364 



[November, 1912. 



SOILS IN RELATION TO GEOLOGY AND 



CLIMATE. 



This is the title of a bulletin issued by the New South Wales Depart- 

 ment of Agriculture. 



Determinations arrived at are the result of research made since 

 April, 1909, by Dr. Jensen under the direction of Mr. F. B. Guthrie of the 

 Chemical Bureau of the Department. 



The object in view was to prepare a soil map, and to this end the 

 investigators used the Geological map of the State as a basis. The 

 analysis of the soil with a view to ascertain the soluble ingredients of 

 plant food was made according to the Hydrochloric acid method. Over 

 2,300 soil samples were examined. 



It would appear that ordinary soil analyses are often quite mislead- 

 ing to the farmer, though they are a valuable aid to the expert, especi- 

 ally when the Geological origin of a soil is known and the depth of the 

 soil, climate and topography are taken into consideration. A knowledge 

 of Geology and Mineralogy thus frequently enables a man to give as 

 valuable an opinion after a short field inspection as after a complete 

 tedious soil analysis. Every type of soil depends on three factors — 

 Geological formation, climate and topography ; i.e., the rocks from which 

 the soils are mainly formed, the rainfall, distance from the sea, aspect 

 of land, whether open or protected, and so on. 



Geological Structure and Chemical Composition. 



The result of investigations made in New South Wales clearly indi- 

 cates this relation between geological structure and chemical composi- 

 tion modified by climate. 



Granite soils contain a good percentage of potash, particularly where 

 there is a minimum of leaching and an active evaporation of soil moisture. 

 In wet districts the surface soil is much lower in potash than the subsoil. 



Basalt soils are particularly rich chemically. Within costal areas 

 with rugged topography they are richest in organic nitrogen and poorest 

 (as the result of leaching) in mineral plant food. 



While the granite soils are of the character of light loams under all 

 climatic conditions, basalt soils vary from triable loam to heavy clay. 



Sandstone soils are mechanically, more or less, uniform, and though 

 slightly better as regards mineral plant food in some parts, are of a 

 very poor description and are only workable under special treatment 

 such as dry-farming, irrigation and manuring. 



Limestone soils, though they exhibit mineral differences due to 

 climate, are rich in humus though containing less valuable matter than 

 is to be expected. 



In the case of alluvial soils though the effects of Geology are 

 somewhat obscured, they are not wholly clouded. The character of the 

 soil is governed by the nature of the rock which contributes the detritus, 

 that is, the formation through a river draining. The effect of 

 climate is more marked, inasmuch as the soils increase in mineral plant 

 food as their distance from the coast increases- this being due to decreased 

 leaching by rainfall. At the same time their moisture content and per- 

 Ventage of organic matter and aitrogen duninian,' 



