THE AGRICULTURAL NEWS. 



January G, 1912. 



hydroxides and organic matter, which surrounds the 

 different particles. 



In regard to the interests of the practical agricul- 

 turist, the colour of a soil may serve as an indication of 

 the presence of some useful plant food constituent. 

 Soils which are light in colour and clayey in texture 

 generally contain greater amounts of potash than those 

 found in the other kinds: while as has been indicated 

 an abundance of (organic matter is usually ])resen: in 

 black soils. From a theoretical point of view, the 

 colour of soils is often useful as an indication of their 

 origin. It has been stated, from the practical f.spect, 

 that the capacity of a soil to absorb heat is indi- 

 cated by its colour, this, however, is not usually of 

 great importance in the tropics, where the soil is not 

 often subjected to cooling, to any great degree. 



A large amount of invest'.igatioii as to the causes 

 of colour in the soil has been made, and the results of 

 the earlier considerations of the subject are summar- 

 ized in the Bulletin mentioned, in the following way. 

 ' Black or blackish soils cif different shades owe their 

 colour to varying amounts of humic or organic sub- 

 stances formed by the decay, under oxidizing condi- 

 tions, of the plant and animal tissues left in the soil; 

 the presence of a considerable proportion of lime car- 

 bonate in the soil facilitates such decomposition and is 

 indicated by a pronounced black colour, lied and yel- 

 low soils owe their colour principally to a eoatincr of 

 ferric oxide, more or less hydrated, on the soil grains. 

 Two theories are extant. According to the first, the 

 ferric oxide is more highly hydrated in yellow soils 

 than in red soils. According to the second, the 

 difference in colour is due to the relative amount of 

 ferric oxide the soil contains, and there is no essential 

 difference in the hydration of the ferric oxide in red 

 and j'ellow soils, respectively.' 



The changes in the iron compounds present in the 

 .soil take place in several ways. Iron sulphides, in the 

 presence of moisture and oxygen, turn to sulphate, 

 which is dissolved, and the sulphuric acid, travelling more 

 quickly through the soil, leaves behind the ferric oxide 

 which is deposited as a coating on the particles. This 

 separation of iron from its salts may be demonstrated 

 readily by filtering solutions of them through thick- 

 nesses of soil. 



Further, the less oxidized iron compounds, known 

 as ferrous salts,-are much more soluble than the ferric 

 compounds, especially in water containing carbon 

 dioxide, and therefore iron is removed by the solution 

 of these. This remoral is aided by the presence of 



dissolved organic matter. It might be concluded that 

 these actions would result in the quick washing away 

 of the iron compounds in a soil: the ease with which 

 the ferrous compounds are oxidized, however, to the 

 almost insoluble ferric compounds causes deposition to 

 take place quickly where air is present, and the loss of 

 the iron compounds is thus prevented. 



The subject reijuires further consideration in view 

 of the circumstance that soluble organic matter, par- 

 ticularly where there is not free access for air, can 

 reduce ferric oxide to the ferrous compounds which, it 

 carbon dioxide is present in solution, are freely carried 

 away. An interesting demonstration of this may be 

 seen in reil soils, near a root or beneath a leaf, where 

 the particles have become whitened because of the 

 reduction of ferric oxide by the organic matter present 

 in such situations, and the subsequent removal of the 

 more soluble ferrous compounds. 



A review of the work done in connexion with the 

 subject and the results of more recent investigations 

 show that the coloiir of a hydrated ferric oxide is 

 partly at least determined by the amount of water 

 combined with it; generally speaking, however, the fact 

 of the difference in colour between red and yellow soils 

 arises from a difference in the thickness of the film of 

 material that colours the particles. Moreover, there 

 exists another indication that the colour of a soil is not 

 due alone to the amount of hydration of the iron oxides 

 l^resent in it, in the circumstance that, though this is 

 comparatively rare, a red soil may become yellow. This 

 can be demonstrated' practically by placing a bright 

 red soil in a long tube, covering it with an equal 

 depth of leaf mould, ar.d pouring distilled water 

 through the tube from day to day; the upper end of 

 the soil column becomes light-yellow in colour, .and 

 iron compounds are found in the water which has 

 passed through. 



The suggestion is made that red soils have resulted 

 from the weathering of rocks having a large content of 

 ferro-magnesian minerals; they may also be formed by 

 the weathering, <luring a long time, of soils that origi- 

 nally contained a smaller proportion of ferric oxide. 

 The source of yello\y soils is probably the acid rocks 

 which do not contain much iron oxide, but possess on 

 the other hand a large proportion of quartz: as has been 

 indicated, the weathering of these gives eventually 

 a red soil. 



The facts brought forward indicate sufHcientiy that 

 the colour of a soil depends mainly on the amount of 



