Distribution of phosphorus in size fractions of sandy soils with different fertilization histories

A major challenge in sustainable crop management is to ensure adequate P supply for crops, while minimizing losses of P that could negatively impact water quality. The objective of the present study was to investigate the effects of long-term applications of different levels of mineral fertilizers and farmyard manure on (1) the availability of P, (2) the relationship between soil C, N, and P, and (3) the distribution of inorganic and organic P in size fractions obtained by wet sieving. Soil samples were taken from the top 20 cm of a long-term (29 y) fertilization trial on a sandy Cambisol near Darmstadt, SW Germany. Plant-available P, determined with the CAL method, was little affected by fertilization treatment (p < 0.05) and was low to optimal. The concentration of inorganic and organic P extracted with a NaOH-EDTA solution (PNaOH-EDTA) averaged about 350 mg (kg dry soil)1, with 42% being in the organic form (P-o). Manure application tended to increase soil C, N, and Po concentrations by 8%, 9%, and 5.6%, respectively. Across all treatments, the C : N : Po ratio was 100 : 9.5 : 2 and was not significantly affected by the fertilization treatments. Aggregate formation was weak due to the low clay and organic-matter content of the soil, and the fractions > 53 mu m consisted predominantly of sand grains. The different fertilization treatments had little effect on the distribution of size fractions and their C, N, and P contents. In the fractions > 53 mu m, PNaOH-EDTA ranged between 200 and 300 mg kg1, while it reached 1260 mg kg1 in the fraction < 53 mu m. Less than one third of PNaOH-EDTA was present as Po in the fractions > 53 mu m, while Po accounted for 70% of PNaOH-EDTA in the smallest fraction (< 53 mu m). Therefore, 16% and 28% of PNaOH-EDTA and Po, respectively, were associated with the smallest fraction, even though this fraction accounted for < 5% of the soil mass. Therefore, runoff may cause higher P losses than the soil P content suggests in this sandy soil with a weak aggregate formation. Overall, the results indicate that manure and mineral fertilizer had similar effects on soil P fractions.