AiKiNS — Factors affecting Hijdrogen Ion Concentration of Soil. 413 



6. Continuous manuring with sulpliate of ammonium, or of potassium, 

 decreases the effective soil alkalinity, even in a calcareous silt, but by a small 

 amount only, about pH 0'2-0'4 in cases examined. 



7. A.n acid soil extract is only slightly altered iDy boiling, from pH 5"4 

 to 5-7 in one instance. Alkaline extracts tend to reach the maximum value 

 for calcium carbonate, pH 9'0. Higher values, such as pH 9-2, appear to 

 indicate the presence of magnesium in small amount. The altered reaction 

 is probably of importance in inhibiting the growth of certain soil organisms 

 in heated soil. 



8. When arranged in order of decreasing alkalinity, soils derived from 

 different materials stand as follows : — Calcareous silt, limestone, sandstone, 

 calcareous tuff (schalstein) with slate, slate, pillow lava (spilite), felsite, and 

 granite. The values are modified in certain places, for proximity to the 

 coast lessens acidity, and a high gradient often increases it. The results 

 relate mainly to Devonian strata and accompanying volcanic or pkitonic 

 rocks. 



9. Eecords ai^e given for the hydrogen ion concentration of the habitats 

 of over a hundred native plants. These show that this measurement is a 

 valuable index of various soil conditions, and that many plants are limited 

 to a short range of pH values. Others, with a wider range, occur mainly in 

 one portion of it, but some plants grow well at widely different soil reactions. 

 A distinction must be made between acid peaty soils, acid clay soils, and 

 acid sandy soils. 



10. The cultivation of wild flowers in a garden is dependent for its success 

 upon the soil reaction of the natural habitats being maintained. 



11. Natural waters, even from the peaty districts examined, contain no 

 acid other than carbonic. Water in a spring may be at pH 64, and the 

 stream flowing from it at pH 8'3, when in equilibrium with the atmospheric 

 concentration of carbon dioxide. Photosynthesis may raise water containing 

 magnesium salts to pH. 9-7. The method of limiting ionic concentrations 

 may be applied to determine the amount of calcium carbonate in a soft water. 

 This method shows Dartmoor water (E. Yealm) to contain 0-004 gram per 

 litre of calcium carbonate in October. The hydrogen ion concentration of 

 natural soft waters tends to increase during winter and to decrease in 

 summer. 



