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African Crop Science Journal, Vol. 9. No. 3, pp. 499-506

EFFECT OF MULCHING CABBAGE WITH BANANA RESIDUES ON CABBAGE YIELD, SOIL NUTRIENT AND MOISTURE SUPPLY, SOIL BIOTA AND WEED BIOMASS

J. K. LEKASI, P. L. WOOMER¹, J.S. TENYWA² and M.A. BEKUNDA²

National Agricultural Research Centre, KARI-Muguga, P.O. Box 30148, Nairobi, Kenya
¹Department of Soil Science, University of Nairobi, P.O. Box 29053, Nairobi, Kenya
²Department of Soil Science, Makerere University, P.O. Box 7062, Kampala, Uganda

Received 14 January, 1998
Accepted 18 February, 2001

Code Number: cs01069

ABSTRACT

Banana (Musa spp.) residues were examined as organic inputs for cabbage (Brassica oleracea L.) production in Uganda with respect to yield and soil biological activities. Cabbage, cultivar Copenhagen, was cultivated on a Ferralsol of low inherent fertility, with (15 t ha^-1 dw) or without banana residues and hand-weeding. Two additional treatments included application of fertiliser (100 kg N, 50 kg P and 100 kg K ha^-1) with weeding and surface mulching with black polythene (no nutrient inputs). Rankings of cabbage yields by management practice were consistent in both seasons, following the order of fertilised + weeded > banana mulched + weeded > plastic mulched > weeded > banana mulched > unmulched + unweeded. Hand-weeding increased yields by 9.3 t ha^-1 (fresh weight). Combined banana mulching and hand-weeding resulted in an additional 12.5 t ha^-1, but this increase was 15.1 t ha^-1 less than that from application of fertiliser. Surface mulching with banana residues was not effective in weed suppression or moisture conservation but increased earthworm population densities. Banana pseudostems decayed more rapidly than leaves, and excluding soil macrofauna from both decaying tissues delayed decomposition. Plastic mulching increased cabbage yields by 14.9 t ha^-1 over the unweeded treatment and improved soil moisture status, but this material is not widely available to smallholder farmers in Uganda. Farmers who seek to improve vegetable production can utilise banana residues as nutrient inputs only in combination with effective weeding; although the nutrients applied through banana mulch may not be utilised effectively compared to chemical fertilisers.

Key Words: Musa spp., Brassica oleracea, earthworms, litter, Uganda

Résumé

Les résidus de banane (Musa spp.) ont été examinés comme intrants organiques pour la production des choux (Brassica oleracea L.) en Uganda en relation avec le rendement et les activités biologiques du sol. Le cultivar de chou Copenhagen, a été cultivé sur le Ferralsol de faible fertilité, avec (15 t ha^-1) ou sans résidus de banane et le sarclage à la main. Deux traitements additionnels comprenaient l'application des engrais (100kg N, 50 kg P et 100 kg K ha^-1) avec sarclage et paillage à la surface avec du polythène blanc (pas d'intrants d'élements nutritifs). L'ordre des rendements des choux par la pratique de gestion étaient consistant dans les deux saisons suivant l'ordre des fertilisants + sarclés> banane paillée + sarclés > plastique paillé >sarclés >banane paillée > non paillé >non sarclé. Le sarclage à la main a augmenté le rendement de 9.3 t ha^-1 ( poids frais). La combinaison de la banane paillée et le sarclage à la main a abouti à un rendement additionnel de 12.5 t ha^-1, mais cette augmentation était faible de 15.1 t ha^-1 par rapport à l'application des engrais. Le paillage à la surface avec les résidus de banane n'a pas été efficace dans la suppression des mauvaises herbes ou la conservation de l'humidité mais a augmenté la densité de la population des vers de terre. Les pseudotiges de banane se décomposaient rapidement plus que les feuilles, et en excluant les macrofaunes du sol de deux tissus décomposant ils ont retardé la décomposition. Le paillage par le plastique a augmenté les rendements des choux dans les traitements non sarclés et ont amélioré l'état d'humidité du sol, cependant ce materiel n'est pas largement disponible chez les agriculteurs en Uganda. Les agriculteurs qui veulent améliorer la production des légumes peuvent utiliser des résidus de banane comme sources d'élements nutritifs seulement en combinaison evec un sarclage efficace, mais les élements nutritifs appliqués par le biais du paillage de banane ne pourraient pas être utilisés effectivement en comparaison des engrais chimiques.

