Phosphorus Dynamic in the Soil-Plant System ... 41Fertilization
Numerous studies have investigated the effects of fertilizer additions on P
fractions and transformation (Niu et al, 2011). Generally, resin-P, NaHCO 3 - Pi
and NaOH-Pi reflect the difference between fertilizer P and crop P removal.
Stable fractions are less affected by inorganic fertilizer P or manure
application (O'Halloran, 1993; Richards et al., 1995; Tran & N’dayegamiye,
1995; Zhang & MacKenzie, 1997). The Po fraction is either unaffected by
excess inorganic fertilizer P (McKenzie et al., 1992; Zhang & MacKenzie,
1997; Zheng et al., 2001) or increased with high rates (Schmidt et al., 1996),
but it increases when inorganic fertilizer is combined with farmyard manure
(O’Halloran, 1993; Tran & N’dayegamiye, 1995; Zhang & MacKenzie, 1997).
Long-term fertilizer application usually enhances P-availability to crops
and compensates for the immobilization that occurs at the beginning. Nineteen
samples of differently textured soils were collected in the sub-humid
Argentinian Pampas. They were analyzed for the amount of P needed to
increase Pe by 1 mg per kg of soil or its equivalent rate (ER). This value was
determined by calculating the remaining soil Pe after P-application at different
rates. ER values across soils ranged from 1.2 to 4.8 kg P ha-^1. Other studies on
multiple soils in Argentina have reported an equivalent rate between 1.5 and
7.0 kg P ha-^1 (Quintero, 2003) and from 3.2 to 6.0 kg P ha-^1 (Rubio et al.,
2008). In the semi-arid Pampas region, an average of 2.7 kg P ha-^1 should be
applied to increase Pe by 1 mg kg-^1. The amount of P needed to increase 1 mg
of Pe per soil kg has been reported to be related to the P-sorption index
(Quintero, 2003) and to the initial Pe (Rubio et al., 2008). Suñer & Galantini
(2012) found that the soil properties that best accounted for ER variability
were initial Pe and mineral-associated organic carbon. This suggests that the
higher the soil Pe content, the higher the P amount needed to increase it by 1
mg kg-^1.
The phosphorus applied in excess would thus become incorporated to the
available P and it would precipitate as low-solubility compounds (Figure 3 &
4). This means that highly available P applied as a fertilizer promotes the flow
of available to less available forms; in these soils, it would precipitate as
Ca phosphates (Galantini et al., 2005).
Also, the higher the SOM, the fertilization rate to be applied must be
major. This confirms that organic matter plays an important role in the
adsorption–desorption balance of the available P-forms (Barrow, 1979,
Börling, 2003, Castro & Torrent, 1998). In terms of the physico-chemical
equilibrium of P, soil characteristics and mineral composition determine the