SOIL ANALYSIS AND ITS INTERPRETATION 329 



soil, and is indeed known to be an excellent bullock pasture. 

 The second is not so good ; it contains too much clay and 

 fine silt, and too little silt and coarse sand. It has no great 

 agricultural value. 



Factors Modifying the Interpretation of a Mechanical 



Analysis. 



The Amount of Organic Matter. — Organic matter at the 

 proper stage of decomposition has the effect of binding a loose 

 soil and lightening a heavy one ; thus it reduces the difference 

 between a light sand and a heavy clay, bringing them both 

 closer to the loams. When 10 to 15 per cent, of organic 

 matter is present it so impresses its properties on the soil that 

 the mechanical analysis loses much of its significance, and all 

 the analyst can do is to point out what the soil would become 

 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 10 or even 15 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 maybe. 



The Amount of Calcium. Carbonate. — In interpreting a 

 mechanical analysis it must be remembered, as shown on 

 p. 117, 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 

 meaning, 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 analy- 

 sis can be interpreted and discussed with any degree of 



