Wild. — A Soil Survey of New Zealand. 487 



Such subdivisions are to be called " subdistricts." It must be emphasized, 

 however, that such subdistricts are not to be recognized unless their soils 

 possess unique agricultural properties, and unless such properties are directly 

 due to geological factors. 



Soil formation : The unit in our proposed scheme is the soil formation. 

 This term is universally employed, though not by any means always in the 

 same sense. We propose the following definition : A " soil formation " is 

 a geographically continuous area covered by a soil uniform throughout as 

 regards mode of origin, climatic conditions, texture, profile, and composition, 

 and therefore as regards all agricultural properties. By " profile" is meant 

 the soil section down to a depth of, say, 4 ft. 



If one held rigidly to this definition a soil formation of any considerable 

 size would not be found : every farmer will declare that he has two or 

 three kinds of soils on his farm. Nevertheless, areas of some magnitude, 

 thirty to forty square miles and upwards, do exist in which the exceptional 

 parts are relatively so small as to be negligible, and such areas can con- 

 veniently be studied as units. 



Soil series : A " soil series " is defined as a group of separate formations 

 alike in the aggregate of their agricultural properties. The formations need 

 not necessarily be identical in every respect, but if they agree so far that 

 their agricultural properties and potentialities are the same they are put into 

 the same soil series. To give an illustration : The Lincoln formation 

 differs in some respects from the Methven formation — their subsoils, for 

 instance, are not alike ; that at Lincoln is a sandy clay, while that at 

 Methven is more porous. But the smaller capacity of the Methven 

 soils to retain water is compensated by a rather greater rainfall, so that 

 agricultural practice on each formation is very similar. These formations 

 are therefore put (provisionally at least) in the same series. 



Fades: It has been said that a formation will usually include small 

 areas with exceptional characters. These are not likely to be delineated 

 or described until the individual formations come to be studied in detail, 

 and when this stage is reached the term " facies " may be appropriately 

 applied to them. Thus the typical soil of the Lincoln formation is a clay 

 loam on a deep sandy-clay subsoil ; but near Lincoln College there is a 

 strip of gravelly soil in a shallow sandy-clay subsoil, on gravel beds. This 

 exceptional area is more or less clearly marked, and constitutes a " facies." 



Nomenclature. 



The soil districts may conveniently be described in geographical 

 language, thus : The Canterbury Plains district, the North Otago district, 

 the Wellington East district, and so on. The series-names obviously 

 should not be place-names, as the idea of a series is simply a set 

 of soil formations of like agricultural properties. It is suggested, there- 

 fore, that the most suitable form of name is one indicative of the more 

 important characters common to the formations constituting the series — 

 as the Loam-on-clay series ; the Alluvial series ; the Scrub-lands series ; 

 the Downs series. The formation, being a geographical entity, is best 

 named after some local place name. This need not be the most important 

 place politically, but should be that place where the soil is being studied 

 by means of continuous field experiments or otherwise. Thus the experi- 

 mental farms should give their names to the local forma ion — as, e.g., the 

 Weraroa formation, the Ruakura formation. 



