SOIL EROSION PROBLEM

Press, Volume XCIV, Issue 28158, 22 December 1956, Page 9

SOIL EROSION PROBLEM

“DIMLY SEEN AND LITTLE DONE”

CONSERVATION OFFICER’S ADDRESS At least three overseas visitors have lately directed attention to the seriousness of soil erosion in New Zealand, and in particular in the South Island high country, and renewed calls have been made for the setting up of a research station in this region. A local soil conservation expert, Mr R. D. Dick, soil conservation officer to the North Canterbury Catchment Board, has also recently been talking publicly about his work. He gave a paper at the second conference of the New Zealand Society of Soil Science which met at Canterbury Agricultural College, Lincoln.

“Soil erosion has been going on since the first herdsman over-grazed grassland and most obviously since the development of the traction plough in Egypt about 1500 8.C.,” he said.

“Over-grazing, fire and cultivation have impaired the stabilising cover of vegetation and this has led to soil erosion. Soil erosion was not recognised and that is understandable when in New Zealand in 1956 soil erosion is dimly seen and little is done to arrest it.

“On the Canterbury plains the problem of overcoming wind erosion has been greatly reduced by the planting of belts of trees.” Mr Dick said. In his paper he described various forms of erosion and the nature of soil conservation.

! “Tree planting is a continuing task and improvements can be made in the siting of new belts and the tree species used. The system of crop rotations has superseded the intensive cereal growing days of 70 and 80 years ago. and this has led to some permanency in the present system of agriculture. Drainage of the wet lands has proceeded rapidly during the last decade, but the most serious current problem is water deficiency for periods during the growing season of pastures and crops on the lighter soils of the plains. The major soil conservation problem on the Canterbury Plains is ‘dry land’ farming on the extensive area of comparatively light porous soils where, incidentally, the system of rotation includes a period of fallow that prepares and exposes the soil to wind erosion. “The hill and mountain country □resents primarily a problem of water control. The raindrop falling on bare ground in this sloping countrv not onlv leads to the transportation of | soil but also to serious flooding on the lower country. Underground water supply, including artesian water, which is used for human consumption and industrial purooses, is dependent on the course of the raindrop. Rapid run-off from bare soil can lead to a thi’-'dv Christchurch city. “The control of water may be attempted by two methods, both of which are complementary, and the use of one method without the other is both unwise and incurs wastage of human effort and money. The first method is by using the soil protective properties of vegetation and the second is by building engineering structures The almost exclusive use of the latter method in New Zealand to deal with floods has led to disappointing rewards and recurring increased expenditure of public moneys.

Stony Creek Catchment “In North Canterbury the Stony Creek became a problem flood stream after rains. The farming system had led to soil erosion and a reduction in fertility. Fortunately in the thirties a local committee caused the Government of the day to buy the land and plant it in trees. An engineering scheme comprising bank protection and groynes was designed to nrevent flooding, but because the catchment had been planted in trees the engineering estimate of cost was reduced by over one-third and the maintenance costs, which had been estimated at about 2 per cent. -of the original capital cost, have in reality been practically nil. “The vegetational treatment of Stony Creek catchment has reduced the peaks of floods and shingle movement in the stream to such an extent that less costly engineering structures can adequately deal with the waters. This is a clear example of the benefits of a combination of soil conservation measures and engineering works in the permanent control of a river

“Over the last 12 years the expenditure of taxpayers’ money on river control works has amounted to approximately £8 000.000. In this past year about £1.500.000 of public money has been spent on river control work out of which considerably less than £lOO.OOO has been spent on soil conservation work. These figures are indicative of the imbalance between the two complementarv methods of river control. Nevertheless, public opinion is awakening to this and it is being realised that more emphasis must be civen to both methods of river control in the upper catchment of rivers and particularly to vegetation and watershed management.

