136 SCIENCE PROGRESS 



too narrow a view of the interrelationship of the plant and 

 the soil ; however necessary it may be for purposes of investiga- 

 tion to concentrate attention on one special factor or group 

 of factors, any division along conventional biological, chemical 

 or physical lines is almost certain, sooner or later, to lead to 

 disaster, because of the risk of overlooking some other factor, 

 outside the purview of the experimenter, that is limiting the 

 effect of the particular factors under investigation. 



Some nine or ten elements of value as nutritive materials, 

 including nitrogen, phosphorus, potassium, sodium, calcium, 

 magnesium, iron and others, are supplied to the plant by the 

 soil. Long experience has shown, however, that addition to 

 the soil of compounds of some of these elements, particularly 

 of nitrates, phosphates, potassium salts and calcium carbonate, 

 commonly leads to an increase of plant growth. Organic 

 matter in the form of plant residues, dung, etc., also increases 

 the crop. All the evidence so far available goes to show that 

 the plant food is taken up from the solution made by the 

 carbonic acid generated by micro-organisms or by plant roots. 

 Thus the soil moisture functions in two capacities : it is the 

 source of water for the plant ; it is also the medium through 

 which the food is absorbed. 



Numerous investigations of the soil water have therefore 

 been made. Its ultimate source of origin is the rain-water falling 

 on the soil : part is held by surface attraction to the particles 

 and part slowly percolates through the soil and runs away 

 into the subsoil ; the rest evaporates. A soil will never drain 

 itself dry but always contains a certain amount of moisture, 

 no matter how long the interval since the last rain. Indeed 

 considerations of surface actions would lead us to expect a 

 movement of water from the moister to the drier parts of the 

 soil or, more strictly, from the places where the moisture films 

 have a flat to those where they have a sharp curvature. How 

 far this capillary movement actually takes place has not been 

 determined. Evaporation is always going on and reducing 

 the thickness of the films but it diminishes in amount very 

 rapidly below the surface of the soil. The minimum amount 

 of soil moisture not further reducible by drainage but only by 

 evaporation depends on the size of the soil particles, their 

 arrangement and the amount of organic matter present. As 

 illustrations the following figures show the lowest amounts 



