160 MANURES. 



than the adulteration of this material. It is best, 

 always, to stipulate that the manure shall contain a 

 certain percentage of soluble and insoluble phospho- 

 ric acid, and to withhold payment until an average 

 sample of the manure received has been tested by a 

 competent chemist. 



SILICIC ACID. 



Silicic acid (or sand) always exists in the soil in 

 sufficient quantities for the supply of food for plants ; 

 but not always in the proper condition. This subject 

 has been so often explained to the reader of this 

 book, that it is only necessary to repeat here, that 

 when the weakness of the straw or stalk of plants 

 grown on any soil indicates an inability in that soil 

 to supply the silicic acid required for strength, not 

 more sand should be added, but alkalies, to combine 

 with the sand already contained in it, and make 

 soluble silicates which are available to roots. 



Sand is often necessary to stiff clays, as a mechani- 

 cal manure, to loosen their texture and render them 

 easier of cultivation, and more favorable to the dis- 

 tribution of roots, and to the circulation of air and 

 water, and in this capacity it is often very important. 

 In my own practice I find it profitable to haul it 

 three miles to use on heavy clay land. 



NEUTRALS. 

 CHLORINE. 



Chlorine, a necessary constituent of plants, and 

 sometimes, though not usually, deficient in the soil, 



