Agricultural soil acidity and phosphorus leaching risk at farm level in two focus areas

Poland has the largest agricultural area within the Baltic Sea drainage basin and reducing the risk of phosphorus (P) and nitrogen (N) leaching from Polish soils to water is therefore essential. Increased acidity is known to reduce soil fertility and may trigger P leaching from non-calcareous soils. As part of advisor training, 25 farms each in Pomerania and north-western Mazovia were visited and 1500 ha arable soil, including 180 ha grassland soil, were monitored in 2013-2014. The soil was mainly coarse textured, but 25% of the Pomeranian farms were dominated by silty or clayey soils. More or less regular drainage systems were found on 20% of the farms, while 50% had simpler, older (>30 years) systems with a few single pipes. The farmers often used only ammonium sulphate or other acidifying N mineral fertiliser. Median pH on the Pomeranian farms, analysed in potassium chloride solution [pH((KCl))], was 5.2 and liming was advised for fields on most (72%) of these farms. Soil P content, measured by double-lactate extraction (P-DL), was positively and significantly correlated (Pearson coefficient 0.57; p < .001) with soil pH and was generally higher for pig farms. Mean P-DL (P given in elemental form) tended to be lower on dairy farms and arable farms and was significantly lower (mean 51 mg P-DL kg(-1) soil) on mixed farms (with just a few cows and poultry) than on pig farms (mean 122 mg P-DL kg(-1) soil). Farm-gate balances indicated deficits of P and potassium (K) on many of the small mixed farms in Pomerania and the soil can be expected to be nutrient depleted. In contrast, the pig farms demonstrated surplus farm-gate P balances (mean 27 kg ha(-1)). The P leaching risk is discussed relative to soil threshold values and to results from Swedish long-term field experiments.