252 GUANO. 



crops with guano, become at last, by the continued use 

 of this agent, frequently so drained of silicic acid and 

 potash, as to lose lor many years their original produc- 

 tiveness. At the same time 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 will at last inevitably come, and it will then be very 

 difficult to repair the damage. 



In 800 cwt. of farm-yard manure with which a hec- 

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

 receives (according to Voelker's analysis) the same 

 quantity 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 Ib. of the latter agent as in 50 Ibs. of farm- 

 yard manure. Guano, therefore, contains these ele- 

 ments in the most concentrated form, and permits the 

 application of them to certain parts of the field more 

 conveniently than by farm-yard manure, as is often ad- 

 vantageously done after putting in the seed. In many 

 places, guano is mixed with gypsum to reduce its over- 

 powerful action. The gypsum divides the guano par- 

 ticles 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 gypsum 

 decomposes the oxalate and the phosphate of ammonia 

 into sulphate of ammonia and phosphate and oxalate of 

 lime. 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 por- 

 tion only of the phosphoric acid is converted into this 

 state, and with the removal of the oxalic acid, ceases, 

 also, the beneficial influence w r hich the latter exercises 

 in promoting the diffusion of the phosphoric acid. 



It will, therefore, be found much more effective to 

 moisten the guano with water to which a little sulphuric 

 acid has been added, and to mix it, after twenty-four 

 hours, with" saw-dust, turf-dust, or mould, instead of 

 gypsum, and to strew this mixture over the surface of 

 the field. The rain water dissolves out the phosphat 



