It is sometimes claimed that materials such as manure are important to attain high yields of crops. While I know that it's possible to farm using only vegan materials, e.g. 'veganic' farming, I'm wondering if the effectiveness has ever been experimentally measured.
Are there studies showing that vegan farming is either as effective or less effective than farming with animal derived fertilizer, when both are done properly, without having to spend significantly more money on vegan farming? How significant is the difference, if any?
