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

Table 6. Nutrient balance (kglha) under two year non·coppicing
fallow species at Msekera, eastern Zambia
Nitrogen Phosphorus· Potassium
limd use systems 1998 1999 1998 1999 1998 1999
Cajanus cajan 27 5 21 8 13 ·9
Sesbania sesban 22 5 39 24 -42 ·32
Natural fallow 8 11 19 15 ·10 ·4
Fully fertilized maize 150 103 57 43 ·19 ·17
Unfertilized maize 31 11 29 20 19 ·1
yields in the second year of cropping after two-year
fallows. The huge amount of N supplied by fallows
could be lost through leaching beyond the rooting
depth of maize. Our leaching studies have clearly
shown substantial inorganic N at depth under
maize after improved fallows. These results imply
that if cropping goes beyond three years after fallows
there will be a negative N balance. Thus the
recommendation of two years of fallows followed
by two years of cropping is well supported by N
balances and maize grain yield trends. Most of the
land use systems showed a positive P balance. This
can be attributed to low offtake of P in maize grain
yield and stover. In addition, this site had a high
phosphorus status. The trees could also have increased
P availability through secretion of organic
acids and the increased mycorrhizal population in
the soil. These issues are under investigation at our
site. In general, we have observed positive P balances
over eight years. However this result needs
to be tested on farm where the soils are inherently
10winP.
Most land use systems showed a negative balance
for K. For tree based systems, sesbania showed a
higher negative K balance compared to pigeonpea.
This is attributed to the higher fuelwood yield of
sesbania with subsequent higher export of K compared
to pigeonpea. The higher negative K balance
for fully fertilized maize is due to higher maize and
stover yield which exports a lot of potassium. This
implies that the K stocks in the soil are very high
and that K mining has not reached a point where it
negatively affects maize productivity. However in
sites with low stocks of K in the soil, maize productivity
may be adversely affected.
Nutrient balances were conducted for coppicing fallows
using gliricidia compared to non-coppicing fallows
using sesbania for four cropping seasons after
fallow clearance. Gliricidia fallows maintained a
positive N balance. This was attributed to resprout
growth, which was applied to maize as a source of
nutrients and deep capture of N from depth by the
well-established gliricidia rooting system. All land
use systems showed a positive P balance. However
from the third season of cropping onwards sesbania
fallows, fertilized maize and gliricidia fallows had a
large negative balance. This was attributed to removal
of nutrients in ·stover maize or leaching of K
from surface soils ..
Overall, the tree based fallows maintained a positive
Nand P balance. However on low· P status, a
negative P balance would be expected. There was a
negative K balance with most land use systems. It
can be hypothesized that as we scale up improved
fallows on depleted soils on farmer's fields, 1