Cropping Systems: Applications, Management and Impact

28 Liliana Suñer and Juan Galantini

fertility diagnosis is necessary to know the different P fractions and evaluate
the impact of the different agricultural practices on the organic and inorganic
fractions of P.
The following points can be made regarding P-turnover:

 The equilibrium among P-pools is usually bidirectional and it depends

on the characteristics of the soil environment (pH, temperature,

moisture, etc.).

 In terms of form, soil P can be divided into organic and inorganic, and

in terms of availability in fast and slow cycling.

Importance of Organic Phosphorus

Even though plants absorb inorganic phosphorus (Pi), the importance of

Po as a reservoir of P available to plants has long been studied (Dormaar,
1972; Stewart & Tiessen, 1987). Organic phosphorus has an indirect influence
on plant nutrition, for it has to be mineralized before being absorbed.
However, when considering the factors controlling Po availability in soil, the
role of organic forms in P-recycling is essential (Anderson 1996, Szott & Palm
1997). Farming practices on virgin lands or grasslands stimulate
mineralization of the labile fractions in Po, thereby causing a relative increase
of the most decay-resistant fraction. This fraction is responsible for slowly
restoring phosphate ions to the soil solution and therefore it fails to
significantly enhance soil fertility and crop nutrition. As result, a longer
farming period decreases the soil capacity to keep P available in a natural way.
Low soil Po levels induced by farming have been observed by a number of
authors (Tiessen et al. 1994). Soil tillage is often likely to enhance soil
aeration and thus fuel microbial activity, which in turn hastens decomposition
of organic matter (Dalal, 1977). It has been reported that in the early 40- 60
years of farming, the P exported by crops used to be far lower than the P
available through organic matter decay (Tiessen et al., 1994). This Pi surplus
during OM mineralization precipitates in the form of low-solubility
compounds such as apatite. Organic phosphorus compounds decompose at
different rates. It is located mainly in the fulvic acid fraction (Dalal, 1977).
Brannon & Sommers (1985) also found that over 40% of soil Po was
associated with humic and fulvic fractions, especially in high molecular-
weight fractions. Acknowledging the importance of organic phosphorus in the