Organic manures
Continuous cropping without addition of fresh or decomposed organic matter results in a decrease of soil organic matter and a decline of soil structure. Organic matter sources are:
- farmyard manure and compost
- green manures , which are incorporated into the soil
- crop residues ploughed back into the soil.
In the sub-tropics the use of farmyard manure is quite common, but is usually only available in small quantities. They are often reserved for crops that react well to the organic manures (vegetables and potatoes) and which give a high return.
Chemical composition
There is so much variation among animals, feeding, handling and storage systems that the data on chemical composition which are presented should only be considered as rough estimates. Micro-elements are often present in small quantities.
Losses
Serious losses of nutrients may occur during handling and storage of the manures. Animal manure left to dry on the soil surface may lose as much as 25% of its nitrogen in one day. Therefore animal manure should be spread and be ploughed back into the soil in on the same day.
Compost
Home made compost from plant residues, household garbage, human and animal excreta, etc is an important contribution towards recycling organic matter and nutrients.
Use of composts and manures is not the only solution towards a balanced soil fertility, since with all products that leave the farm soil nutrients are also exported.
To maintain soil fertility these removed elements have to be replaced by e.g. fertilizers, cattle food, farmyard manure or compost from elsewhere, etc.