The growth rate of crops is related to the amount of moisture that their roots can access. plants with better acess to moisture grow taller and can also turn a different colour than plants around them. if archaeological features are burried under a field it it can either produce a negative or positive cropmark.
E.g. a burried ditch with its infill of humus and topsoil will often hold moisture creating a dark green line in the crop above. This will look like a 'positive' crop mark that is visible from the air.
The opposite happen with plants over a buried wall, the plants will most likely be stunted and produce a yellow negative cropmark.
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