32 CHEMICAL MANURES. 



Thanks to this community of expression, we all appeared to agree, 

 when in reality we did not agree at all. 



Let us avoid this danger by being faithful to our programme. Let 

 us put words aside to go to the bottom of things, and draw our light 

 and information from experience. 



How and in what case does humus show a favorable action ? 



The first of its good effects come from the property it has like clay 

 of absorbing much water, and thus contributing to maintain the 

 humidity of the soil. 



If we remark, however, that earth contains hardly a small per cent, 

 of hunius, it is difficult to believe that so small a quantity has the 

 power of modifying the physical condition of the soil. 



Humus possesses a more useful property : it is quick at fixing the 

 ammonia in the soil, which it subtracts from the washing of the rains, 

 and which it returns later to vegetation. 



Its offices in this respect are analogous to those of clay. 



Here is where the importance of its office begins : the humus ab- 

 sorbs the oxygen of the air, and immediately submits it to- a slow, 

 inapparent, but real combustion. It thus becomes for the soil the 

 source of a slow but uninterrupted formation of carbonic acid, less 

 useful by the carbon which it furnishes than by the dissolvent action 

 it exercises in respect to certain minerals, and particularly the phos- 

 phates and chalks. 



We will, if needed, find the proof of this in a very simple experi- 

 ment : Begin two cultures in burnt sand one with the aid of humus 

 and the other without this body, both having received equal quantities 

 of chemical manures. In the two cases the returns will be the same, 

 but analysis will show more phosphate of lime in the yield from the 

 sand containing humus than from that without it. 



Humus can, in certain cases, produce a more useful effect ; it can, 

 in a certain measure, increase the return : this effect takes place when 

 the humus is associated with carbonate of lime. 



To prove it let us make four new experiments. 



Institute a culture in burnt sand, the soil being provided with 

 azotic matter and all the minerals proper to employ in this condition, 

 excepting the carbonate of lime. If we sow 22 grains we will gather 

 from 360 to 396 grains. Add the humus to the sand, the harvest 

 does not change. Substitute carbonate of lime for the humus, still 

 no change. Add the humus and carbonate of lime together, and the 

 return rises to 558 grains. 



These facts are of fundamental importance to practice. Permit 

 me, then, to sum them up in this little table : 



Nature of Soil. Returns. ^ 



1. Complete manure, burnt sand 396 grains. 



2. " " sand with lime 396 " 



3. " " " and humus 396 " 



4. " " and lime 1.3 oz., or 558 grains. 



The excess of th* ifletum obtained in this last case is due to the 



