THE CLASSIFICATION OF VIRGIN LANDS OF THE NORTH ISLAND.

New Zealand Journal of Agriculture, Volume XXIII, Issue 5, 21 November 1921, Page 266

THE CLASSIFICATION OF VIRGIN LANDS OF THE NORTH ISLAND.

B. C. ASTON,

F.1.C., F.N.Z.lnst., Chemist to the Department

The classification of virgin lands in a new district with no settlement or accumulated experience to guide the surveyor is often a difficult matter, and one which may require aid from more than one science in determining what the probable agricultural value of any given area may be. The following brief article will endeavour to set forth in the. order of their relative importance the principles which should guide the soil-surveyor in arriving at a decision..

In the 1907 Annual Report of the Department, of Agriculture, page 100, the writer summarized the principles which in his opinion should guide those who might in the future undertake a flying soilsurvey of New Zealand. These remarks applied to the whole country, and the work was classified into a consideration of (1) climatic conditions, (2) conditions of surface, (3) geological origin and relation, (4) -physical and chemical characters, (5) flora, and (6) recognized economic adaptations. . The present paper is restricted to the classification of virgin lands in the North Island, and by confining the consideration to this area it becomes easier to generalize. In the North Island the climate is more equable, there being no arid or semi-arid districts like those of Central Otago and Marlborough, and no large extent of abnormally moist areas such as those of the Fiordland and of Westland, while in temperature there are not the great extremes found in the South Island. The soils are generally derived from the old sedimentary rocks (poor in lime), similar to those forming the Southern Alps, the main exceptions being the papa ” and other sedimentary rocks (rich in lime), the eruptives of the Central' Volcanic Plateau, and those of Mount Egmont and the Auckland Isthmus and Peninsula. Compared with the South Island there is little variation in the plant covering of most of the North Island. With the sinking into uniformity of the climatic and geological conditions, and to a large degree of the plant covering, there emerge in importance the conditions of surface and the texture and chemical composition of the soil. By dividing the North Island into districts this elimination can be further accentuated, and climate and geological origin diminished as factors or left out of account altogether.

The most important characters to be taken into consideration' in classifying lands are those which are included under the heading of “ Topography and position.” These deal with (1) the contour of the surface whether it is flat, undulating, hilly, or steep and broken- — and (2) the situation of the land, dealing with its position relative to the sun (latitude, . altitude, and aspect) and to the sea or great lakes, or, in the case of . arid country, to means of irrigating it (rivers, artesian supply, &c.). . - Next in order is a consideration of the composition of the soil itself (including the subsoil), which includes a knowledge of the mechanical

composition or the relative amount of the differently sized particles composing it, conveniently grouped in the descending order of their fineness under the names of clay, silt; sand, and gravel (wholly peaty soils, mechanically considered, stand in a class by themselves), and a knowledge of the chemical composition, which in most cases will resolve itself into a question of the relative amounts of available lime and ’ phosphates the soil contains. An accurate knowledge of the origin of the — i.e., from what, class of rock it has been derived, and whether it is a transported or sedentary soil (one formed in situ from the parent rock) —may be of value 'in. assisting the surveyor to diagnose the class to which any area of land belongs.

Excepting unstable dune-sands and some recent eruptive country, New Zealand lands in a state of nature support a more or less continuous plant covering of some kind, the quality and quantify of which is some indication of the quality of the soil. The quality of the plant covering is most largely dependent on the water-supply for its character, and hence a knowledge of the climate should be helpful to a surveyor in forming his opinion. The science of the soil is in its infancy, and it is possible that as a knowledge of soil micro-biology increases this branch of science may be of more value in differentiating types of soil than any of the other means which are not applicable in the field at present. However, this aspect must here pass unconsidered.

What has been said may now be summarized under the following headings : I. Topography. (i.) Contour : (a) Flat, (&) undulating, (c) hilly, (d) steep and broken. ' (2.) Position: (a) Latitude, (6) altitude, and (c) aspect (relative to the sun), (d) distance from sea or great lakes (relative to great bodies of water). 11. Composition of Soil. ■ ■ (1.) Mechanical composition : (a) Clay, (6) silts, (c) sands, and (d) gravel (mineral or inorganic), (e) peat (organic). - , (2.) Chemical composition (a) Presence of available lime, (&■) amount of phosphates, potash, and nitrogen present. 111. Plant Covering {dependent mainly on Climate'). IV. Geological Origin and Mode of Formation.

