Digestate use and having problems (maybe with FieldCalculator mod)

So i have a field 1.96 ha and the mod says i need 28,188 digestate to fertilize it and i am using the slurry spreader VT130 with it sprayer. It carries ca. 13-14k of digestate but the thing is i am already on my 5th full tanker that's double the amount what mod told me to use (seeds calculation works fine) . Is the mod broken or is that i am playing on hard a problem? i also know that using store bought fertilizer would be more efficient but i wanted to use some other gameplay to have fun then just profits