40 



The Ottawa Naturalist. [June 



fact has within the last few years led to many important changes 

 in the use and application of the so-called chemical fertilizers. 



The amount of water used by crops in their feeding, that is, 

 absorbed by their roots and lost by transpiration through their 

 leaves, is enormous, equalling several hundred of tons per acre. 

 Taking the mean of a number of determinations, Hellriegel found 

 that for every ton of dry matter produced in plants, in the neigh- 

 bourhood of 325 tons of water were required. An acre of Indian 

 corn probably uses in this way during its growth i,qoo tons of 



water. 



Notwithstanding these considerations, it is, from the practical 

 standpoint, those elements, or rather compounds, other than 

 water, withdrawn from the soil that, as agriculturists, we must 

 regard as most important. It is their removal by successive 

 cropping without any concomitant return, that results in soil 

 exhaustion and reduces the yield below the mark of economical 



production. 



What are these elements? First, there are the mineral or 

 ash constituents. These comprise calcium, magnesium, iron, 

 potassium, sodium, silicon, sulphur, phosphorus and chlorine, 

 and occasionally traces of several ot the rarer elements. As 

 already explained, these are found in the plant variously com- 

 bined, and not in the elemental condition. They form, say, from 

 .1 per cent, to 3.0 per cent, of the weight of fresh plant tissue, 

 the proportion depending largely on the part examined. Though 

 crops differ m their demands for ash constituents, the amount 

 withdrawn per acre by average yields of farm crops usually lies 

 between 200 lbs. and 300 lbs. 



Now with regard to the above-mentioned elements, the 

 majority of them are present in soils in quantities so abundant, 

 and the amounts required by plants by comparison so 

 extremely small, that their return to the soil by the farmer may 

 be neglected. Indeed, as the result of scientific research as well as 

 of practical experience, it is known that to maintain fertility — as 

 far as these inorganic constituents are concerned— it is only 

 necessary generally to replace two or, at most, three of them. 

 They are commonly spoken of in agriculture as potash and phos- 

 phoric acid, with lime as third in importance. 



