ITS ESSENTIAL ELEMENTS- 18 1 



of magnesia, and silicate of potash. In the straw 

 which has served as litter, we add a further quantity 

 of silicate of potash and phosphates ; which, if the 

 straw be putrefied, are in exactly the same condition 

 in which they were before being assimilated. 



It is evident, therefore, that the soil of a field will 

 alter but little, if we collect and distribute the dung 

 carefully ; a certain portion of the phosphates, how- 

 ever, must be lost every year, being removed from the 

 land with the corn and cattle, and this portion will 

 accumulate in the neighborhood of large towns. The 

 loss thus suffered must be compensated for in a w^ell- 

 managed farm, and this is partly done by allowing 

 the fields to lie in grass. In Germany, it is con- 

 sidered that for every 100 acres of corn land, there 

 must, in order to effect a profitable cultivation, be 

 20 acres of pasture-land, which produce annually, on 

 an average;, 651 lbs. of hay. Now assuming that 

 the ashes of the excrements of the animals fed with 

 this hay amount to 6*82 per cent., then 376 lbs. of 

 the silicate of lime and phosphates of magnesia and 

 lime must be yielded by these excrements, and will 

 in a certain measure compensate for the loss whichi 

 the corn-land had sustained. 



The absolute loss in the salts of phosphoric acid,, 

 which are not again replaced, is spread over so great 

 an extent of surface, that it scarcely deserves to be 

 taken account of. But the loss of phosphates is 

 again replaced in the pastures by the ashes of the 

 wood used in our houses for fuel. 



We. could keep our fields in a constant state of 

 fertility by replacing every year as much as we re- 

 move from them in the form of produce ; but an in- 

 crease of fertility, and consequent increase of crop 

 can only be obtained when we add more to them 

 than we take away. It will be found, that of two 

 fields placed under conditions otherwise similar, the 

 one will be most fruitful upon which the plants are 

 enabled to appropriate more easily and in greater 

 16 