Discussing various types of erosion Mr Dick said that sheet erosion was one of the commonest tyoes of soil erosion in New Zealand The removal by water of thin layers of topsoil, it was the most inconspicuous and insidious type of erosion. In Canterbury it was prevalent on the loessal soils of Banks Peninsula, on the stiff clay soils of the foothills and on the soils, of bigh silt content on the downs west of Timaru The beating action of raindrops combined with surface flow was tbe principal cause of sheet erosion. Action of Raindrops The action of raindroos was sufficiently important for some people to use a term “raindrop erosion” ... The raindrop, said Mr Dick, snlashed soil □articles into the air and on some bare soils it had been estimated that up to 100 tons of soil to the acre had been splashed into the air by heavy rains On sloping soils considerably more soil was splashed downhill than uphill. not only because the soil particles moved further but also because the 1 angle of impact caused the splash re- ■ action to be in a downhill direction.

i Rill erosion was the removal of soil I bv water in small but well defined | channels when there was a concentra- ! tion of overland water. Rills were i small enough to be easily obliterated by the ordinary methods of cultivation. This type of erosion was most serious where high intensity rains occurred on soils having high run-off □reducing characteristics and loose shallow topsoil. In Canterbury rill erosion was readily seen every year in the regions noted for sheer erosion, particularly on fallow land or on soils under final oreparation for the sowing of seed during the summer and early autumn seasons.

Gully erosion, which was limited m extent* in Canterbury, was the advanced stage of rill erosion. The gul lies could not be obliterated by the normal methods of cultivation. Discussing wind erosion. Mr Dick said said that the atmosphere had a tremendous capacity foi transporting soil, particularly those soil fractions less than 0.1 millimetre in diameter llt had been estimated that the potential carrying capacity of one cubic mile of atmosphere was up to 126,000

tons of soil. In 1937 at lowa as much as 2001 b of soil an acre had been deposited from a dust storm originating in Texas Panhandle. In 1859 a dust storm originated by a sirocco in the Saraha was reported to have deposited 86 tons of material to the square mile in Westphalia in Europe. “If some records were available of the soil deposited in the Pacific eastwards of the Canterbury plains during some of the dust storms that occurred during the extensive wheat farming days of the eighties and nineties they would show a considerable loss of topsoil,” Mr Dick said. Soil Creep Soil creep had been defined as the slow downward movement of superficial soil and rock debris, usually imperceptible except to observations of long duration. It progressed by a multitude of minute downslope movements of the individual soil particles or aggregates due to gravity. Frequent alternation of freeze and thraw produced soil creep. Where there was sufficient moisture and freeze needle ice was formed which lifted the overlying soil. The needle ice, which was often up to about three inches in □eight in the Broken river basin, of the Waimakariri catchment, melted at its base and threw the uplifted soil downslope. Earth flows such as slips or j- lumps were not common in Canterbury. They occurred on the foothills. Banks Peninsula an dthe downs when the soil became saturated with moisture and rains of intensity of six inches or more fell in 24 hours. Particular care had to be taken before spending large sums of public money in an effort to reduce or prevent mass movement by mechanical methods. Mr Dick said. “The first* requisite to conservation of the soil is to fit the crop—whethei it be trees, cultivated crops, or grazing plants—to the capabilities of the soil and the water available,’’ he continued “Equally important is the use of agronomic and engineering practices which, conjointly, will control and conserve water and counteract the erosive action of both wind and watei on the soil. In conjunction with these two basic principles there must be re turned to the soil organic matter, nitrogen and mineral plant nutrients which have been removed by the different methods of land use. Proper amounts of moisture for plant growth must be maintained through soil conservation measures, including irrigation, or drainage where it is needed and feasible. It is on these premises that so’’ conservation is planned and developed.”

In planning soil ■ conservation farming the first step was to make a land inventory. Land inventory mapping was the presentation of the physical 'actors which influenced the use and conservation of a parcel of land. The principal factors examined were the soil, climate, vegetation, slope, present land use, and soil erosion. The land inventory map was the basis for the land capability plan for a farm, or a ’’atchment. The land capability plan divided land into eight capability classes.