30 Liliana Suñer and Juan Galantini
determine soil structure. The POM on the other hand is the more dynamic
component, playing an active role in nutrient flows and therefore closely
associated with the availability of nutrients to crops (Lützow et al., 2002;
Galantini et al., 2005; Galantini & Rosell, 2006). Considering that both
fractions are crucial to soil fertility, it is important to determine the organic
(Po) and inorganic (Pi) phosphorus contents in each size fraction and to relate
them to soil texture. Differences in P content are expected to be found in both
particle size fractions: the fine fraction is likely to contain the more stable Po
(Po-MOM) and the more available Pi (Pi-clay and silt sizes); the coarse
fraction, instead, is assumed to contain the more labile Po (Po-POM) and Pi (Pi-
sand size) with low availability in the short term.
INFLUENCE OF NATURAL FACTORS ON P STATUS
In semiarid Pampean region of Argentina, two of the most important
characteristics that modified the equilibrium among different forms of P, as
well as the total content, are soil texture and caliche layer depth. The last one
had an important effect on soil pH, for this reason, it can modify soil P forms
equilibria.
Soil Texture
Texture influences soil physical conditions by affecting pore-size
distribution and pore continuity, which in turn control soil water availability,
gas diffusion and the activity of soil organisms (Hassink et al., 1993). The
interaction of these processes determines the optimum soil water content for
microbial biomass distribution and activity. Furthermore, the distribution, size
and continuity of pores determine the soil space available for microorganism
activity and the possible protection of SOM from mineralization (Zibilske &
Bradford, 2003). Organic matter typically decomposes more rapidly in sandy
soils than in clay soils where the amount of soil microbial biomass is larger,
thus suggesting a greater physical protection of SOM in fine textured soils
(Franzluebbers & Arshad, 1996). Furthermore, specific respiratory activity is
greater in coarse-textured soils, thus providing plant available nutrients more
rapidly than in fine textured soils (Hassink, 1994; Franzluebbers et al., 1995).
Soil texture also affects N and P availability by influencing total SOM
accumulation (Schimel et al., 1985) and microbial activity (Hassink et al.,