One of the least obvious pitfalls is residual herbicide, applied to a previous crop or fallow some time in the last few months or even the last few years, depending on the herbicide. Some of the residual herbicides (such as atrazine, Tordon®, Ally® & Glean®) can remain active in the soil for well over a year, even though they are having no obvious effect, but they can still have a nasty effect on cotton!