456 ANNUAL REPORT OF THE OfE. Doc. 



if the crops are fed to live stock it is well to know about one- 

 fourth of the nitrogen and one-fourth of the phosphorus are retained 

 in the tlesh and bone of the animal, while three-fourths of the nitro- 

 gen and phosphorus and practically all of the potassium are returned 

 iu the solid and liquid manure. 



Thus we have another process of separation by which part of the 

 needed nitrogen and phosphorus leaves the farm with the animals, 

 while the potassium is again returned, even though it may not be 

 needed. 



It should be a plain fact that manure made from animal excre- 

 ments with straw or stalks for bedding must be deficient in nitro- 

 gen and still more deficient in phosphorus, but rich in potassium, as 

 compared with the requirements of the crop; and this is especially 

 noteworthy when the manure is to be used on land already deficient 

 in nitrogen and phosphorus but well supplied with potassium. 



In the case of nitrogen the difficulty can be overcome by making 

 a libera] use of clover or other legumes in the crop rotation and as 

 catch crops, turning under these crops and crop residues so far as 

 practicable. Legume crops may also be used in pastures to a con- 

 siderable extent, thus receiving nitrogen from the air to balance the 

 deficienc3^ in the manure. 



With the phosphorus the difficulty is greater, because the propor- 

 tion contained in the manure is less and there is no such ever-pres- 

 ent inexhaustible supply as in the case of nitrogen. 



It must be apparent that to increase the value of farm manure we 

 should add phosphorus to it. Thus we can balance manure and 

 when added on soils rich in potassium in rotation with nitrogen-fix- 

 ing legume crops we can provide plant food in a balanced ration to 

 meet the needs of the maximum crop yields. By these means we 

 can check the progress of soil exhaustion and even gradually in- 

 crease the fertility and productive capacity of the land. Indeed, we 

 can thus profitablv enrich such land even beyond its virgin fer- 

 tility. 



By far the cheapest form of phosphorus is the fine-ground raw rock 

 phosphate. This material is but slightly available for the use of 

 crops if applied to soils deficient in decaying organic matter; but, 

 if applied in iutimate connection vrith rotting manure, it is thus 

 made soluble and available for plant growth. 



Certainly one of the most profitable and probably the very most 

 profitable methods of maintaining the necessary supply of phos- 

 phorus in the soil is to put back into the manure in the form of 

 fine-ground raw rock phosphate somewhat larger amounts of phos- 

 phoius than the animal has retained in his bones. It is well for a 

 time, at least, to put back larger amounts than the animal retain, 

 because the soils are already deficient in phosphorus and also because 

 there may be some waste of manure. 



These statements are based both upon the chemical analysis of 

 soils and crops and manures and also upon carefully conducted field 

 experiments covering many years. 



The Maryland Experiment Station furnishes some valuable data 

 from probably the earliest systematic investigations still being con- 

 tinued, and a large amount of information is rapidly accumulating 

 from our more extensive work in Illinois; but the most complete ex- 

 periments of long duration are reported by the Ohio Experiment 

 Station. Where 40 pounds of fine-gronnd rock phosphate, costing 