Mots Clés: Musa spp., Brassica oleracea, vers de terre, humus, Uganda

INTRODUCTION

By the year 1994, annual fertiliser use in Uganda was only 2,000 metric tonnes (FAO, 1995a) with fertilisers being largely provided through foreign aid (Gerner and Harris, 1993); this figure represents an average per capita fertiliser use of <0.1 kg y^-1 (calculated from FAO, 1995a; 1995b). Insufficient replacement of soil nutrients removed in harvests inevitably results in fertility decline (Smaling, 1993), which has been identified as the leading biophysical cause of food insecurity in sub-Saharan Africa (Sanchez et al., 1997). One manifestation of soil nutrient depletion in Uganda is matoke (banana) decline (Bekunda and Woomer, 1996), a widespread condition whereby banana yields have declined from greater than 50 kg bunch^-1to less than 10 kg bunch^-1 (Bananuka and Rubaihayo, 1994; Gold et al., 1999). Matoke decline is a complex syndrome comprising of nutrient deficiencies, insect and nematode pests and sigatoka (fungus) disease (Pseudocercospora musae, Zimm).

Banana residues, consisting of cut pseudostems and leaves, are arguably the most abundant farmer-available organic resource in Uganda. Based upon national banana production figures of 9.2 million t y^-1 (FAO, 1995a), plant partitioning information from Stover and Simmonds (1987), and assuming that residues consist of 40% leaves and 60% pseudostems, we estimate that an average 9.2 t ha^-1 of pseudostems and 6.1 t ha^-1 of leaves are available to Uganda's banana farmers per annum. Banana stalks and leaves are rich sources (and sinks) of plant nutrients, containing 1.0 and 2.8% N and 7.7 and 4.9% K, respectively. Banana residues in the Lake Victoria Basin are often, used as surface mulches for banana and associated intercrops (97% of households), as livestock feed (10%), and as a compost ingredient, yet one infrequently reported household practice (4%) is the application of banana residues as mulch in cash crop production systems (Bekunda and Woomer, 1996).

One such cash crop is cabbage (Brassica oleracea var. capitata), originally from temperate Europe but well suited to Uganda's tropical highlands (Purseglove, 1969). Leafy vegetables are intercropped by 3% of banana farmers (Bekunda and Woomer, 1996), and cabbage is particularly favoured for its high potential yields, high market demand and relative ease of storage and transport. Yet little is known concerning the comparative advantage derived from surface mulching of cabbage with banana residues. In this study, we attempted to partition the nutrient addition, weed control, moisture conservation and other soil benefits resulting from the use of banana mulch in the cultivation of cabbage, and the consequent effects on cabbage yield.

MATERIALS AND METHODS

The experiment was conducted at the Mukono District Farm Institute in Uganda (0^o'N, 32^o'E). The site receives bimodal (March-June, September-December) rainfall averaging 1,375 mm yr^-1, with average annual minimum and maximum temperatures of 15 and 27.5°C, respectively. The soil is classified as a Ferralsol with a sandy clay loam surface texture. Selected soil characteristics are presented in Table 1.

Six treatments were arranged in a randomised complete block design with four replicates. Treatments consisted of: (i) unweeded, (ii) weeded, (iii) banana-mulched and unweeded, (iv) banana-mulched and weeded, (v) black polythene mulched, and (iv) intensively managed cabbage plots. The intensively managed cabbage plots received 50 kg N ha^-1 as urea, 25 kg P ha^-1 as single superphosphate and 50 kg K ha^-1 as muriate of potash in broadcast application and periodic hand weeding. Plot size was 4 m x 6 m separated by 1 m alleys in all four directions. The second season crop was planted on the same plots but without further addition of mulch or fertiliser.

Banana residues consisting of leaves, pseudostems and thinned suckers were obtained from a nearby field. Prior to the first cropping season, the residues were chopped (10 to 15 cm in length), pseudostems were split lengthwise, and applied to selected treatments at a rate of 5 t ha^-1 dry weight.

Likewise, prior to the second crop, pseudostems were split but the remaining mulch was not chopped and these residues were applied at a rate of 10 t (dry weight) ha^-1. During the first season, a sub-sample of the mulch was analysed for plant nutrient contents following procedures described by Okalebo et al., 1993.

Prior to application of mulch, nutrient and carbon contents of the mulching materials were determined by the complete wet digestion method (Okalebo et al., 1993). Lignin was measured according to van Soest's (1963) method. The decomposition of banana leaves and pseudostems was characterised using litter bags of two different mesh sizes. Plastic litter bags (5 mm mesh, 30 cm x 30 cm) containing 50 g dry weight, the equivalent of 2 t ha^-1, were arranged in a randomised complete block design with four replicates adjacent to the cabbage mulching experiment. Crop residues were also placed into finer mesh stainless steel litter bags (2 mm mesh, 10 cm x 10 cm) that exclude soil macrofauna and were placed into the larger litter bags. The bags were deployed in the envelope configuration on the soil surface and recovered after 2, 4, 8, 16 and 32 weeks as described by Anderson and Ingram (1993). After recovery of the litter bags, the residues were repeatedly washed with tap-water to remove adhering soil particles. The residues were then dried at 72°C for 24 hr, weighed and ground to pass through a 2 mm sieve. A 1.0 g sub-sample was combusted in a porcelain crucible at 550°C for 2 hr and the residual mass expressed on an ash-free basis. The mass loss of litter was fitted to a first order exponential decline function (litter remaining/initial litter = exp^k*years). The decline coefficients (k) were obtained by non-linear regression and the values were compared by calculating time to 50% decomposition where t₅₀ = ln0.5/k, which serves to normalise data, and then subjected to analysis of variance (ANOVA) using Genstat computer software.

