The effect of different fertilizing strategies on soil organic matter, soil P fractions and P leaching potential in Flanders and the Netherlands.

Despite actions taken by the government of Flanders to lower nutrient leaching in agricultural soils, nutrient losses are still substantial. Further restrictions on the fertilizer and manure input were imposed in 2011. The total organic carbon (TOC) content of agricultural soils has declined over the last 20 years. This decrease is due to several factors, including the increasing use of slurry instead of farm yard manure. With a further reduction of the organic fertilizer use, the soil’s organic carbon content might fall even faster. The nutrient supply to the soil and nutrient losses must be lowered, simultaneous with an increasing supply of organic matter to the soil.
We have measured differences in soil P condition and P leaching during 2 long-term field trials. The hot water extractable P (HWP), the ammonium lactate extractable P (P-AL), the organic P and the calciumdichloride extractable P (P-CaCl2) were measured on soil samples. The differences in P leaching were examined via a percolation experiment. Soil columns were placed in the dark at a constant suction pressure of 100 hPa below atmospheric pressure (field capacity of a sandy soil) at 18°C. A constant water flow (6.5 l/m².day) was supplied to the top of these columns. We then measured the total P, total dissolved P (TDP), total dissolved C (TDC) and orthophosphate concentrations in the percolated water.
One field trial has been ongoing since 2005 at the Vredepeel experimental farm (the Netherlands; black sandy soil). Three management types have been compared: Integrated Management (IMa) (mineral fertilizers + animal slurry), organic farming (OF) (farm yard manure + allowed mineral fertilizers) and Integrated Management (IMb) without the use of organic fertilizers and reduced use of mineral P fertilizers (P-mining). The TOC level was positively influenced by the use of organic fertilizers, being 2.34% for IMa, 2.28% for OF and 2.15% for IMb. The P-AL of the IMa (255.9 mg/kg) and the OF (244.0 mg/kg) soils were significantly (p<0.05) higher than for the IMb (209.3 mg/kg) soils. This was expected, because there was a lower P input in the IMb soils. The same conclusions can be made for HWP (IMa 14.5, OF 13.6 and IMb 12.0 mg/kg), the organic P (IMa 453.9, OF 465.7 and IMb 380.8 mg/kg) and P-CaCl2 (IMa 1.7, OF 1.8 and IMb 1.5 mg/kg). The P leaching was significantly (p<0.05) higher in the IMa soil. The mean total P concentration in the percolation water was 1.3 mg/l for IMa, but only about 0.8 mg/l for the IMb and OF soil. We expected to find more P in the OF soils due to the higher P input. We have not found an obvious explanation why the P leaching in the OF –where the P input was even higher than in the IMa soils - is lower than in the IMa soils. The P leaching could possibly have been influenced by the different fertilizer types that were used.
Another field trial was conducted from 1997 to 2010 at the experimental farm in Melle (Belgium, sandy loam). The value of compost –as a stable carbon source- was tested in combination with cattle slurry and mineral fertilizers under a maize monoculture. The yearly addition of cattle slurry
(35 Mg/ha), compost (22.5 m³/ha) or cattle slurry (35 Mg/ha) in combination with compost
(22.5 m³/ha) led to higher TOC levels of 1.57%, 1.82% and 1.87%, respectively, in comparison to non-organic fertilizers only (1.42%). This was significant (p<0.05) for compost and cattle slurry in combination with compost. The soil P was only slightly influenced by the addition of cattle slurry. The additions of compost –in combination with cattle slurry or not- increased the soil P content. The P concentration in the percolation water was significantly higher (p<0.05) in the soils with compost addition. The addition of cattle slurry did not influence the P leaching. The addition of compost is a good option to restore the TOC level of the soil, but contains an extra amount of P susceptible for leaching.