3.2 Quantitative description of fields
Crop mixtures and species numbers per unit area were surveyed following
the technique described by Mutsaers et al. [1981].
First-year fields were surveyed in October, about 6 months after cultivation
started and 3 months before the yam crop was harvested. At that
moment all crops were well-established, albeit in different stages of their
respective life cycles. In each of the fields visited a measuring tape of 50 m
was laid out diagonally across. Along the tape all crop plants in a corridor
2m wide were recorded. In each field surveyed strips of 100m 2 were thus
sampled in detail. The main advantage of this sampling procedure is that
one arrives at a higher degree of randomness in sampling site selection as
compared to choosing squares while moving through the field.
Based on literature studies for each crop a 'standard pure stand density'
was adopted which indicates the optimum density in sole-cropping (Table
1). Actual plant numbers in mixed cropping were expressed in terms of
relative densities. If for example 4,200 Xanthosoma sagittifolium plants are
found per hectare, while the standard pure stand density is 15,000 plants
ha -~ the relative density is 4,200(:)15,000 or 0.28.
As species in crop mixtures have different time-spans to reach maturity
and space actually occupied by the plant changes over time, relative densities
have to be adjusted for the stage of crop development.
For crops that complete their cycle within one year the adjustment factor
is 1.0. For Xanthosoma sagittifolium, a tuber-crop harvested the second year
an adjustment factor of 0.5 is adopted. The crop is planted just after the yam