuries as mechanisation increased.
You can't really increase the number of farmers, there is a limited amount of land and it's difficult (without an invading army) to remove farmers from the land.
Generally in times of economic downturn farms have suffered as much as anywhere else, there are no customers for their crops, but farmers always have the option to curt production back to what they can sell - or even just to subsistence. So in hard times workers have moved from the county to the city - in a big city there might be a chance of some sort of work.
Although if you include people growing some food in garden crops themselves to try and help the food budget then there probably is a rise in "agriculture" during recessions.