SILT DEPOSITS NOT LACKING IN FERTILITY ELEMENTS

Gisborne Herald, Volume LXXV, Issue 22665, 16 June 1948, Page 6

SILT DEPOSITS NOT LACKING IN FERTILITY ELEMENTS

131GGEST FLOOD IN N.Z. HISTORY

'[’HE settlers on the Gisborne flats whose properties were

covered with silt in the recent flood were fortunate in that they did not receive heavy deposits of sand as a result of the flood. The deposits left over an area of about 13 square miles of the most productive lands were not lack-

ing in most of the elements o: well to treatment.

fertility, and should respond

This view was expressed by Mr. W. I. J. Pohlen, of the Soil Survey Division, Scientific and Industrial Research Department, in a discussion with members of the P.B. Catchment Board yesterday.

If the situation had been the same as in Hawke’s Bay, with very extensive hill slipping, most of the 13 square miles covered by the floods of May 14 would have been heavily silted. Susceptibility to flooding must remain one of the major obstacles to development of agriculture on the Gisborne flats, he added.

Mr. Pohlen is in Gisborne in connection with a survey of the Gisborne flats, and in his remarks to the board intimated that in his opinion the Gisborne flood was probably the most extensive in the history of New Zealand. People in other parts of the Dominion, even as close as Hawke’s Bay, had not realised the magnitude of the disaster, he said.

©lt was obvious that only a small proportion of the most productive land had escaped inundation, and equally obvious that until control of flood hazards was established there must always be an obstacle to development. Drainage for Deep-root Crops

Drainage was particularly important for deep-rooting trees and crops, he pointed out, and unless drainage was good it was impossible to anticipate the development which the high fertility of the Gisborne flats seemed to warrant.

Soil surveys have been made of the whole of the North Island, on a scale of four miles to the inch, and the maps new prepared give a very clear picture of the soils of the whole island, said Mr. Pohlen.

In reply to Mr. E. A. Madden, Mr. Pohlen stated that an opinion on the recently-deposited silts would have to wait upon analysis, but from analyses made of previous silt deposits it appeared that they had reasonably high contents of phosphates, potash and lime. Nitrogen was the only element in which they seemed to be lacking. However, it was obvious that there would be a progressive deterioration in the quality of silt deposits in successive floods. The tendency was for the sands to move out of the riverbeds and spread over the flats, as erosion increased. This process would bring a progressive and serious deterioration in the quality of the land. Good Grass Cover Essential The fineness of the silt deposited in the recent flood would tend to prolong drainage difficulties on silted lands. That would apply to the balance of this year, at least, and while time would mellow the silt the effect might be felt next winter. It was important to get a good grass cover on the silt as quickly as possible, as it was the grass roots that set up a correction in the surface-soil structure, improving the drainage and reinstating normal conditions. Mr. A. D. Todd, engineer to the board, stated that measurements taken by the board’s staff had established that about 40,000,000 tons of silt had come down the Waipaoa River. The chairman of the board, Mr. J. R. Hair, asked Mr. Pohlen whether cultivation would not provide a quicker aeration of the silted land and a consequently quicker return to normal conditions? Mr. Pohlen remarked that that was a question for agricultural experts. His own observations applied to lands previously in grass, and only lightly covered by silt.

With regard to the hill country in the Gisborne district, the work was not well -advanced owing to shortage of qualified staff, but the Gisborne fiats were regarded as of especial importance in view of their fertility and the likelihood of a great advance in production here.

35 Soil Types Identified Already some work had been accomplished in establishing the main soil types, of which . abou t 35 had been identified.

The Tolaga Bay flats were also scheduled for early attention, said Mr. Pohlen, and he hoped to get on with that job after the Gisborne flats were completed.

Flood conditions produced difficult problems for the settlers, continued Mr. Pohlen, who complimented the officers of the board on the early production of charts and maps showing the extent of the flooding.

He, like most other people outside the Gisborne district, had had no idea of the magnitude of the innundation, until he had seen these records. A personal inspection of the flood area had enabled him to estimate that about 2500 acres below Te Karaka had been silted to a depth which wipea out the grass altogether. 8,000,000 Tons of Silt Laid

He understood that another 1200 or 1500 acres above Te Karaka was in the same condition. If they considered that the average depth of silt was only Ift. —probably an under-estimate —it was obvious that some 8,000,000 tons of silt had been deposited by the flood on that 4000 acres. Further, there had been lighter deposits over a very much greater area and the effect of this was to hold up drainage and must interfere with production at least until the spring. This was a significant factor in the situation which must be taken into account. © Siltation here was undoubtedly much heavier than in the recent Hawke s Bay floods, and, indeed, the Gisborne flood must be one of the greatest ever experienced in the history of New Zealand. Sterile Sands Not Dominant He had taken samples of the silt from different points, said Mr. Pohlen, and these would be analysed and the results publicised. He did not think the proportion of sterile sands deposited was significant, and in that respect the district appeared to have been lucWl'. They had not had the experience of the Esk Valley, for instance, where 3ft. to sft. of sand had been deposited on good loamy soil. The degree of slipping in the hillcountry was not extensive, said Mr. Pohlen, who was of opinion that the main sources of silt-burden m the fioodwaters were comparatively localised.