eluded that, at current low levels of
nitrogen application, crop response
to nitrogen is frequently around 15
kg grain/kg N in forest zones and
33 kg grain/kg N in savanna zones
(the difference is partly explained
by the higher natural level of
nitrogen in forest soils). Data from
FAO trials show a crop response of
around 10-20 kg grain/kg N at rates
of 20 kg N/ha, falling to 6-14 kg
grain/kg N at rates of 40 kg N/ha
(McIntire 1986).

Whether or not these crop re-
sponses are sufficient to justify the
increased cost of purchasing and
applying fertilizer depends on a
number of factors, including the
price of fertilizer, the price of maize
grain, and the cost of additional la-
bor required. Partial budget analy-
ses done in several countries indi-
cate that fertilizer use on maize is


often profitable at prevailing
market prices. On the other hand,
researchers, extension agents, and
especially policy makers should not
be lulled into believing that it is al-
ways profitable to use fertilizer on
maize. On-farm trial data from a
number of sites suggest that, when
other factors that limit yields are
present (e.g., lack of water, inade-
quate weed control, or deficiencies
of complementary nutrients) maize
may respond only modestly to in-
creased applications of fertilizer. If
such constraints are present,
increased fertilizer use on maize
may be unprofitable, and farmers
would be making an economically
rational decision in choosing not to
apply additional fertilizer.

Subsidies often encourage ineffi-
ciently high levels of fertilizer use.
In a series of on-farm trials at 222
sites in different agroclimatic zones


Table 6. Economic profitability of fertilizer use on maize in Ghana
under two levels of fertilizer prices

 -...-- .. -:~~aie n practice from:


throughout Ghana, two levels of
fertilizer application were com-
pared to the practice of using no
fertilizer (Table 6). The marginal
rate of return to the resources
invested in fertilizer is positive for
both fertilizer treatments, if 1987
prices for inputs and maize grain
are used in the calculations. How-
ever, 1987 market prices in Ghana
included a large fertilizer subsidy,
which reduced the nutrient-to-
maize grain price ratio to approxi-
mately 2:1. If the subsidy had not
been present and fertilizer had
been sold to farmers at a price
reflecting its true import cost, the
nutrient-to-maize grain price ratio
would have risen to approximately
4:1. Using this higher ratio, the
marginal rate of return to the
resources invested in fertilizer
would have been positive for the
lower fertilizer level (Treatment 2),
but negative for the higher one
(Treatment 3). These data support
the view that fertilizer use on
maize in many parts of Africa can
be justified economically only at
relatively modest levels (Carr
1989).


rtwilizer price level


 T Treatment la
- tr to
 AWreatment 2b


Treatm
 -to
Ireatm


Nitrogen-to-maize grain price ratio = 2:1


Marginal costs (cedis/ha)
Marginal benefits (cedis/ha)
Marginal rate of return to
 additional investment ( h)


8,866
32,934


7,
11,


Nitrogen-to-maize grain price ratio = 4:1


Marginal costs (cedis/ha)
Marginal benefits (cedis/ha)
Marginal rate of return to
 additional investment (i%)


14,796
27,004


13,
5,


lent 2 In addition to high cost, a second
 obstacle to increased fertilizer use
ent 3C in Africa is limited availability.
 Many farmers cannot obtain
 fertilizer when they need it, or in
 the formulations they desire. Many
222 factors contribute to fertilizer
778 supply problems. Planning and
 administering a national fertilizer
 63 program requires skills that are not
 always available in the government
 agencies that oversee input supply,
 and the private sector also may ex-
740 perience problems in distributing
260 fertilizer (Shepherd 1989). Further-
 more, fertilizer imports have to be
 -62 financed with foreign exchange,
 which is often in short supply.
 These problems are compounded by
 lack of facilities for importing,


Source: Ghana Grains Development Project.
a Treatment 1 = no fertilizer; yield = 1.6 t/ha maize.
b Treatment 2 = 50 kg N/ha, 25 kg PO/ha; yield = 2.7 t/ha maize.
c Treatment 3 = 100 kg N/ha, 50 kg P,05/ha; yield = 3.2 t/ha maize.