Cultivation effects on phosphate forms and sorption in loess-soils of Argentina

Phosphorus behavior in unfertilized soils of the Semiarid Argentinean Pampa s is not well understood. The objective of this study was to evaluate the e ffect of cultivation on qualitative and quantitative phosphate concentratio ns as well as on P sorption of these soils. For this purpose topsoil sample s (10 cm) of 10 unfertilized plain soils with different textured parent mat erials (Ustipsamments, Haplustolls, and Hapludolls) from five sites with in creasing mean annual rainfalls (550 to 650 mm) and temperatures (15 to 16 d egrees C) were studied. Undisturbed (under Prosopis caldenia forest) and cu ltivated paired pedons (under rotation of annual crops and pastures for mor e than 30 years) were compared at each site. Cultivation decreased the inor ganic P fractions (P-i) 16 to 19% in one Haplustoll and the Hapludoll, decr eased the organic P fraction (P-o) 7 to 54% in both Ustipsamments, and decr eased the Bray-Kurtz I extractable P (P-a) 62 to 73% in one Ustipsamment, a Haplustoll, and the Hapludoll. Only one Haplustoll, cultivated during long periods with alfalfa pastures, did not show changes in any of studied P fr actions. This soil lay for long periods under cattle-grazed alfalfa pasture s, under which conditions P losses by wind erosion were decreased, and P re cycling caused by additions of cattle and plant residues to the soil, as we ll as P translocation from the subsoil to the topsoil by deep alfalfa roots , were increased. Total P losses from the four other cultivated soils were larger than calculated plant removal, indicating that other processes than plant uptake decreased P amounts. The Hapludoll and one Ustipsamment were c ultivated with annual crops for longer periods than with alfalfa pastures. This produced wind erosion (thinner A-horizons, and losses of easily erodab le textural fractions), which depleted P-i and P-a in the Hapludoll and P-a and P-o in the Ustipsamment. A Haplustoll and an Ustipsamment, both cultiv ated for longer periods with alfalfa pastures than with annual crops, did n ot show wind erosion features but did show P decreases. The Haplustoll lost P and P-i, probably because of plant removal without P recycling to the so il by a highly extractive production system (seed and grass-parcel producti on). The Ustipsamment lost P-o because of high mineralization of organic P by tillage, Larger P losses that occurred in both Ustipsamments were attrib uted to the higher mineralization rates of organic P compounds, which are k nown to be high in these textural soil types. The C/P-o below 200 of all st udied soils indicated that organic P compounds can supply P to the plants b y mineralization. This process will be more effective in cultivated than in undisturbed soils because of their lower C/P-o quotients. Solubility diagr ams showed that the phosphates predominant in all studied soils were fluora patite (FA) and hydroxiapatite (HA). Cultivation did not change phosphate f orms significantly, although when pH values were increased as a consequence of lime accumulation and organic matter (OM) losses, cultivation decreased P sorption capacity of the soils, which contributed to reducing available P to plants, Cultivation with annual crops using conventional tillage syste ms will increase losses of inorganic P reserves by wind erosion in Hapludol ls and of organic P reserves by mineralization in Ustipsamments, Conservati on tillage, which leaves large amounts of plant residues on the soil surfac e, can reduce these losses. Cattle grazing on alfalfa pastures seems to be the best system for reducing losses of available P in any soil type because it reduces wind erosion and increases P recycling to the soil.