48 



Essay on the Analysis of Soils* 



from stoneSj, should be accurately weighed ; and it is convenient to 

 take some determined quantity of grains, as 1000, 500, or 250, 

 according to the accuracy of the instruments at hand. This por- 

 tion should be put into a shallow earthen or metal vessel, and 

 heated over the fire, or a lamp for about ten minutes, stirring it 

 with a chip of dry wood : the heat should not be so great as to 

 discolour the wood. It may then be allowed to cool, and be 

 weighed again ; the loss of weight indicates the water which re- 

 mained uncombined after the soil appeared quite dry. This is the 

 first thing to be noted. 



The power of retaining water, without any external appearance 

 of moisture, is greatest in humus, next in clay, both of which readily 

 absorb it from the atmosphere ; carbonate of lime does so in a 

 less degree, and siliceous sand least of all. This moisture occu- 

 pies the pores of the soil, and is very different from the water 

 which is combined with clay as a part of its substance, and to 

 which it owes its ductility : for when this last is expelled by a 

 great heat the clay loses its quality, and approaches to the nature 

 of sand. Pounded brick will not bind with water ; and porcelain 

 reduced to a fine powder has all the properties of siliceous sand 

 in the soil. The finer the division of the particles of the soil, 

 the greater will be its power of absorbing and retaining water ; 

 but in a soil where clay greatly predominates the lumps some- 

 times become so hard and baked by the sun that the moisture 

 cannot penetrate, and in this case the power of absorption is 

 much diminished. Hence loams in which there is a good pro- 

 portion of humus have a greater power of absorption than the 

 pure earths. Taking all- circumstances into consideration, it 

 will be found that the soils which most readily absorb moisture 

 are also the most fertile, and therefore it is important to ascertain 

 their power of absorption. 



This can be found by comparison. Equal portions of different 

 soils, dried as before, are placed in the opposite scales of a good 

 balance, and left exposed for some time to a moist atmosphere. 

 That which preponderates has the greatest power of absorption ; 

 the degree is measured by the difference of the acquired weights. 



Another important circumstance is the specific gravity of a 

 soil. The different earths have very different specific gravities ; 

 and humus being lighter than any mineral earth, the lightness of 

 the soil is a sure indication of its richness, excepting where this 

 lightness is occasioned by an excess of undecomposed vegetable 

 matter, or peat. Humus, when nearly pure, has a specific 

 gravity varying from 1.2 to 1.5; fine porcelain clay, 2; chalk, 

 about 2.3 ; siliceous sand from 2.5 to 2 7: mixed soils have 

 specific gravities varying according to the proportions of their 

 component parts. Those in which clay, chalk, and humus 



