DISCUSSION OF THE OBSERVATIONS MADE OF BANANA PLANT GROWTH AND THE CONCEPT OF "SOIL QUALITY"

Essentially the term "soil quality", which I have used for convenience, probably can be equated to the "limiting factor" of THOMASON & CRESWELL (1987).

The standard text-book on bananas (STOVER & SIMMONDS, 1987, or the earlier editions by SIMMONDS, 1959 and 1966) contains relatively little information on plant growth and limiting factors. Perhaps this is because the emphasis is on the crop as it is grown on commercial plantations, where the level of crop management has reached a very high level of sophistication. Correspondingly, the findings which have been published are on commercial varieties, notably "Dwarf Cavendish" (summarised by SIMMONDS, 1959), "Gros Michel" (HASSELO, 1962), and "Valery" and "Grand Nain" (summarised by STOVER & SIMMONDS, 1987). Additionally, the principal factors which have been evaluated are temperature, water supply and intermat (inter-stool) competition. STOVER & SIMMONDS (1987) comment on mineral nutrition being a factor that remains to be coupled to simulation models of plant development.

The results obtained in the Kagera studies, therefore, appear to be the first on the upland AAA types and, perhaps, the first on bananas grown by subsistence farmers. In Kagera Region the mean temperature varies very little right through the year and rainfall, although showing some seasonality, also does not vary sufficiently to affect growth rate in the areas where most banana shambas are located. Inter-stool competition also probably is not of great significance, except where farmers do not adhere to the traditional spacing of stools some 3 m apart, or do not keep the traditional practice of thinning out of suckers.

TABLE 10, although giving very limited results, shows that, as in the field surveys and the pre-trial site examinations, there is no evidence to support the banana weevil having any great effect on yields. The data on nematode infestation permits only limited conclusions to be reached; but at Kalonge the toppling of a mature bunch-bearing plant in treatment F was a total loss due to nematode destruction of the roots, which can be compared with the sterile plant from treatment C2 (repeated application of carbofuran) which had completely clean roots and was of a similar size to the toppled plant.

With the evidence available by May 1990, the overwhelming factor determining successful establishment and growth of banana stools appears to be the availability of nutrients. The consistently better growth exhibited in the high manure treatments suggests that nitrogen of organic origin is the key factor. When examined more critically this, however, seems not to be the whole answer.

The two factorial trials showed that at an impoverished site, such as Kibeta, a relatively limited addition of macronutrients (N, P or K), of organic or inorganic origin, would enable plants to attain maturity; albeit more slowly than when high quantities of organic nutrients were supplied. The apparently good pre-trial state of the soil at Nyakato allowed the stools with no added nutrients to produce plants which grew as well as the low manure treatments. What high manuring did do at both sites was to give much greater production of suckers. The essential routine of thinning out, or pruning, of excess suckers might seem to negate the merits of the greater production but a good number of vigorous suckers emerging in the earlier stages of crop establishment enables a much earlier and better pattern of fruiting.

The results from the simpler demonstration and evaluation trials equally showed this inter-site difference, with plants growing more vigorously and attaining maturity sooner in high manure treatments. The actual height at which maturity, that is flowering, was attained, however, seems to have been strongly site-specific.

Flowering at Izimbya and Rubale, sites only some 10 km apart, commenced at almost identical times (from week 55-56) but the height of plants flowering at Izimbya was around 370-390 cm whereas at Rubale it was 330-340cm. Without sound rainfall records it can only be speculated that this may have been due to the more severe rainshadow at Rubale.

At Kalonge and Kalema the flowering commenced at weeks 58-59, but with heights of around 350-370 cm and 310-330 cm respectively. Repeated applications of carbofuran appeared to be of greatest significance at the sites with the least vigorous all-round growth (Kalema and Ibaraizibu).

Taking all the sites for which results have been given, a grouping based on growth rates can be deduced as follows:

• High manure and high soil quality, with growth around 6.10 cm per week.
• Low manure and high soil quality, with growth around 5.45 cm per week.
• High manure and low soil quality, growth 5.00 cm per week.
• Low manure and low soil quality; or high manure and very low soil quality, growth 4.23 cm per week.
• Low manure and very low soil quality; or zero manure and low soil quality, growth 3.17 cm per week.

Clearly, the determination of growth and attainment of maturity of the first pseudostem is due to a combination of factors and it would be over-simplistic to assert that the level of nutrients is all-important and the addition of organic manure is all that is required to ensure a good crop. Caution has to be expressed concerning the potentially devastating effects of root nematode infestation, as exhibited at Kalonge.

In view of the foregoing evidence that plant nutrition perhaps is the key factor in banana growth in Kagera, it is unfortunate that very little information was readily available from elsewhere to help interpret the results. HASSELO (1962) working with "Gros Michel" in the Cameroons, found a high correlation between pseudostem circumference and bunch weight. If true in Kagera this gives an indicator for the final prospects for the studies reported here. What is of particular interest in Hasselo's findings, however, is that certain factors did appear to lead to low correlation coefficients; for instance, corm damage attributed to borers and leaf damage due to Sigatoka disease, plus certain soil fertility deficiencies. He concluded that such adverse factors might affect bunch weight more than circumference. One has to assume that this would reflect different nutritional demands for the full growth of the different plant structures. Recently, the research by Daniel Rukazambuga, see Postscript Research has confirmed some of my findings.

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