"Spreu" is often used in the phrase "die Spreu vom Weizen trennen", so it's the "worthless" part of the harvest (just like the money the "poetic I" got is worthless). And of course, in summer, if you are in the countryside, you can also lie down on it if you want to lie down on something soft (but prickly). But even though you can lie down on it, it's mocking you with its worthlessness. You understand the connotations of "Herbstmanöver"? (The Bundeswehr, sometimes the whole NATO used to have bigger military exercises in autumn, when the harvest was already in and moving through fields wouldn't destroy anything). to rely on s.th. = sich auf etwas verlassen (not "auf etwas liegen").