Green manure crops are tilled under before they mature. Their high water content speeds decomposition. Green manure are an excellent source of nitrogen. Chicken manure is higher in nitrogen than other manure, whereas sheep manure is a better source of potassium. Cow and horse manure contains less nitrogen and potassium than sheep manure.
Most animal manure is low in phosphorus.
Fresh manure applied too close to planting time can burn plants due to rapid decomposition and high ammonia content. Fresh manure is bulky, and it can interfere with plowing and cultivation. Soil microorganism populations can explode and tie up the available nitrogen in the soil. Poorly integrated fresh manure can create pockets of poor water and air infiltration. Fresh manure can be applied in the fall and do have certain advantages.