INTRODUCTION xxi 



ductive soils contain as much as 20 per cent of calcium carbonate, 

 corresponding to 200 tons of limestone per acre. 



(8) A loo-bushel crop of corn takes from the soil about 100 

 pounds of nitrogen, 17 pounds of phosphorus, and 19 pounds of 

 potassium, in the grain, and about 48, 6, and 52 pounds of these 

 respective elements in the stalks or stover. 



(9) One ton of average fresh farm manure contains about 10 

 pounds of nitrogen, 2 pounds of phosphorus, and 8 pounds of 

 potassium; and 100 pounds of the most common " complete " 

 commercial fertilizer contains about 2 pounds of nitrogen, 4 pounds 

 of phosphorus, and 2 pounds of potassium. 



(10) One ton of clover hay contains about 40 pounds of nitro- 

 gen, 5 pounds of phosphorus, and 30 pounds of potassium. When 

 grown on soil of fair productive capacity, the roots and stubble 

 of the clover plant contain no more nitrogen than the soil has 

 furnished to the plant; but for each ton of clover plowed under, 

 the soil is enriched by about 40 pounds of nitrogen. 



(n) Roughly estimated, the plant food liberated from an aver- 

 age soil during an average season with average farming is equiva- 

 lent to about 2 per cent of the nitrogen, i per cent of the phos- 

 phorus, and ^ of i per cent of the potassium, contained in the 

 surface stratum (about 6f acre inches, or 2 million pounds of 

 average soil). 



(12) As an average in live-stock farming, the animals retain about 

 one fourth of the nitrogen and phosphorus and destroy two thirds 

 of the organic matter of the food consumed, and large loss is likely 

 to occur in the manure produced, especially in nitrogen and or- 

 ganic matter, a loss of one half of these constituents being easily 

 possible during three or four months, in part from fermentation, 

 which may occur even under cover, and in part from leaching 

 where the manure is exposed to the weather or where too little 

 absorbent bedding is used. 



(13) It is less difficult to maintain or increase the organic matter 

 of the soil by means of legume crops and crop residues in a good 

 rotation for grain farming than in any system of live-stock farming 

 which does not include the purchase of feed. 



(14) Some satisfactory rotation plans for grain farmers are 

 wheat, corn, oats, and clover ; or wheat, corn, and cowpeas ; or 



