178
General remarks and recommendations.
It will be observed that the mechanical analyses of these soils have been carried out on those portions of the original soils which passed through a 2 mm, sieve, in accordance with the recently instituted International Method for Mechanical Analysis. The chemical analyses, on the other hand, were performed on the portions of the samples which passed through a 1 mm. sieve. This arose owing to much of the chemical work having been completed before the International Method for Mechanical Analysis was adopted here. It may be remarked, however, that there is not at present any official recommendation as to the size of the material which should be used for chemical analysis.
The percentages of material in the soils passing a 1 mm. sieve expressed in terms of the "fine earth" passing a 2 mm sieve are as follows:-
Material passing 1 mm. sieve, as percentage of that passing 2 mm. sieve.
Sample
E
ABOARKOI
В
D
G
H
90.75
94.29
97.88
92.95
93.44
95.57
95.22
91.15
It is probable that the material of diameter between 1 and 2 mm. (which would be very coarse sand) would be inert as regards plant nutrition, and hence the approximate chemical composition of the material passing a 2 mm. sieve may be calculated from the above figures, if it is desired to express the chemical and the mechanical analyses on the same basis. This would not affect the conclusions to be drawn from the results.
The soils, although not all of the same type, show no extreme variations in mechanical composition. They vary from clay loam to sandy loam and in no case is one or more of the separate fractions very low in amount.
With the exception of sample E, all the soils have an acid reaction and the acidity is higher in the top-soils than in the sub-soils; this is as would be expected, especially in the case of uncultivated soils. In the case of sample E, in which the top-soil is faintly alkaline while the sub-soil is slightly acid and the land has been used for the cultivation of vegetables, it seems most probable, as already mentioned, that the soil has recently been limed.
There appears to be a certain amount of correlation between the available bases i.e. those soluble in citric acid solution) and the exchangeable bases as determined by the Hissink method (leaching with ammonium chloride solution). As regards lime, the amounts found by these two methods are very similar, while the magnesia shows consider- able variation; the amounts of potash extracted are similar, the exchangeable' being very slightly higher on the whole, while the amounts of soda are fairly similar in most
cases.
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The ratio of carbon to nitrogen in the soils is usually 12 or 13, except in samples C and G, which would be regarded from other considerations as likely to be the least fertile of the series. A carbon to nitrogen ratio of about 12 appears to be normal.
The composition of the clay fractions separated from the various top-soils shows considerable variation. The clays from samples A, D and E are fairly similar, those from samples B and F are similar, and those from samples C and G are also somewhat similar. The clay from sample H does not show any close resemblance to the others. The silica to alumina ratio falls below 2 only in the clays from samples C and G, indicating a tendency in those samples to lateritisation, which is often associated with low fertility. The high percentage of alumina which is soluble in hydrochloric acid in samples C and G is note- worthy.