The field was tractor ploughed and harrowed one week prior to transplanting the cabbage seedlings. Cabbage was grown during each of the two rainy seasons of 1995. Cabbage seeds (cv. Copenhagen) were sown in wooden boxes containing soil from the experimental site previously passed through a 5 mm sieve, and were transplanted into 7 cm diameter plastic pots filled with site soils. After three weeks, the seedlings were selected for uniformity and transplanted at a spacing of 40 cm x 60 cm. Selected plots were hand-weeded using a hoe, as normally done by vegetable farmers in the study area.

The above-ground biomass of ten cabbage plants was sampled after 3 and 8 weeks, the fresh weight recorded and chopped and a sub-sample oven-dried to constant weight at 72⁰C. The sub-samples were then ground and analysed for total N, P, and K contents using a wet digestion procedure (Okalebo et al., 1993). The final harvest was obtained after 11 weeks from an area of 2.8 x 1.6 m², the fresh weight measured and a sub-sample recovered for similar nutrient analyses. Weed biomass from 1 m² of the unweeded plots was obtained 8 weeks after transplanting cabbage, a sub-sample was oven-dried at 72°C and total dry matter calculated.

Soil macrofauna were assessed at the conclusion of the experiment. A 30 cm monolith cube was excavated (Anderson and Ingram, 1993) at randomly selected locations in all treatments. Macrofauna were hand-collected, sorted and fresh weights recorded.

Cabbage yields were analysed with ANOVA procedures using the following model: yield = constant + block + season + treatment + (season x treatment). Soil macrofauna were compared using similar procedures after removing season and interaction terms from the model. Means were compared using the Least Significant Difference method (Little and Hills, 1978).
Soil moisture was measured using the gravimetric method, while N mineralisation was measured using the in situ core technique. Both methods are described by Anderson and Ingram (1993). The nutrient use efficiency (NUE) was estimated as:

NUE = [(mulch yield increase/fertiliser yield increase) x (fertiliser nutrient input/mulch nutrient input) x 100)].

RESULTS

Significant differences in cabbage yields were observed between crop management practices (P<0.001) tending to be greater in the first growing season (P<0.10) (Fig. 1). The least yields (0.389 t ha^-1) were observed in the unweeded + unmulched treatment during the second season, and the greatest (29.3 t ha^-1) were recorded in the fertilised + weeded treatment during the first growing season. Cabbage yield ranking by management was consistent in both seasons following the order of: fertilised + weeded > banana mulched + weeded > plastic mulched > weeded > banana mulched > unmulched + unweeded. When differences in yield and nutrient inputs were compared for banana mulch plus fertiliser, estimates of nutrient use efficiency (NUE) were respectively, 18, 92 and 5% for N, P and K applied as organic inputs. Weeding had a greater influence on cabbage yields (P<0.004) than mulching with banana trash (P<0.035) with marginally significant weeding x mulching interaction (P<0.081) when selected treatments were compared by ANOVA in a 2 x 2 factorial arrangement (i.e., with the plastic mulched and fertilised treatments excluded).

Banana leaves contained more nitrogen than did pseudostems (2.75 vs 1.01%) and less potassium (4.90 vs 7.25%) (Table 2). Based on the typical proportion of leaves to pseudostems (Stover and Simmonds, 1987), this corresponds to the addition of approximately 256 kg N, 12.3 kg P and 960 kg K ha^-1 within the 15 t of banana mulch during the two cropping seasons.
The time to 50% decomposition (t₅₀) of banana trash (Table 3) differed between plant parts (P<0.008) and litter bag mesh size (P<0.001), with marginal significance observed for the part x mesh interaction (P=0.082). Banana pseudostems decayed more rapidly than leaves. Reduced mesh size greatly reduced the rate of leaf decomposition, while having a lesser but still significant effect on pseudostem decomposition.

Earthworm population density, in situ nitrogen mineralisation and soil moisture content exhibited significant differences among management practices (Table 4). The population densities of earthworms were increased through addition of banana residue mulch. Total macrofaunal biomass tended to be more with banana mulch, but the effect was not significant.

