South Sister St. Marys, Tasmania

South Sister

soil sampling & recommendation

[text of letter from Owen Ingles to SOS member recommending caution over logging on South Sister]

Owen Ingles Pty. Ltd.
Tasmania, 7275
17 January 2007

Dear Ross,

Re: South Sister Soil Profile

I have now examined the soil profile samples that you took at South Sister (GPS 0597789/5401063), and found as follows.

This is a Gradational Brown Forest Soil which Northcote would class Gn 4.3, but which I consider to be Gn 2.1; by the local DPIWE 2004 classification it is a Ferrosol (and indeed is highly feruginous, especially in the upper 300 mm level). As received on 30.11.06 in sealed containers, it would be a profile described as follows:

150 mm   dark brown, fibrous, friable
300 mm   red brown, fibrous, friable (higher than normal S.G.)
450 mm   brown-red, some fibres, friable, very fine
600 mm   brownish-red, fine, friable, a little small gravel
750 mm   brownish-red, fine, friable, some gravel

For purposes of grading test, two subsamples were made up: one of equal parts of the 150 and 300 mm samples, and the other of equal parts of the 450 and 600 mm samples. It was assumed that the dry, friable condition of these samples were as a result of the current severe drought. However, determination of the field moisture content was a major surprise, the 150/300 sample returning 39.5% dry soil basis (duplicates 39.3, 39.6), and the 450/600 sample returning 39.4% moisture content dsb. These are very high moisture contents for a seemingly dry soil, and suggest these samples are rich in either zeolite (not unusual for a dolerite or basal parent rock) or in hisingerite (an hydrated iron oxide found in some Victorian soils).

The friability of these samples despite a high moisture content permitted an unusual grading comparison to be made between sieving at field moisture content and with pure water (i.e. no dispersant, so as to simulate rainwater). The results are shown on the attached diagrams (150 & 300mm 450 & 600 mm). I have also marked on those diagrams the outer limits for highest erosion susceptibility (erosion at velocities less than 20 cm/sec). It can be seen that both the shallower and deeper samples are significantly less erodible at field moisture content than subjected to rainfall. And in neither case is there any appreciable clay content. The soil must necessarily be described as a SILTY GRAVELLY MEDIUM SAND.

The implications of the above findings are, in my opinion, as follows. Firstly, that this soil in its natural condition and vegetation state is of fairly low erodibility, and any clay that forms is not accumulated but washed out of the soil. If, hwever, the understory vegetation is removed and rain can access bare soil, this soil will rapidly erode, and must be classed as highly erodible. It is therefore a soil where clearfelling and understory removal should not be contemplated. Due to the high natural moisture content in the soil, ground cover will be maintained even in drought periods; but once removed subsequent drought breaking rains are likely to cause major soil loss both in quantity and fertility, especially on higher slopes (greater than about 12°).

Yours sincerely,

 

Owen G. Ingles
B.A., M.Sc., Ph.D., F.R.S.C., F.I.E.Aust., M.I.E., M.A.I.E.N., C.Chem., C.Eng., C.P.Eng.
Owen Ingles P/L,
Soil Engineering and Risk Management Consultants,
Swan Point, Tasmania

Default Colours Less Contrast More contrast

70206-4273 (1, 3, 9, 277)