TOPOGRAPHY.

With regard to “ Contour,” it will no doubt be true generally that these classes are set down in the descending order of their relative value, although occasionally it does happen that higher, steeper lands will be of greater value than the lower flats. This often occurs in recent volcanic country, such as that formed in the Volcanic Plateau of the North Island, an altogether abnormal area, concerning which little accurate information is available. In this article the North Island Volcanic Plateau and the soils derived or modified by the. volcanoes of that region must be treated as an exception to which some of the generalizations are inapplicable. Another exception which must be ranked with the pumice soils is that of the dune-sands. The pumice soils are highly abnormal, and, so far as the writer can learn, are unique in the history of agriculture. The dune-sands, however,

occur in many parts of the world, and the treatment necessary to bring them under profitable occupation has been well worked out in Europe. The pumice gravels or sands consist of highly porous, coarse rock-particles, loosely consolidated (having been deposited from the air), and have not had the advantage of having been sorted out and acted upon by running water, neither. have they the geological. age which would have enabled them to become weathered to a much finer state. Owing to the high and well-distributed rainfall these pumice soils support a natural vegetation, which may be either forest, heath, tussock-grassland, or fernland. The grazing-capacity of the improved land varies greatly, apparently being dependent .bn the fineness of the particles, the degree of consolidation, and the amount of humus which is present. Where such soils have had the advantage of being submerged or sorted out by lake -or river waters, or mixed with finer particles by more recent eruptions, or mixed with the abundant decaying vegetation of lakeside situations, the pasture which grows on these lands when (where necessary) the water is drained off is highly nutritious as food for ruminants.. Where, however, the soil occurs in some other situations uninfluenced by the benign agencies stated the luxuriant pasture will not support ruminants, which become unthrifty, and finally, in the course of months, if not removed, become affected with malnutrition disorder.

yrhe dune-sands are a comparatively unimportant portion of the North Island, and suffer from the same mechanical defectsbe., an excess of the larger particles of soils and a deficiency of organic matter —a character which renders it advisable that when a pasture is established on these soils its improvement should be effected by top-dress-ing rather than by tillage methods. The accumulation of humus which always takes place in grasslands should be encouraged on all undulating and hilly sand-land. Ploughing has the tendency to destroy the good which has been brought about by the accumulation of organic matter in the soil.

Steep and broken country is placed lowest in the series. An apparent exception is papa country of the right sort, which is often steep and broken, and on which slips are frequent. The pastoralist, however, views the slips with equanimity, as the pasture which speedily appears on the new scar is better than the original, owing probably to the larger amount of carbonate of lime which is to be found in the new soil. Certainly the growth of clover on the slips is often phenomenal.

In regard to “ Position,” owing to the longer period of active growth, land in a lower latitude should be more productive than in a higher latitude. Altitude must be similarly considered, land at a lower being classed better than land at a higher altitude. Aspect is sometimes of importance, the value of land with an inclination to the sun and which intercepts a maximum of the sun’s rays being higher than that of land which .receives less. Position with regard to the sea, and to a less degree to large bodies of fresh water, is important, as in such cases the temperature of the soil and air is more equable. Sea littoral pastures are usually better than those on the same rock and soil farther inland. This is possibly partly owing to the chemical influence of sea-spray ■or to the past influence of sea life (usually abundant on sea-coasts) in increasing the content of phosphates and carbonates of lime in the soil.

COMPOSITION OF SOIL.

