210 



THE AGRICULTURAL NEWS. 



July 6, 1912. 



It has been known for a long time that the soil 

 supplies to the plant several elements that are of value 

 in nutrition, including among others nitrogen, potas- 

 sium, phosphorus, sudium, calcium, magnesium andiron; 

 there has also bee a the recognition that an increase 

 of plant growth is brought about when certain com- 

 pounds of these elements, notably nitrates, phosphates, 

 potassium salts and carbonates, are added to the soil; 

 further, that the crop may be increased by the use of 

 organic matter in the form of green dressings, composts 

 and natural manures. The results of all the observations 

 that have been made have gone to show that the food 

 bodies are taken up by the plant either after being 

 dissolved directly in the water in the soil or after being 

 made to dissolve through the presence, in the water, of 

 carbon dioxide that has been excreted by roots or 

 formed by bacteria. All that has been said demon- 

 strates sufficiently the supreme importance to the plant 

 of the soil moisture. It is the source of the water 

 required by plants, and the means by which the food 

 entering through the roots is absorbed. 



This is the reason why so much work has been 

 done for the purpose of learning as much as possible 

 concerning the water in soils. It must be considered, 

 in making the investigations, that four things happen 

 to the water that falls upon the soil: part of it clings 

 to the particles, through surface attraction; part travels 

 slowly downwards and escapes into the subsoil; part 

 evaporates; and where much rain falls in a short time, 

 a portion runs off the surface without entering the soil. 

 The water that clings to the particles is of the greatest 

 importance to plants. By surface action, it is supposed 

 to travel from the wetter to the drier places in the soil 

 though little is actually known about the matter; and 

 it is constantly reduced in amount through evaporation 

 and absorption into plants, but to an extent to which 

 there is a definite limit that its removal cannot exceed. 



It is easy to demonstrate that the spaces in the 

 soil are not only occupied by water, but to a greater 

 extent by air. Foi' the present purpose, however, much 

 consideration of this matter is not required. It may be 

 sufficient to realize that owing to the actions of plant 

 roots and micro-organisms, mentioned above, oxygen 

 is continually being taken from this air and carbon 

 dioxide put back in its place. New supplies of oxygen 

 flow in from the outer air, and the ultimate result is 

 that the air in the soil continually resembles ordinary 

 air in its composition, ixcept that it contains a little 

 more carbon dioxide. 



In iis relation to plants, the soil derives much of 

 its fitness as a medium for their growth from the 



extent to which it contains calcium carbonate. Where 

 the proportion of calcium carbonate is small or wanting, 

 the soil is usually called 'sour', and is realized as being 

 but poorly adapted to the raising of crops. It is neces- 

 sary, therefore, that much account should be taken of 

 the effect of the presence of this substance, when 

 investigations of the soil are being made. 



These matters have been brought forward in order 

 to indicate the circumstances of the soil that agricul- 

 tural workers are called upon to investigate. They 

 show that the conditions which go to make the soil 

 useful to the plant bear an intimate relation with one 

 another: they must all be served in sufficient degree. 

 If therefore, in an investigation, attention is given to 

 one or more ot them to the exclusion of the rest, useless 

 and mi.sleading conclusions will be reached. The view 

 of the experimenter must be broad, lest he attribute 

 results to the particular factors that he is investigating, 

 when they are actually due to the limiting effect of 

 some other factor that, in his restricted survey, he has 

 overlooked. 



Short refepnce to the hypotheses of the United 

 States Bureau of Soils has been made already. These 

 hypotheses depend on the recognition of the great 

 importance to plants of the soil moisture, and study of 

 this has led to most interesting conclusions, among 

 such conclusions being: that plants will grow in culture 

 solutions that vary greatly in strength; that variations 

 in the proportions of the nutrient salts in the solutions 

 result in a much more marked effect on the growth and 

 organization of plants; and that the strength of the 

 soil solution is practically the same in all soils. Of 

 these, the last conclusion is the most suggestive, from 

 the practical aspect, and requires consideration at 

 length. 



The supposed constancy in composition of the 

 soil moisture is accounted for by Whitney and Cameron 

 by the fact that most soils are dei'ived from a relatively 

 small number of minerals which are very similar in 

 composition, so that when they are dissolved into the 

 water in the soil, similar solutions are always formed. 

 They explain that the constitution of such solutions is 

 not affected by the addition of .artificial manures con- 

 taining definite chemical compounds, for any propor- 

 tion of the latter that may dissolve is balanced in the 

 solution by the removal of an eipial quantity of similar 

 matter that was present already. Further, the mineral 

 particles in the soil serve two purposes with respect to 

 this solution: they supply it with the substances that 

 are removed from it by plants, and they afford a large 



