Soil Fertility as a Contingent Rather than Inherent Characteristic - Principles of Sustainable Soil Management in Agroecosystems - page 147

TABLE 6.1
Populations of Azospirillum Associated with Rice Yield and Tillering
Using Alternative Cultivation Practices with Different Nutrient
Amendments, on Clay Soil, Anjomakely, Madagascar, 2001

Azospirillum CFUs
in Roots a
(10 3 mg -1 ) Methods Yield (t ha -1 ) Tillers Plant -1

Conventional cultivation methods:
without nutrient amendments

1.8 b 17 65

SRI cultivation methods:
without nutrient amendments

6.1 45 1100

with NPK amendments 9.0 68 450

with compost amendments 10.5 78 1400

Source: Andriankaja, A. H., Mise en evidence des opportunites de developpement de la riziculture
par adoption du SRI et evaluation de la fixation biologique de l'azote: Cas des rizieres des
hautes terres . Memoire de fin d'etudes. ESSA-Agriculture, University of Antananarivo,
Antananarivo, 2001.
Note: The data reported here are means from six replications of the respective treatments on ran-
domized plots, 2.5 × 2.5 m.
a CFUs: colony-forming units.
b Note that average yield of conventional cultivation methods with NPK amendments was 3 t ha -1 ;
Azospirillum counts were not made for this treatment as the researcher focused on without-
amendment comparisons.

• With conventional practices and no soil amendments , the average

paddy yield was 1.8 Mg ha -1 , approximating the national average yield
in Madagascar. The associated population density of Azospirillum was
65,000 colony-forming units (CFU) mg -1 of sampled root biomass. When
conventional practices were used with NPK fertilizer, the grain yield was
3.0 Mg ha -1 , two-thirds more than with no fertilization. Unfortunately, the
researcher did not analyze root-sample bacteria from these plots.

• SRI practices without soil amendments gave much greater grain yield,

6.2 Mg ha -1 , and were accompanied by a 20-fold increase in the density
of associated Azospirillum , to 1.1 million CFU mg -1 . This was twice as
much yield without soil amendments as resulted from conventional prac-
tices supported by inorganic fertilizer applications. No causal connection
was examined between microbial populations and yield, but some causal
positive association could be hypothesized.

• When NPK fertilizer was used together with SRI practices, rice grain yield

increased by another 50%, to 9.1 Mg ha -1 . However, the population density
of Azospirillum in the root samples declined by 60%, an effect not surpris-
ing when soil is supplied with inorganic nutrients (Pariona-Llanos et al.
2010). The substantial yield increase observed would probably have been
mostly attributable to the inorganic N supplied.