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174 SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962
Some aspects
On many field surveys in the tropics in New Guinea our attention was
attracted by the rotation period, which apparently was bound up with several
traditional and ritual customs of the local people. Investigation (Reynders,
Neth. J. Agr. Sci,. Vol. 9 (1961) No. 1) made it clear that the rotation period,
or period of fallow, was adapted to the fertility level of the soil and to the time
it required for regeneration.
In a number of different regions investigations were made into the rotation
period, both in the field and from the picture of the regeneration patterns of
the vegetation visible on the aerial photographs. In our experience, when
dealing with a longer rotation period, the aerial photos give a better picture of
the stages as shown by height differences, than can be obtained in the field.
The investigations mentioned above, appear to agree (Reynders, Nova Gui
nea, (1962) No. 3).
The following example is given as an illustration (see aerial photograph
and the pedological analysis made of the same area). The picture shows a
region in New Guinea, south of the Star Mountains, near the confluence of the
rivers Digul and Iwur, at an elevation of 200 m above sea level. Terraces are
recognizable on the banks of the rivers. Between the rivers the spurs of a
mountainous area are found. There is no agriculture on the alluvial soils, with
the exception of the triangle in the lower course of the Ok Katem. Here,
younger and other deposits give better soils and gardens are made.
Shifting cultivation patterns can be seen on the slopes of the spurs. The
first agricultural conclusion which can be drawn is that the soils on the slopes
are much more fertile than those on the terraces. Field observations made it
clear that the latter are strongly leached podzols while the former are rather
young soils, due to the rejuvenation caused by erosion.
Two shifting cultivation areas are selected for analysis. Probably two families
or clans were cultivating the areas A and B at the time the photos were taken.
The dotted lines are the boundaries of the regeneration stages. The area C,
between A and B, shows a much older regeneration pattern. The surface area
of all the stages of regeneration, as far as they were recognizable, were meas
ured. The results, given in mm 2 , are presented in the first three columns of
Table I.
If there is neither increase nor decrease in the clan over a long period - and
this assumption is generally true in those regions - there must be a greatest
common divisor of the units, generally equal to the number of units cultivated
at the moment when the picture was taken, that is, stage one. In the fourth
column the ratios of the different stages to stage one are given.
If the average time of occupation of a garden is known, the total age of all
the stages can be calculated. In this case the average period of cultivation of
a plot is one year. In the last column the ages of the stages are given in years.
The result is that in both cases the clans cultivate the area for about 12 to 13
years. The total units of area C are equal to the total of A or B. If area C was
