The reason why isotopes are useful when we’re trying to find out whether crops were manured or not in the past is that nitrogen, or more precisely the lighter isotope of nitrogen 14N, from manure volatilises very easily (that is the smell you get when you pass a cow’s field). Manured soil therefore contains relatively more of the heavier isotope 15N than unmanured soil, giving it a higher δ15N value. The crops grown on the manured soil take up the nitrogen and therefore also have a higher δ15N value.
Fraser et al. (2011) investigated just this, measuring the δ15N values of crops grown on experimental farming sites (like Rothamsted) and traditional farming plots (like in Romania). They found that regardless of location – they sampled from Denmark to Syria – the manured crops always had δ15N values higher than their unmanured counterparts. It therefore makes sense that if we measure the δ15N value of cereal grains found on an archaeological site, we could learn something about the amount of manure (or lack of it) that was applied to help the cereal grow.