Agroforestry for Small Landholders of Eastern and Southern Africa - Soil Management of Smallholder Agriculture - page 249

et al. 1999; Sileshi et al. 2008). In these systems, soil fertility benefits from fallowing
are derived from the use of annual, biannual, or perennial N-fixing trees or “legu-
minous fertilizer trees,” which are either planted in rotation (e.g., improved fallows)
or together with crops (e.g., alley or intercropping). Leguminous fallow trees that
have been used successfully include coppicing perennials such as Gliricidia and A.
angustissima and others such as Sesbania and pigeon pea ( Cajanus cajan ), which do
not coppice well and need to be replanted during the fallowing phase of crop-fallow
rotations.

11.2.1.1 Nitrogen Fixation
Species such as Sesbania , Gliricidia , and Tephrosia replenish soil fertility through
biological fixation of atmospheric N into available N, root uptake, and recycling of
nutrients. Nitrogen that accumulates in the aboveground biomass (especially leaves)
of planted tree fallows and intercrops is released when the biomass decomposes
after being incorporated into the soil, and it is utilized by crops during the cropping
phase. Different tree species can fix different amounts of N, and the total amount of
N that is released is also dependent on the decomposition rates of the leguminous
tree biomass (Mafongoya and Dzowela 1999). For example, Mafongoya and Dzowela
(1999) reported that biomass from S. sesban decomposed faster than biomass from
A. angustissima under similar field conditions.
A study in Zimbabwe by Chikowo (2004) showed that total fixed N (estimated
using Hyparrhenia spp. as reference plant) in A. angustissima (nonwoody compo-
nents + leaf litter) was 122 kg N ha −1 during the 2-year fallow period, while pigeon
pea, Sesbania , and cowpea fixed 97, 84, and 28 kg N ha −1 , respectively (Table 11.2).

TABLE 11.2
Total N Fixed by Different Legumes during the Fallowing Phase

Species Total N (kg ha −1 ) Location Reference

Improved Fallow

A. angustissima 122 Zimbabwe Chikowo 2004

S. sesban 84 Zimbabwe Chikowo 2004

C. cajan 97 Zimbabwe Chikowo 2004

S. sesban, T. vogelii 150 Zambia Ajayi et al. 2005

S. sesban (1 year) 60 Malawi Chirwa and Quinion 2012

S. sesban 128 Kenya Sanchez and Palm 1996

C. cajan 75-200 Malawi Kumwenda et al. 1996

Rotational Woodlot

A. crassicarpa 78 a Tanzania Kimaro et al. 2008

A. mangium 87 a Tanzania Kimaro et al. 2008

A. polyacantha 104 a Tanzania Kimaro et al. 2008

G. sepium 114 a Tanzania Kimaro et al. 2008

a Maize uptake during the following 3 years, rather than supply.