A soil suffering from one of the more obvious defects of ’mechanical composition, such as an excess of sand or gravel, will be noticed in the field, but as a rule a mechanical analysis will be required before the class to which any soil belongs may be determined. The difficulty in surveying soils is not so much in recognizing types as in being able to determine the boundaries of those types which so often imperceptibly shade off at the margins in admixture with the adjoining types.' The value of mechanical analysis is more to correlate soils of similar composition than .to derive any positive information as to the excess or deficiency of a given fraction whose particles vary only within definite limits. British authorities agree that the properties of the finest fraction of soils—clay—are of great importance to the fertility of the soil, no constituent being more necessary in proper proportions or more harmful in excess. From Bto 16 per cent, is stated to be a satisfactory proportion in the soil where the rainfall is 20 in. to 30 in. a year. Hall (“ The Soil ”) states that good wheat-land or land that will form sound permanent pasture will contain at least 30 per cent, of silt and clay. “ Fine silt,” like clay, has a great water-holding power, and amounts above 10 to 15 per cent, are held to be excessive, increasing the difficulty of working the soil, especially if much clay is present. The stickiness of fine silty soils, not having the colloidal and plastic properties of true clay, is said to be less altered by lime. The fraction “ silt ” appears to be a very valuable one, judging by the large proportion in which it exists in many of the most fertile soils in England, 30 or 40 per cent, of which consists of this fraction. * The lighter soils, which contain only from 10 to 20 per cent, of silt, will probably require some treatment having for its object the increase of the organic matter of the soil.

Fine sand forms usually from 20 to 30 per cent, of nearly all soils, but coarse sand is perhaps the fraction most variable in soils, its properties having the reverse effect of clay, keeping the soil open and friable. An excess of coarse sand4o per cent, or moreor less than 5 per cent, of clay, unless the water-supply is exceptionally good, would render a soil hard to cultivate, owing' to the difficulty in retaining moisture. As the amount of coarse sand increases, this difficulty becomes greater until conditions are reached under which only specialized plants are capable of growing. Hence such soils are barren for .ordinary crops, special methods of treatment having to be adopted, such as growing lupins, in order to form a plant covering. Finally we have the gravelly soils, which in extreme cases, where unmixed with finer soil, are hopeless for anything except forestry. Some of the foregoing are opinions with regard to lands of the United Kingdom, and they are . possibly true only of countries of similar climate. What effect the different climate of New Zealand may have in modifying the above conclusions is yet to be decided. Peaty soils are invariably the result of swampy conditions, and usually require draining and some form .of after-treatment,, such as liming or burning, before they can be brought to their maximum productiveness. In some cases, however, in which the organic matter has become. mixed with mineral matter by flood-waters or air-currents in the vicinity of volcanoes, draining is the only treatment required to convert the swamps into high-class grazing-land.

The chemical composition of the soil is one which can be satisfactorily determined only in the laboratory. The presence of available lime when present as' carbonate could be detected, however, in the field by merely pouring some acid upon the soil, an effervescence, of course, indicating the presence of a carbonate, which is usually carbonate of lime. Some recently formed soils, however, contain calcium in a form which is not carbonate, but which yet dissolves in dilute hydrochloric acid without effervescence. The natural presence of carbonate of lime in the virgin surface soil will be a very rare occurrence in the North Island, being probably restricted to papa country and lands flooded by waters bearing papa silt. The classification of soils by means of their content of plant-food is tabulated in the following form. Maercker’s figures refer to the total amount of plant-food which may be dissolved out' with strong acids, and which may therefore be supposed to become ultimately available. B. Dyer's figures are obtained by a method intended to give some idea as to the amounts-of plant-food which are immediately available.

Classification of Soils by Plant-food Percentages (Maercker’s Rating); cAi Phosphoric Acid. Nitrogen. Grade of Soil. Pr Cent . Per c s . Poor . . . . . . ... Below 0-05 Below 0-05 Medium ... .. ... 0-05 to o-xo 0-05 to o-io Normal .. .. .. o-xotoo-i5 o-iotoo-15 ■’ Good . . . . .. .. 0-15 to 0-25 0-15 to 0-25 Rich . . : . . . . . Above 0-25 Above 0-25 Rating adopted for Available Plant-food, as determined by B. Dyer’s Method. r- c c -i Potash. . Phosphoric Acid. Grade of Soil. Per Cent . f e Cent . • Deficient .. .. .. Below 0-005 Below o-oi Normal .. . . .. 0-005 to o-ox o-oi to 0-015 Good . . ... . . . . Above o-oi Above 0-015