Abstract

Media summary

Key words

Temperature, Daylength, Rainfall, Altitude, Tropical environments, Performance.

Introduction

In Limpopo province of the Republic of South Africa, the rural economy is heavily reliant on agriculture, and 85% of the population in this area is involved in farming (White Paper on Agriculture 1995). With average farm sizes of 1.5 ha, agricultural production in the area is predominantly carried out by smallholder farmers at subsistence level. Productivity levels in these farms are still very low and the key factor which was identified by BASED (Broadening Agricultural Services and Extension Delivery) project was declining soil fertility (Ramaru et al. 2000). One of the causes of declining soil fertility is continuous cropping without the use of either organic manure or inorganic fertilizers. Majority of the farmers are resource poor and can therefore not afford to purchase fertilizers. For the few who can afford to purchase fertilizers, the rates of application are often below recommended rates. Legumes have great potential for improving soil fertility at a relatively low cost compared to chemical fertilizers. Legumes can also be used successfully and effectively as components in intercropping systems (Ayisi et al. 1997). The use of green manure legumes may be an option for improving soil fertility. Green manure adds nitrogen to the soil and organic matter which improves soil water holding capacity, nutrient content, nutrient balance, friability and pH. The objective of this study was to assess the biomass production and nitrogen accumulation of some dual-purpose tropical legume species in relation to time of planting and to determine their potential role in smallholder farming systems in Limpopo province.

Methods

Field studies were conducted during October 2002 to February 2003 and July toNovember 2003 at the University of Venda, School of Agriculture experimental farm on a deep well-drained clay, Hutton form soil type (Soil Classification Workgroup 1991). Some physical and chemical properties of the soils (0-20 cm depth) of the experimental site are shown in Table 1. The experimental design was a randomized complete block design (RCBD) with three replications. Four dual-purpose legume species: Mucuna pruriens (31000 plants ha^-1), Lablab purpureus (Cv. Rongai) (53000 plants ha^-1), Vigna unguiculata (Cv. IT93K2046-1) (66000 plants ha^-1), and Clitoria ternatea (butterfly pea, var. Milgara) (66000 plants ha^-1) were planted as treatments. During the July-November 2003 planting season, an additional cowpea variety (Var. Agrinawa) (66000 plants ha^-1) was included. Unit plot size was 6m x 4m. At 120 days after planting (DAP), whole plant samples were obtained from an area of 1m². Plant samples were oven-dried at 70^oC to a constant weight and dry biomass was determined. The plant samples were then ground and analyzed for total N concentration. Nitrogen accumulation was calculated as a product of biomass and N concentration. Using the randomized complete block design (RCBD) model, analysis of variance was conducted using the general linear model (GLM) procedure of SAS (1996). Where significant differences among treatment means was observed at p=0.05, treatment means were compared using the least significant difference (LSD) procedure.

Table 1. Some physical and chemical properties of the soils (0-20 cm) of the experimental site.

Results

Biomass production

The total dry biomass production was significantly different among the legume species in the first growing season (Table 2). Lablab consistently produced more than 2.2 t ha^-1 of biomass in both the growing seasons. With the exception of butterfly pea, all legumes produced more than 2 t ha^-1 of biomass in the 2003 season.

Table 2. Legume biomass production.

NP= Not Planted , ¹Cv. IT93K2046-1, ²Var. Agrinawa.

Nitrogen concentration

Nitrogen concentration ranged between 12 to 24 g kg^-1 in 2002/2003 growing season, and between 31 to 40 g kg^-1 in 2003 season (Table 3). In 2003 season, lablab and the two cowpea varieties had significantly higher N concentration than mucuna and butterfly pea.

Table 3. Legume biomass N concentration.

NP= Not Planted, ¹Cv. IT93K2046-1, ²Var. Agrinawa.

Nitrogen accumulation

Nitrogen accumulation followed a similar pattern to biomass production. Nitrogen accumulation ranged between 12 to 41 kg ha^-1 in 2002/2003 growing season, and between 4 to 106 kg ha^-1 in the 2003 growing season (Table 4)

Table 4. Nitrogen accumulation by different legume species.

NP= Not Planted, ¹Cv. IT93K2046-1, ²Var. Agrinawa.

Biomass production by individual species between the two growing seasons was different. The 2002/2003 growing season was during summer. The drought during that season may have lead to the low biomass production in mucuma. Consequently, this was also reflected in the N accumulated. The 2003 growing season was during winter/spring period. Butterfly pea seems to have been affected by the reduced daylength and low temperatures resulting in significantly lower biomass production than all the other legumes. Lablab performed well consistently in both growing seasons. Lablab is a legume well suited to most tropical environments, as it is adaptable to a wide range of rainfall, temperature and altitude. Several authors have reported that lablab grows well under warm and humid conditions at temperatures ranging from 18° to 30°C and is fairly tolerant to high temperatures (Hendricksen and Minson 1985; Kay 1979; Cameron 1988). Below 20°C, the plant reduces growth; leaves begin to drop at minus 2°C, but the plant can survive in frost for a limited period (Kay 1979; Mayer et al. 1986). The average daily maximum temperature during the two growing seasons ranged from 28° to 31°C and 24° to 29°C in the 2002/2003 and 2003 growing seasons, respectively. Average daily minimum temperatures ranged from 16° to 20°C and 10° to 18°C in the 2002/2003 and 2003 planting seasons, respectively. Hence, winter period in this region is fairly mild and this can allow for favourable growth of lablab. Average rainfall in this region is 600 mm. Lablab is drought hardy and has been grown in arid, semi-arid and humid regions with rainfalls between 200 and 2500 mm (Hendricksen and Minson 1985; Cameron 1988).

Conclusion

Influence of growing season on growth performance of legume species was evident in this study. Lablab biomass production was consistent in both growing seasons, indicating that it has the potential to be incorporated into cereal monoculture system in this region when planted in summer or it can be used as a green manure when planted during winter and incorporated before the summer crop is planted. In addition, with its deep taproot, lablab is able to bring minerals, otherwise not available for annual crops, from the depths to the topsoil.

References

Ayisi KK, Putnam DH, Vance CP, Russelle MP and Allan DL (1997). Strip intercropping and nitrogen effects on seed, oil, and protein yields of canola and soybean. Agronomy Journal 89, 23-29.

Cameron DG (1988). Tropical and subtropical pasture legumes. Queensland Agricultural Journal 114, 110-113.

Hendricksen RE and Minson DJ (1985). Lablab purpureus – A Review. Herbage Abstracts 55, 215-227.

Kay DE (1979). Hyacinth Bean – Food Legumes. Crop and Product Digest No. 3. Tropical Products Institute. 16,184-196.

Mayer L, Chandler DR and Taylor MS (1986). Lablab purpureus – A fodder crop for Botswana. Bulletin of Agricultural Research in Botswana 5, 37-48.

Ramaru J, Mamabolo Z and Lekgoro J (2000). Improving soil fertility management in South Africa: Learning through participatory extension approaches. Managing Africa's Soils No. 19. Rusell Press, Nottingham.

SAS (1996). SAS/STAT User’s Guide, Version 8, Volume 2. SAS Institute Inc, SAS Campus Drive, Cary, North Carolina.

Soil Classification Working Group (1991). Soil classification. A taxonomic system for South Africa. Department of Agricultural development. Pretoria.

White Paper on Agriculture (1995). Northern Province Department of Agriculture, Pietersburg.