94 STATE BOARD OF AGRICULTURE. 



meet the requirements of all plants, so far as ash constituents are concerned. 

 "When lime is used as a manure it is usually for some other purpose than to 

 directly supply the lime needed in plants, and I will speak of the special uses 

 of lime in another place. All our soils contain a small amount of sulphate of 

 calcium or plaster, which supplies sul^^hur to plants, though a few classes of 

 plants require an extra supply beyond what the soil contains. 



THE TRIPOD OF AGRICULTUEE. ' 



We thus see that of the thirteen chemicals of agriculture, Nature has pro- 

 vided some in exhaustless abundance, and others so liberally that the toilers 

 of the soil need feel little anxiety, so far as the direct food of plants is con- 

 cerned. Ten are thus summarily disposed of. But the case is far different 

 with the remaining three, nsimely, potassiut7i, p7wsp7iorus,a7id nitrogen. These 

 most costly and precious substances constitute the golden tripod of agricult- 

 ure. Plant life never unfolds a greenback, except upon the substantial bank- 

 ing basis of these chemicals of agriculture. They are the specie basis of 

 Nature's greenbacks. Not only are they indispensable for all growth, but 

 their limited supply correspondingly limits all the other conditions of 

 growth. In manurial value they hold front rank. To accumulate in the soil 

 and present to the plant in active form these three substances, are the essen- 

 tial chemical conditions of extraordinary cropping. With a sufficient supply 

 of these three substances in soils of ordinary composition, there is no limit to 

 production save those imposed by the physical conditions of growth. 



MANURE. 



Manure is any substance added to the soil to increase its fertility by chang- 

 ing its chemical composition, or by affording an increased supply of plant- 

 food. 



Some writers seem to regard manure as the crude material to be manufac- 

 tured into crops by the machinery of the plant, just as wool and cotton are 

 made into cloth by the factory. They seem to regard the soil as a passive 

 agent in this process — useful merely as the receptacle to hold the manurial 

 materials for the plant. But the soil is something more than the platter to 

 hold the plant's dinner. So far from being the mere dish to hold the food, 

 the soil is itself the roast beef — the principal dish of the meal — while the 

 manures we use with profit are merely the salt and mustard to make the roast 

 beef palatable and digestible. To make this clear let us count the cost of a 

 crop on the theory of the passive condition of the soil. Let us estimate the 

 cost of the crude materials to make a ton of clover hay if they were bought in 

 separate form and at market rates. A ton of clover hay contains 



39 lbs. potash (K2 0) @ 5c - U 95 



nibs, phosphoric acid ( Po O5 ) ®l^c - --- 1 38 



44^ lbs nitrogen @ 20c 8 90 



Cost of materials $13 23 



This would be the cost, saying nothing about labor and rent. It is evident 

 that the soil itself must furnish the body of the feast or we shall go hungry. 

 The nutritive power of the soil and the great diversity of soils in this respect 

 are factors of prime importance in discussing the subject of manures, and 

 serve in part to explain the discordance of results reached by different experi- 

 menters with the same manures but on different soils. We shall not go far 