Banana residue mulch treatments had greater weed biomass production relative to the unweeded control during the second cropping season (Table 5). During the second season, weed biomass was greater with banana mulch treatments, presumably in response to nutrients released during the first season. Plastic mulching of cabbage improved yields (Fig. 1) as a result of near-complete weed suppression (data not presented) and improved soil moisture status (Table 4). Hand-weeding was a necessary component of successful cabbage cultivation, resulting in weed suppression, improved moisture status and increased N mineralisation (Table 4). Hand-weeding resulted in 9.3 t ha^-1 productivity gain over the two cropping seasons. Hand-weeding in absence of organic inputs reduced earthworm populations and macrofaunal biomass, but this trend was reduced through the addition of banana mulch. Hand-weeding operations effectively incorporated surface mulch resulting in increased biological activities in the soil (Table 4).

The stepwise improvement of cabbage production as a result of crop management practices is listed in Table 6. Minimum management (i.e., no inputs, no weeding) resulted in little or no economic yield, nor did mulching without subsequent weeding. Combining banana mulching and hand-weeding resulted in an additional 12.5 t ha^-1, but this increase was 15.1 t ha^-1 less than was obtained by applying fertiliser.

DISCUSSION

Cabbage yield was improved most through nutrient application, either as banana plant residues or fertiliser, but only when weeds were controlled. Weed control whether by hand weeding or suppression with plastic mulch did not result in yield increase unless nutrients were supplied. More N and K, but less P, were applied in the plant residuals than in the fertiliser. Based on plant deficiency symptoms, P availability may have constrained cabbage yield with banana mulch application, but these results were inconclusive. A previous glasshouse investigation indicated that P was the most limiting nutrient to cabbage at this site (Lekasi, 1997). The inefficiency of N and K as organic inputs may be partly attributed to the low availability of P, suggesting that the use of N and K would improve with the addition of supplemental P. Thus, yield increases largely resulted from both nutrient additions (Table 2) and rapid decomposition (Table 3) of the banana pseudostem, and, to a lesser extent, the leaves. Combination of the two decomposing tissues allowed for a more even spread of nutrient supply over the two growing seasons. Greater synchronisation of nutrient supply with crop demand is considered one of the challenges facing organic resource management (Myers et al., 1994; Palm et al., 1997) and the observed differences in decomposition rate between banana pseudostem and leaves may present an opportunity in that regard.

Soil macrofauna increased litter turnover of both pseudostems and leaves (Table 3), but their influence upon nutrient recycling may only be inferred by this experiment. Based upon the trophic categories of Lavelle et al. (1994), these earthworms are either epigeics which dwell within the surface litter, or anecics which feed on surface litter but dwell within verticle burrows in the soil surface or in the upper soil. Nutrients ingested by earthworms are unlikely to be lost from the soil system, and in some cases plant nutrients become more rapidly and completely mineralised (Fragoso et al., 1997). Termites operate in a different manner, however, by transporting litter to nests which may be located below or away from crop root systems (Jones, 1990; Lavelle et al., 1994) reducing nutrient availability. Termites were the most abundant insects recovered from the soil (data not presented), but insect populations varied greatly within as well as between treatments (Table 4) suggesting marked spatial heterogeneity.

Plastic mulching, as a water conservation and weed control measure, improved yields but is not presently feasible as polythene sheeting is not widely available in Uganda. However, the yield improvement from plastic mulching (5.6 t ha^-1) in contrast with weeding with hand implements, may signal a potential market for this mulching product.

While this study offers insights into improved cabbage management practices within banana-based cropping systems, it remains inconclusive in terms of formulating exact management recommendations. A detailed economic analysis of the treatment effects was not conducted, in part because technologies employed within the study (i.e., fertilisers and plastic mulch) are not widely available to smallholder farmers. Furthermore, the transfer of banana residues from one farm component (banana fields) to another (vegetable plots) has implications at the whole-farm level, particularly for interactions between pest cycles and nutrient recycling, that are beyond the scope of this study. Nonetheless, farmers who wish to improve or enter vegetable production can effectively utilise banana residues as nutrient inputs in combination with hand weeding; however, the nutrients applied in the banana mulch may not be utilised efficiently over the short-term as compared to chemical fertilisers. The results of this study must not be viewed as a radical departure from recommended intensive management of market vegetables involving the application of mineral fertilisers and careful control of weeds, but suggest that banana residues may be useful organic inputs within the vegetable enterprises.

ACKNOWLEDGEMENTS

This research was conducted in partial fulfilment of an M.Sc. degree training in the Department of Soil Science, Makerere University. It was funded by the Rockefeller Foundation's Forum on Agricultural Resource Husbandry. Both organisations are gratefully acknowledged for their assistance in this work.

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