[Read at the New Zealand Institute Science Congress, Dunedin, 30th January, 1926; received by Editor, 5th March, 1926; issued separately, 8th March, 1927.]
A Soil-Map is designed primarily to show the geographic position and extent of soil-types, howsoever they may be differentiated. The first requirement of a soil-survey of any country is a method of cartographically representing different soil-types. Various methods have been proposed for mapping the soils of New Zealand, but most of these methods have already been proved to be inadequate in other countries where soil-surveys are in progress. The reason for this is that certain factors which differentiate soils are left out of account.
Take some extreme cases as examples:—If soils be differentiated according to texture (1)* a clay, rich in iron, etc., derived from a basic volcanic rock would belong to the same soil-type as a clay, with low content of potassium, phosphorus, iron, etc., derived from a white claystone. If climate (2) is the primary consideration the shingle soils, the loessic down-soils, the limestone upland-soils, and the alluvial flats in the neighbourhood of Timaru would all fall into the same group. If plant assemblages (3) or the revenue-earning capacity of the land (4) be used as a basis, such a survey is hardly a soil-survey.
Grouping of soils according to the quantity of some particular substance they contain, e.g. the percentage of iron (5), or iodine (6), or sodium chloride (7), gives only partial results, useful for a particular purpose, but inadequate for most purposes.
It remains to suggest a method which will differentiate soils and which at the same time can be applied in mapping them. A method was outlined by me in 1915 (8), based on geologic-hydrologic considerations and since then other suggestions have been made (9).
In the study of the fauna or flora of a country the origin of that fauna or flora is one of the first points to be considered. In the study of the soils of a country, knowledge of their origin is of fundamental importance. Now all soils are derived from rocks, and may be either resting upon the rocks from which they are derived, or they may have been transported, mixed with other soils, and deposited upon another rock-formation. A geological map portrays these processes. The Geological Survey map of, say, Purua Survey District in North Auckland, or the map of Tiger Hill Survey District in Central Otago, is a ready-made basis for a more detailed soil-survey. Anyone who
[Footnote] * Figures in parenthesis refer to the list of literature appended.
knows either of these areas can see that these maps differentiate certain groups of soils, e.g. greywacke hill-country soils, basalt soils, ‘gum-land’ soils, in the case of North Auckland, or mica-schist tussock country, quartz-gravel terraces, loessic soils and alluvial flats (meadow-lands) in the case of Central Otago.
Each soil-group may be given a formational name such as is indicated. These names tend to group the soils geographically, texturally, chemically, and agriculturally. The topographical map which forms the basis of the geological map shows the situation and aspect of each soil-group or soil-type, and can be made to include climatic data and other desirable information.
If the practice obtaining in other countries be noted, it becomes manifest that a geological map provides the most useful basis for a soil survey of New Zealand.