This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Farmyard manure is a bulky manure, but in a good condition it is probably the best and safest of all manures, natural or artificial. Although 1 ton of it only contains from 9 to 15 lb. nitrogen, 4 to 9 lb. phosphates, 9 to 18 lb. potash, and 39 lb. carbonate of lime, its fertilizing value must not be judged from these quantities on the unit system applied to artificial manures like sulphate of ammonia or nitrate of soda. While it is in itself a complete manure, containing all the foods from the soil, water, and air, it possesses mechanical advantages superior to any other manure. Being bulky, when dug into the soil it pushes the clods asunder and allows fresh air and water to enter freely. By its decomposition or fermentation heat is generated, carbonic acid gas is given off, minerals and metals are rendered soluble in conjunction with lime, and millions of bacteria are brought into being to produce other foods in the soil. These important functions cannot be performed or brought about by any chemical manure by itself, and it would be courting disaster to use them exclusively on any soil.
The quantities of farmyard manure necessary to keep a soil in a fertile condition vary according to the soil and its nature. On loamy soil in a well-cultivated condition from 12 to 16 tons may be regarded as a fair dressing. In a heavy loam, or clayey soil as it is often called, from 16 to 24 tons to the acre would not be too much. And in light, sandy, or gravelly soils, which are notoriously hot and hungry, from 30 to 50 tons per acre would be hardly sufficient to obtain good results. It will thus be seen that although a light sandy soil may be had at a very low rent, this advantage will vanish completely when the expenses of manuring and cultivating are compared with those of loamy and clayey soils.