SOIL REACTION 409 



organisms, if insufficient, must be aided by the addition of 

 natural or artificial fertilisers. The complexity of even a 

 single factor such as this is evident when it is remembered that 

 the very poorest of soils apparently contains sufficient nutrient 

 material for many hundreds of crops, and the rate at which the 

 potential foodstuff is made available for the plant is apparently 

 of more importance than the actual amount of nutrient material 

 present. The micro-organic population of the soil is also of 

 supreme importance, and not merely as food producers, although 

 the conditions governing the equilibrium between the various 

 genera and species form an almost untouched field of work. 

 The relation of the soil to air, water, and temperature form 

 another group of limiting factors of no less interest and 

 importance than the others. All these and many more are 

 intimately related and mutually dependent — the simple addition 

 of a few cwts. of soluble fertiliser means far more than a mere 

 trifling addition to the store of plant food in the soil, for such 

 an addition is followed by an alteration in many physical and 

 biological properties of the soil. 



Importance of Soil Reaction as a Factor in Soil 



Fertility 



Anything that producer an alteration in any of these 

 numerous and mutually dependent factors will have its effect 

 also on many of the others, and the complex system of equilibria 

 existing or tending to exist in the soil will be disturbed. In 

 particular the growth of soil organisms, as well as that of the 

 plant itself, is very sensitive to the reaction of the medium, and 

 it frequently happens that the presence or absence of a base 

 will act as a limiting factor in crop production not merely through 

 the effect of acidity or alkalinity on the plant itself, or on the 

 soil organisms, but on account also of the varying displacements 

 produced on all the factors that go to make up the complex 

 chain of soil equilibria. 



The question of soil reaction cannot therefore be over- 

 emphasised, and one aspect of it, viz. soil acidity or " sourness," 

 to use the farmer's phrase, has attracted attention from very 

 early times. The use of lime as oxide, hydroxide, or carbonate, 

 the last-named as chalk, limestone or marl, in correcting soil 

 acidity, or adjusting soil reaction, is among the oldest of agri- 

 cultural operations ; while of late years the study of soil acidity 

 has occupied an important place in agricultural research. It 

 cannot be said, however, that the enormous amount of work 

 done has either solved the practical problem involved or clarified 

 our ideas as to what exactly soil acidity is. This is illustrated 

 by the almost indiscriminate use of such terms as " soil acidity/' 

 27 



