Good read. I think there are (at least) three concerns with the proposition.
The first is urbanization. In most countries, farming is a rural endeavour with urban farming still a marginal source of food. We are seeing a trend of many people moving to cities for work and social oppurtunities. When we combine these trends, we may end up with a segregated society; the well off live in cities and ones who are on the guaranteed work program in rural communities. This isn’t an ideal arrangement both economically and socially.
The second aspect is that most agricultural jobs are guaranteed so to say by the current labour market. In California and Ontario, many locals are unwilling to do the labour required for picking fruits, vegetables and cash crops at the current price. As a result, many temporary worker programs exist for this purpose. So if you wanted to pick peaches in the summer, there would be no shortage of farmers willing to hire you. But, working in the son for 10 hours a day isn't a pleasant experience so most people aren’t willing to do it for minimum wage.
Even in the more skilled agriculture jobs, fewer people want to engage in such work as it isn't seen as socially or economically favourable (often disregarding the fact that many farmers, including chicken farmers can make quite a fair amount of money).
Moving these jobs to city centres would also be difficult as it would require shifting our city planning, which has proven to be very difficult. Within highly concentrated population centres or wealthy neighbourhoods, there is also no shortage of demand for minimum wage service sector jobs. Issue is people can’t afford to live there.
These jobs are also often done much more efficiently by machines. If you can double the production of crops by the precise use of fertilizer and watering, is it efficient and moral to lower those yields to create job? A person either eyeballing or meticulously measuring water usage would significantly reduce yields, undermining the viability of urban or small land use farming.
The third issue is the structure of society. People need something to do, this I agree with. Structuring a social program as a low paying job is not ideal. People are more likely to do pro bono work then work at a fraction of their job rate because they are now doing so for a cause/purpose rather than sustenance. If a program is instead structured as a community engagement or passion project, we can potentially avoid the motivational and productivity limitations that will be encountered by a guaranteed work program. So the pairing of UBI with more community and entrepreneurial projects would be beneficial.
To wrap it up, if 20% of population is put out of work, it will be very difficult to set up any sort of program without significant planning. So as mentioned, pilots and social policy changes are essential as more and more low skill jobs are eliminated.
