138 SOIL CONDITIONS AND PLANT GROWTH 



if by persistence in certain methods of management the organic matter 

 were reduced below a certain point. 



It is, however, essential that the organic matter should be properly 

 decomposed. Barren sandy wastes not infrequently contain 5 to lo or 

 even 1 5 per cent, of organic matter, but much of it is simply dried 

 bracken or other vegetation that has not broken down and has no value, 

 but rather the reverse, in improving the physical conditions. If the 

 drainage is bad a good deal of peat may form ; further, the water fills 

 up the soil, making its condition bad whatever its composition may be. 



The Amount of Calcium Carbonate. — In interpreting a mechanical 

 analysis it must be remembered, as shown on page 61, that i or 2 per 

 cent, of calcium carbonate may greatly modify the clay properties and 

 give a considerable degree of friability to a soil which otherwise would 

 be very intractable. When the percentage rises to much higher amounts 

 the soil becomes very chalky and the mechanical analysis loses its mean- 

 ing, just as when much organic matter is present. 



Water-supply and the Interpretation of Mechanical Analysis. 



It has already been pointed out that a mechanical analysis can 

 be interpreted and discussed with any degree of completeness only in 

 terms of the water-supply ; the rainfall, the coolness of the climate, the 

 presence of moving underground water, and the nature of the subsoil all 

 have to be taken into account. 



Effect of the Rainfall. — The effect of a high rainfall is to bring into 

 prominence the " sticky " properties of the fine ^fractions, and to put 

 into the background their water-holding capacity. Thus a light soil 

 under a high rainfall behaves like a heavier soil under a low rainfall ; it 

 is as well supplied with water and on the whole behaves in the same 

 kind of way on cultivation. For example, the Stedham soil (Table 

 LIV.) is rather lighter than the Swanley soil, and yet in virtue of its 

 extra rainfall is more useful for farming purposes ; indeed the Swanley 

 soil is essentially a market-garden soil, requiring large dressings of dung 

 for successful cultivation. The North Chapel soil is physically as good 

 as the East Farleigh soil but agriculturally much inferior ; owing to the 

 higher rainfall it becomes somewhat too sticky to cultivate profitably 

 and so is in rather poor grass ; the East Farleigh soil, on the other hand, 

 is from a highly fertile hop garden. 



Coolness of Climate. — Soils containing so much coarse sand or fine 

 sand that they would scorch or burn in a dry warm district may prove 

 very suitable for cultivation in a cooler district where evaporation is 

 lessened. Potato soils afford some good illustrations ; potatoes require a 



