Agricultural History Review, Volume 50 Part 2 (2002), pp. 164–180

In medieval times sheep were commonly grazed on pasture land by day but folded on arable each night. This was recognized as a way of improving the soil fertility of the arable. This paper calculates how much nitrogen, phosphorus and potassium per sheep would be transported to the arable by this practice, and hence how much grain export from the farm could be supported without soil fertility declining. The answer it is suggested, is about 3-5 bushels of grain per year per sheep. Applying these figures to data from demesne accounts of the thirteenth and fourteenth centuries shows that on some demesnes the number of sheep was fully adequate to maintain soil fertility, but on others it fell short. 

Complete Volume PDF