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

above-ground biomass was not translated into
higher grain yields, suggesting poor photosynthate
partitioning to pods. This was despite Soil P increasing
with P application, reaching ad~quate levels for
plant growth at greater than 60 kg P20S ha- l for both
sources of P. The low shelling percentage indicated
that there was need for liming. PAPR was 2.5 times
more effective in increasing the level of soil P at the
highest rate of P applied (120 kg P20s ha- 1 ). For
MAP, soil P decreased at this high application rate.
The higher rates of P applied as PAPR tended to increase
soil P values and in tum tended to produce
higher grain yields and increased shelling percentage.
In the second cropping season, adding nutrients
other than P (N, K, S) increased biomass yield over
the absolute control, indicating that these nutrients
were limiting. The effect of P on biomass yield depended
on lime. Lime depressed biomass yield of
the absolute zero control treatment, while increasing
the yield of the PO control where P was not applied
(Figure 9). The significant depression of biomass
yield of 1.4 times with liming was probably
due to a Ca/Mg imbalance. Magnesium was low in
these soils, and therefore adding an excess of Ca
through P APR and lime probably offsets the balance.
The comparative higher yields of the unlimed
compared to the limed absolute control treatment
was because the exchangeable aluminium was low
and not detrimental to plant growth even though
the soil pH was strongly acidic. Overall, liming increased
the pH from 4.4 without liming, to 5.1 wi'th
liming. The change of pH with lime was confined
mainly to the topsoil. The pH was higher for PAPR
than MAP when P was applied at 60 kg P20S ha- 1
and in the sub soil of the limed plots at both 60 and
120 kg P20 S ha- 1 (Table 2). This suggests a liming
effect of PAPR that did not occur at the higher rate
of PAPR.
Response of biomass yield to fresh applications of P
over the absolute control with liming and the PO
control without liming that were observed for MAP
without liming and with liming for PAPR was corroborated
by the low levels of soil Pfor the Absolute
zero and PO controls (Figure 10). Although residual
application of MAP and PAPR with and
without liming did not increase soil P beyond that
of the control treatments, response to residual P
was, however, observed for the two fertilizers. The
response was obtained for MAp without liming. For
PAPR, the residual effect was greater and more effective
than that of MAP without liming.
Cowpea
At Lusitu On-Station site, there was a significant (P
> 0.1) response of cowpea grain yield to application
of P above 80 ~g P20S ha- 1 with P APR and above 120
kg P20S ha- 1 with MAP (Figure 11) in the first cropping
season. The yield response to P was consistent
with the inherent P deficiency in the soil at this. site
and therefore the need for P application to increase
yields. This is corroborated by the available soil P
values which increased to levels adequate for plant
growth with application of P above 80 kg P20S ha- 1
for MAP and above 40 kg P20S ha- 1 for P APR
(Figure 12).PAPR was more effective in increasing
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'0 1500
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UI 1000
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E 500
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P and Lime (Level kg PzOs ha- 1 )
The soil P was lowest for the limed non-P fertilized
control compared to the absolute control whether
limed or unlimed. Crop production of maize and
groundnut during the 2000/01 and
Figure 9. Effect of source of P, level of Pand lime on groundnut
biomass yield at Mwanachingwala On-Farm site. Means followed by
the same letter are not significantly different.
2001/02 cropping seasons depleted soil P Table 2. Response of soil pH to application of lime
compared to the absolute control. Recur­ P level Depth (cm)
rent applications of P as MAP or PAPR kg PzO~ hal (0-15)
increased soil P by 3.7 and 4.9 for MAP MAP MAP PAPR PAPR MAP
and PAPR with liming and by 2.8 and 4.2 l Ul l Ul l
for MAP and PAPR without liming re­ 60 4.9 bcd 4.1 fgh 5.5 a 4.6 cde 3.9 gh
spectively. The increase in soil available 120
P occurred primarily in the topsoil, espe- CV _ 4.35 %
5.3 a 4.5 cde! 5.7 a 4.1bcd 4.6 cde
cially with PAPR with liming and to a LSD _ 0.4412
lesser extent with MAP without liming. Means followed by the same letter are not significantly different.
The increase in soil.P was highest for Key
PAPR with or without liming.
MAP L . MAP Limed MAP UL· MAP Unlimed
PAPR L . PAPR Limed
PAPR UL . PAPR Unlimed
(15·30)
MAP PAPR PAPR
Ul l Ul
3.9 h 4.7 cde 4.5 cdef
4.1 fgh 5.2 ab 4.1 fgh
194
Grain Legumes and Green Manures for Soil Fertility in Southern Africa