What do you mean by "sludge"? Typical hydroponic solution just looks like water, thus the name. The fertilizers used are higher purity ("greenhouse grade", "ult

What do you mean by "sludge"? Typical hydroponic solution just looks like water, thus the name. The fertilizers used are higher purity ("greenhouse grade", "ultra-soluble", etc.) than those applied to soil, since impurities risk clogging the fertigation equipment.