Soil Fertility as a Contingent Rather than Inherent Characteristic - Principles of Sustainable Soil Management in Agroecosystems - page 150

rice plants having root systems that are larger and healthier, with less or delayed
senescence, and also larger, more photosynthetically active canopies (Thakur et al.
2010; Barison and Uphoff 2011). Larger root systems together with more leaf area
and light interception (Thakur et al. 2010) are associated with increased root exuda-
tion that delivers more carbohydrates, amino acids, and other compounds into the
rhizosphere, supporting larger populations of microbes and the web of fauna that
feed upon them.
It is not self-evident why SRI practices should enhance the populations of benefi-
cial soil organisms, rather than of pathogens, in the root zone. The empirical results
so far simply show increases particularly for known beneficial organisms. We do not
know of any studies that compare population densities of pathogens in response to
SRI management practices; but the evidently better health and measurably higher
yields of SRI plants suggest that numbers of adverse organisms and the diseases they
cause are not increased nor intensified.
Aerobic organisms tend to be more beneficial than anaerobic ones for plants. A
study done at the China National Rice Research Institute examined the impact of
two key SRI management practices—intermittent versus continuous irrigation, and
the degree of organic fertilization, keeping constant across all trials the total amount
of N added to the soils—on the populations of actinomycetes in the rhizospheres of
rice plants (Lin et al. 2011). Actinomycetes are a Gram-positive order of soil bacteria
known to enhance soil fertility when there are sufficient organic sources of nutrients
in the soil to sustain their active populations. They also commonly have antagonistic
antimicrobial activity against a wide range of soil and plant pathogens.
Table 6.3 shows that when 100% organic fertilization was used with intermittent
irrigation so that aerobic soil conditions were maintained, the population density
of actinomycetes was almost five times greater than with 25% organic fertilization
(same total N) and continuous flooding giving anaerobic soil. Similar research done
at Zhejiang University in China comparing the rhizosphere soils of rice plants grown
with SRI methods (more organic matter applications and intermittent irrigation)

TABLE 6.3
Effect of Organic Matter Treatments and Water Regime
on Actinomycete Population Densities in Rhizosphere
Soil (g -1 dry soil)

Actinomycetes (×10 6 )

Fertilization Intermittent Irrigation Continuous Irrigation

25% organic 66.3 cC 52.3 Bb

50% organic 119.7 bB 84.4 aAB

100% organic 259.6 aA 93.3 aA

Source: Lin, X.Q. et al. Paddy Water Environ 9:33-39, 2011.
Notes: Lower-case letters indicate significance at 5% level; capital letters
show significance at 1% level.