Bruce --

This was a practice allowed by the military law of Feb. 17, 1864. The Secretary of War was allowed to detail soldiers as agriculturalists. Here's the pertinent section of that law:

In addition to the foregoing exemptions, the Secretary of War…may exempt or detail such other persons as…ought to be exempted on account of public necessity, and to insure the production of grain and provisions for the army and the families of soldiers. He may also grant exemptions…to such overseers, farmers or planters, as he may be satisfied will be more useful to the country in the pursuits of agriculture…: provided that such exemption shall cease whenever the farmer, planter or overseer shall fail diligently to employ…his…labor exclusively in the production of grain and provisions, to be sold to the government and the families of soldiers….

Such persons were detailed from Confederate military service, which meant that they were exempt from militia duty the same as any other soldier.