Scientific Agriculture. 



232 



[March, 1912. 



venient in that it also indicates the lines 

 of cleavage in the discussions that have 

 centred round the hypotheses now 

 under discussion. 



All these factors are of equal import- 

 ance. A deficiency of any one of them 

 sets a limit to the effectiveness of the 

 others and consequently to the crop 

 growth. Thus, no matter how much 

 food may be supplied, the plant cannot 

 make a corresponding growth if the 

 water supply be insufficient or the 

 temperature too low. It is therefore 

 fatal to our subject to take too narrow 

 a view of the inter-relationship of the 

 plant and the soil ; however necessary 

 it may be for purposes of investigation 

 to concentrate attention on one special 

 factcr or group of factors, any division 

 along conventional biological, chemical 

 or physical lines is almost certain, 

 sooner or later, to lead to disaster, be- 

 cause of the risk of overlooking some 

 other factor, outside the purview of the 

 experimenter that is limiting the effect 

 of the particular factors under investig- 

 ation. 



Some nine or ten elements of value as 

 nutritive materials, including nitrogen, 

 phosphorus, potassium, sodium, cal- 

 cium, 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 eoil : 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 mois- 

 ture not further reducible by drainage 

 but only by evaporation depends on the 

 size of the soil particles, their arrange- 

 ment and the amount of organic matter 

 present. As illustrations the following 

 figures show the lowest amounts of 

 water found in certain soils periodically 

 examined by the writer :— 



Lowest amount of Water found in certain 

 Soils after periods of Drought per 

 cent, by Weight. 

 Clay soil, 

 Rothamsted. 

 No Organic Manures Annual Dressing of 

 supplied. Farm-yard Manure. 



11-9 18-0 



Sandy Soil, Heavy Clay 



Millbrook. Soil, Woburn. 



7-6 20'0 

 The air relationships are bound up 

 with the water content of the soil, Only 

 a limited amount of space is available in 

 the soil for air and water. In some of 

 the above cases the figures are : 



Volume available for Air and Water in 

 100 Volumes of Soil in 



Rothamsted. Rothamsted . 

 No organic Farmyard 

 Manure. Manure. 

 Total space unoccupied 



by soil particles ... 84 38 

 Minimum of water al- 

 ways, present 23 30 



Difference, being maxi- 

 mum volume of air 

 that can be present 11 8 



The volume of air present in the soil 

 fluctuates from the maximum figures 

 given above to the very small volume 

 found during and shortly after heavy 

 rain. Oxygen is continuously being ab- 

 sorbed and carbon!,dioxide evolved, but 

 usually atmospheric oxygen diffuses 

 sufficiently rapidly into the soil to main- 

 tain there a partial pressure approxi- 

 mately equal to that outside. Thus the 

 difference between the soil air and atmo- 

 spheric air is that the former contains a 

 few more volumes of carbon dioxide in 

 10,000 parts than the latter. 



Injurious and inhibiting factors are 

 of various kinds and form a highly vague 

 group, but some of them must be put out 

 of action by calcium carbonate, because 

 of the striking effect it has on soil fertil- 

 ity. Indeed from the vegetation stand- 

 point, soils can be divided into two main 



