264 GUANO. 



to lose for many years their original productiveness. At 

 the same titae it cannot be denied that there may be 

 many soils which, for several years, by the aid of guano 

 alone, might be made to produce high cereal crops before 

 this state of exhaustion appears ; but it "vvill at last inevi- 

 tably come, and it will then be very difficult to repair the 

 damage. 



In 800 cwt. of farm-yard manure witJi which a hectare 

 of land is manured in a rotation of crops, the soil 

 receives (according to Voelker's analysis) the same quan- 

 tity of phosphates and of nitrogen as in 800 kilo- 

 grammes (15-7 cwt.) of guano ; in other words, there is 

 as much of these two elements of food for plants con- 

 tained in 1 lb. of the latter agent as in 50 lbs. of farm- 

 yard manm^e. Guano, therefore, contains these elements 

 in the most concentrated form, and permits the apph- 

 cation of them to certain parts of the field more 

 conveniently than by farm-yard manure, as is often 

 advantageously done after putting in the seed. In 

 many places, guano is mixed with gypsum to reduce 

 its over-powerful action. The gj^sum divides the guano 

 particles and causes them to be more equally distributed 

 over the field ; but there is no real diminution of the 

 chemical action of the ammoniacal salts ; the g}^3sum 

 decomposes the oxalate and the phosph^ate of ammonia 

 into sulphate of ammonia, and phosphate and oxalate 

 of hme. The phosphate of lime formed in this way 

 is in a state of infinitely fine division, in which it is 

 most suitable for the roots of plants ; however, a small 

 portion only of the phosphoric acid is converted into 

 this state, and with the removal of the oxahc acid, ceases, 

 also, the beneficial influence which the latter exercises in 

 promoting the difilision of the phosphoric acid. 



It wiU, therefore, be found much more effective to 



