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The nitrogen replacement value
of soybean on a following maize crop
N
itrogen is one of six macro elements that is taken up by
grain crops from the soil and as the demand usually ex-
ceeds the supply, application of nitrogen containing ferti-
liser is required. Leguminous crops, of which soybean is a
member, however, is an exception to this rule.
They have the unique ability to supplement the soil supply through
a symbiotic nitrogen fixing process with
Rhizobium
bacteria. The
bacteria penetrate root hairs on young growing roots and forms
the well-known root nodules (
Photo 1
).
Symbiosis is a beneficial cohabitation between organisms. In the
case of
Rhizobium
and soybeans, the bacteria live on the energy
supplied by the photosynthetic process of the plant. The bacteria in
turn, fix nearly inert atmospheric nitrogen, N
2
, into compounds use-
ful to the soybean plant.
To take advantage of this symbiotic process, nitrogen fertilisation
of soybeans is not recommended, except on sandy soil where the
residual nitrogen supply is usually low. Small amounts of nitrogen
fertiliser are therefore recommended on such soils for enhanced
growth of young plants.
Inoculation of the seed or soil, with a specific
Rhizobium
specie,
Bradyrhizobium japonicum
, associated with soybean, is very impor-
tant as it does not occur naturally in South African soil.
The advantage of the symbiotic nitrogen supply does, however,
not end with the soybean crop, but the following maize crop also
benefits – or so it seems. The yield of maize following soybean on
a particular land is often noticeably better than that of maize follow-
ing maize.
The yield increase over years is about 13% on sandy textured soils.
Rotated maize usually shows no nitrogen deficiency symptoms
while it often appears in monoculture maize during the reproductive
part of the growing season (
Photo 2a
and
Photo 2b
).
The popular interpretation is that some of the symbiotic fixed nitro-
gen is still available for uptake by the soybean following maize crop.
Closer investigation has, however, showed that this is unlikely.
Firstly, soybean is fairly nitrogen neutral. Roughly the same amount
of nitrogen that is fixed is removed by the grain. Increases in the
soil’s residual nitrogen after soybean are usually too small to explain
maize yield improvements. Some results even showed that soy-
bean can even decrease the amount of residual nitrogen in the soil.
The yield of monoculture maize is usually lower than that of maize
preceded by a soybean crop, no matter the amount of fertiliser ni-
trogen applied. This shows that other factors aside from nitrogen are
responsible for the yield increase of soybean rotated maize.
Several possible explanations, some with supporting evidence,
have been presented. Among them are the carry-over of soil mois-
ture from the soybean to the follow-up maize season, an improve-
ment of the soil’s physical properties, a decrease or increase in
growth suppressing or promoting substances respectively and a
decrease in disease pressure.
Decreases in maize root diseases probably also play a role in our
environment. According to pathologists, root rots with varying in-
tensity and extent can be found on nearly all maize plants annually.
Root rots damage and kills parts of the root system especially during
the reproductive phase. The root system is consequently restrict-
ed and the uptake of moisture and nutritional elements is limited,
eventually affecting the growth and yield.
Trials at Viljoenskroon showed that the root system of maize follow-
ing soybean measured after pollination were 16% larger than that
of monoculture maize. This value agrees well with the maize yield
increase found.
Generally, maize take up only about 50% of applied fertiliser nitro-
gen. This efficiency of nitrogen uptake can however, vary between
20% and 80% depending on circumstances. The enhanced perfor-
mance of maize following soybeans is most probably the result of
an increase in the efficiency of nitrogen uptake, through a larger
and probably healthier root system rather than a larger nitrogen sup-
ply in the soil.
In addition to the maize yield improvement, the nitrogen fertiliser
application rate on maize following soybean can be lowered there-
fore improving the net return even further. The question is with how
much?
The answer is explained through
Graph 1
where the yield response
of maize grown in rotation with soybean and in monoculture is
compared from trial work done on a sandy soil near Viljoenskroon.
Note that the term ‘nitrogen replacement value’ of soybean is used
rather than the more popular ‘nitrogen credit’ due to the crop’s ni-
trogen neutrality.
The yield response of maize in crop rotation with soybeans (upper
curve) is different from that of monoculture maize (lower curve) as
shown in the graph. Point A and point B represent yield at the eco-
nomically optimum nitrogen fertiliser rate for maize in rotation and
in monoculture, respectively.
The yield at these points is 6,53 t/ha for the soybean rotated
maize and 5,9 t/ha for the monoculture maize, while the correspond-
ing optimum nitrogen application rates are 54 kg/ha and 95 kg/ha
respectively.
The nitrogen replacement value of soybeans in this case is
95 kg/ha - 54 kg/ha = 41 kg/ha. This value agrees with that found in
the USA, but is not applicable everywhere as it can be influenced
by various factors.
Indications exist for example, that soil with higher clay or organic
material content will have a lower nitrogen replacement value than
sandy soil with a low organic material content, such as those on
which the trial was done. Some evidence also indicates that the
nitrogen replacement value will be smaller where no-till is practised
in comparison with conventional tillage.
Contrary to what was previously thought, the nitrogen replace-
ment values have no relationship with the yield of soybeans. Con-
sequently replacement values cannot be estimated from the yield
of soybeans.
ON FARM LEVEL
Nitrogen / Soybean / Maize / Replacement value
Fertiliser
ANDRÉ NEL,
ARC-Grain Crops Institute, Potchefstroom