Calcium carbonate, in the form of ground limestone, spread on the patches, or mix it with water
some will dissolve, then water it in. Get it from horticultural suppliers or from garden centres, or even flea bay.
Cheap and fairly effective especially if used directly on the urine, if there are any small grains not dissolved it won't hurt the chooks if they pick it up. In fact it has the same calcium content as oyster shell.