582 SCIENCE PROGRESS 



underground water-table by capillary attraction. The soil 

 loses water by downward percolation into the water-table, 

 by evaporation from the surface, by transpiration through the 

 leaves of vegetation of the moisture taken up by plant roots, 

 and by the running off along the soil surface of part of a heavy 

 rainfall. The actual amount of water held by the soil at any 

 time depends, of course, on the type of soil — a clay soil holding 

 more than a sandy soil. Typical measurements of moisture 

 content for a heavy clay soil and a light sandy soil showed 

 that the former varied from 35 per cent, to 13 per cent., and the 

 latter from 14 per cent, to i per cent., according to season. 

 Soils of a loamy nature have intermediate values. The presence 

 of organic matter in the form of farmyard manure also increases 

 the water-holding capacity of the soil, and this additional 

 amount of moisture may be of vital importance in seasons of 

 drought. This is well illustrated by two plots growing mangolds 

 on the Rothamsted Experimental Farm, Both receive ample 

 plant food — one plot in the form of farmyard manure, the other 

 in the form of artificial fertilisers. In droughts the plants 

 on the former plot continue to grow owing to the extra moisture 

 in the soil, whereas the plants on the latter are seriously 

 affected. Actual measurements will show that there is still 

 some water in the soil of the latter plot, and the conclusion is 

 that the amount is insufficient, or unavailable for the plant 

 roots. From the beginning of agricultural science the question 

 has been discussed of how much soil moisture was available 

 for the plant, and it was well recognised that the texture and 

 structure of the soil were concerned in this matter. 



Early investigators treated the subject from the point of 

 view of water-films spread over sand-grains, and divided soil 

 moisture into three broad divisions. The best exposition of 

 these views was given eventually by Briggs in 1897. He 

 distinguished between : gravitational water — that which is in 

 excess of the amount normally held by the soil, and can drain 

 away under gravitational forces ; capillary water — that which 

 is retained by the soil under natural conditions and is capable 

 of movement through capillary action ; hygroscopic moisture 

 — the thin film in intimate contact with the surface of the 

 particles, and not capable of movement under capillary and 

 gravitational forces. Movement of the capillary water would 

 be controlled by the surface tension forces in the curved film 

 stretched over and around the soil-grains. A system of this 

 type is easily treated by the well-established physical laws 

 dealing with the surface tension of thin films, and Briggs 

 showed how the movement of water from places of high moisture 

 content to regions of lower content could take place under these 

 forces, whenever the moisture content was reduced at one 



