346 SOIL FERTILITY 



Aside from supplying readily decomposable vegetable material of a wide nitrogen- 

 carbon ratio, green manures may conserve plant food, and, under certain conditions, 

 increase the nitrogen resources of the farm. They are usually grown during a portion 

 of the season when the land is not required for one of the principal crops. Hence they 

 may be used in the fall and early spring for protecting the soil against leaching. When 

 legumes are used as green manures the soil's store of combined nitrogen is increased, 

 usually by 25 to 100 pounds per acre. 



Animal manures are most frequently applied at the rate of 5 to 10 tons per acre. 

 Market gardeners employ larger amounts, occasionally up to 30 or more tons per 

 acre. The progressive farmer knows that barnyard manure contains, aside from 

 nitrogen and ash ingredients, 350-450 pounds of organic matter per ton of material 

 and also enormous numbers of bacteria. When added to the soil, manure enriches it 

 in plant food, furnishes energy material for the micro-organisms, and serves as an 

 inoculum, thanks to the mass infection caused by its application. Horse manure con- 

 taining a large proportion of straw and used at the rate of more than 20 tons per acre 

 may temporarily depress plant growth. This is attributable primarily to the rapid 

 transformation of ammonia, nitrates, and amino compounds into insoluble, microbial 

 cell substance because of the abundant supply of cellulose and other carbohydrates. 

 The competition for available nitrogen compounds between the crop and micro- 

 organisms is, then, usually an unfavorable one for the farmer. When the manure is 

 well rotted before application the depressing effects just referred to are not apparent. 

 The practice of composting manure and other refuse materials of plant and animal 

 origin is still quite common in the Far East and in some of the European countries. 

 The significance of animal manure as a soil inoculant is more readily appreciated 

 when we remember that a gram of fresh manure may contain from fifty to several 

 hundred million bacteria; that an adult beef or dairy animal will produce 10 to 12 

 tons of manure per annum, and a horse or mule 5 to 6 tons; and that the animal 

 population of the United States, including cattle, horses, mules, sheep, and swine, 

 consists of more than 180,000,000 units. In recent years there has been developed a 

 process for making "artificial" manure from straw or other waste vegetable materials 

 with the aid of sulphate of ammonia or of other nitrogen salts. The cellulose and 

 carbohydrates in the straw furnish energy for the formation of proteins. The making 

 of artificial manure is, therefore, a composting process. 



Commercial fertilizers are made up principally of mono-calcium phosphate, 

 potassium chloride, ammonium sulphate, sodium nitrate, and animal and vegetable 

 by-product materials such as meat scrap, fish scrap, bone meal, cottonseed meal, etc. 

 Other salts furnishing nitrogen, potassium, and phosphorus are used in smaller 

 amounts. The mono-calcium phosphate is derived from phosphate rock by treating 

 the latter with sulphuric acid. The resulting product is known as acid phosphate or 

 superphosphate. It contains, aside from the mono-calcium phosphate, relatively 

 small amounts of di-calcium and tri-calcium phosphate and a very considerable pro- 

 portion of hydrated calcium sulphate. Since commercial fertilizers consist largely of 

 soluble salts, their application increases the concentration of the soil solution and, 

 among other things, stimulates the activities of the micro-organisms. The quantities 

 of fertilizer applied range from about 100 to more than 3,000 pounds per acre. The 



