Grain Legumes and Green Manures for Soil Fertility in ... - cimmyt

Most interventions involving green manure crops
have emphasized the role of the legumes in the
maize-based cropping systems without looking at
improving the legume itself. It is iffiportant to note
that for the leguminous crops to fix nitrogen, they
need good phosphorus and sulphur nutrition, in
addition to other nutrients. The aim of supplying
. green manure legume crops with phosphorus, sulphur
and zinc is to boost early root development
that would take up soil nutrients for plant development,
and subsequent biomass production and biological
nitrogen fixation (BNF). With low soil nutrient
contents, most legume manure crops do not produce
sufficient quantities of biomass to supply the
required levels of nutrients upon mineralization
(Palm et al. 1997). Many organic materials when applied
in modest amounts, i.e. 3-5 t ha·1 dry matter
contain sufficient N to meet the requirements of a 2
t maize crop. However, they cannot. supply the P
requirements of maize, hence legumes must be supplemented
by P in areas where P is deficient (Palm,
1995). Application of inorganic fertilizer to legumes
would thus improve biomass production and nutrient
recycling, thereby releasing higher amounts of
plant nutrients upon decomposition.
The objectives of the experiment were (a) to evaluate
the effect of phosphorus and sulphur application
on biomass production by three legume green manure
crops; MUCllna pruriens, Cajanus cajan and
Tephrosia vogelii and (b) to screen a green manure
legume crop that can result in higher yields for the
subsequent maize crop.
Materials and Methods
Experimental sites
The on-farm, farmer-managed, researcher-designed
experiment was conducted in two Extension Plan­
ning Areas (EPAs) of Mzuzu Agricultural Develop­
ment Division in Northern Malawi. The sites were
Champhira EPA in Mbawa Rural Development Pro­
ject and Nchenachena EPA in Rumphi Rural Devel­
opment Project.
Soils of Champhira (Loudon series) are classified as
weakly Ferallitic Latosols and those of Nchenachena
(N chenachena series) are Ferrisols (Young and
Brown, 1962). Champhira lies at an elevation rang­
ing from 1216 to 1338 ill above sea level and located
12° 24' Sand 33° 40' E while Nchenachena is 1216 to
1307 m above the sea level and located at 10° 30' S
and 33° 50T
Experimental design
The experiment was laid out in a split-split plot ar­
rangement in a randomized block design. The two
sites of the experiment were the main plots. In Year
1 (1999 - 2000), three green manure legume crops; (i)
Pigeon pea, hybrid variety ICP 9145 (Cajanus cajan
(L) Mellsp.), (ii) Velvet bean (Mucuna pruriens) and
(iii) Fish bean (Tephrosia vogelii) and (iv) Maize hybrid
MH 18 (Zea mays (1.)), were the sub-plots. The
sub-plots measured 15 m long with five ridges
spaced at 0.90 m apart (67.5 m2). There were three
sub-sub plots for each crop with five ridges each 5m
long and spaced at 0.90m (22.5 m2). Treatments for
sub-sub plots were (i) without phosphorous and
sulphur, (ii) 20 kg phosphoru~ ha·1 plus 4 kg sulphur
ha- 1 and (iii) 40 kg phosphorus ha··1 plus 8 kg
sulphur ha- 1 • Plant density was as shown in Table 1.
Immediately after harvest (3 rd week of May and 2 nd
week of June, 2000 for Champhira and Nchenachena
respectively) in year 1 (1999-2000), the green
manure and m,aize stover were ploughed into the
soil in the individual treatment plots. In Year 2
(2000 - 2001), a maize crop (MH 18 hybrid) was
grown in all the plots to assess the residual effect of
the green manure legume crops.
Soil sampling
Soil sampling was done at each site before the start
of the experiment. Soil samples were randomly
taken from 0-15 cm and 15-30 cm soil depths from
each of the smallholder-farmers' plots using an auger.
From each farmer's plot, five samples were
taken at each of the soil depths. The soils from the
same depths with the same farmer were mixed and
after several splits, about 500 g of the soil was obtained
and stored in plastic bottles. The initial soil
samples were for characterizing the two sites. These
samples were analyzed for general soil physical and
chemical properties (Table 2). All soil samples were
air-dried, sieved through a 2 mm sieve and stored
in plastic bottles before laboratory analyses.
Plant sampling
Three plants from the middle ridge of each treat­
ment plot were sampled eight weeks from planting
and at mature harvest for both seasons. These sam­
ples were oven-dried at 65°C for 48 hours, then
ground to powder (passed through a 0.1 mm sieve)
using an electric grinder and stored in plastic bot­
tles. The samples were analyzed to determine nitro­
gen and phosphorus in the plant tissue.
Biomass was estimated at harvest for both the leg­
ume and maize stover after the end of the first sea-
Table 1. Spacing of the crops between and within ridges (em)
Crop Within ridges Between ridges Plants per station
Maize 50 90 2
Mucuna 15 90 1
Pigeon pea 90 90 3
Tephrosia 75 90 3
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Grain Legumes and Green Manures for Soil Fertility in Southern Africa