Is it permissible to urinate or defecate on grass or dirt during Shabbos?
It is permitted to urinate on grass (O.C. 336:3). It is also permitted to defecate on grass or dirt in a city. In either case there is no concern that human waste is helping the plants grow (see last Biur Halocho 336:3). On a field that was just plowed and is ready for planting (and there are usually large clumps of earth around), one should not defecate since the Gemoro (Shabbos 82b) explains there is a concern that while one is relieving oneself one will forget oneself and move a clump of dirt from a high point in the plowed field to a low point in order to even out the area. Flatten out the field would assist in the planting and is therefore a subcategory of plowing (O.C. 312:9).