64 PLANT PRODUCTS 



every soil exactly that treatment would be the ideal one, 

 but investigation on the soil itself will probably show what 

 is most required. Similar results have been obtained on 

 light soils by combined potash and phosphate fertilizers, 

 whilst on some soils wild white clover seed harrowed in has 

 produced the desired ejBfect. 



Owing to the colloids in a soil, it is difficult to filter a 

 soil extract through paper. A soil will, however, always 

 filter itself clear, since any sized particle will always find some 

 particles a little coarser than itself, the interstices between 

 which will always be smaller than itself. By fitting up a 

 funnel with a filter disc and cloth, to which is adapted a 

 long fall tube for suction purposes, and pouring the soil, 

 mixed with water, into the funnel, the first cloudy runnings 

 can be returned to the funnel and then a clear solution will 

 be obtained. Hence the finest colloids do not penetrate 

 very deeply into a soil. 



Specific Gravity. — The true specific gravity of a soil — 

 that is, the specific gravity of the particles of which the soil 

 is composed — is not in itself a matter of much practical 

 importance, though referred to in nearly all text-books. 

 The crude gravity — that is, the weight of a given volume of 

 soil, including air spaces — is, however, a distinctly useful 

 figure. Commonly this measure is expressed in pounds per 

 cubic foot. A sand will weigh no pounds per cubic foot 

 when dry, a good arable soil from 80 to go pounds, a heavy 

 clay 75 pounds. A soil containing very much decomposed 

 organic matter will weigh about 70 pounds, whilst a peaty 

 soil containing much fibrous organic matter will only weigh 

 from 30 to 50 pounds per cubic foot. The soil on Tree Field, 

 at Cockle Park, in its unimproved condition, weighs between 

 84 and 8y pounds per cubic foot, and, though a clay, contains 

 a few stones and a little organic matter. The apparent 

 heaviness of all the soils of the Cockle Park type is due 

 rather to utter lack of balance than to the strict physical 

 properties of the fundamental ingredients, a fact which is 

 borne out by the above figures, which would classify this 

 type of soil as having a much higher value than it has in its 



