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The critical value of soil Olsen-P (mg Olsen-P kg-1) is the value of soil Olsen-P above which the probability of crop response to P fertilization is small or null. The aim of the present study was to determine the critical value of soil Olsen-P for potato tuber yield in volcanic soils. The data used in this study were taken from 41 experiments of P fertilization conducted in southern Chile from 1977 to 2015 by the National Potato Center at the Instituto de Investigaciones Agropecuarias (INIA). To identify the critical values of soil Olsen-P, relationships between relative yields and initial soil P-Olsen were assessed using three models (linear-plateau, boundary and Mitscherlich function). Across experiments, the average fresh tuber yield was 41.9 Mg ha-1 and varied between 4 and 89 Mg ha-1. Fresh tuber yield decreased up to 92% by P deficiency; however, yield reduction was concentrated between 20 and 40%. The critical Olsen P values determined were 41, 33 and 11 mg Olsen-P kg-1 obtained (P < 0.01) with linear-plateau, boundary function and Mitscherlich function, respectively. However, linear-plateau and Mitscherlich functions showed lower coefficient of determinations (R2 = 0.23 - 0.31) than the boundary function (R2 = 0.97). Therefore, we suggest that the critical Olsen-P value of 33 mg P kg-1 determined with the boundary function is more appropriate for potato growers than the critical value determined with the other models. This information will be valuable for optimizing strategies for P fertilizer management in potato production systems aimed at achieving optimal crop yield and minimizing potential environmental risk.

In the frame of the Land Use/Land Cover Area Frame Survey sampling of topsoil was carried out on around 22.000 points in 25 EU Member States in 2009 and in additional 2 member States in 2012. Beside other basic soil properties soil phosphorus (P) content of the samples were also measured in a single laboratory in both years. Based on the results of the LUCAS Topsoil survey we performed an assessment of plant available P status of European croplands. Higher P levels can be observed in regions where higher crop yields can be expected and where high fertiliser P inputs are reported. Plant available phosphorus levels were determined using two selected fertilizer recommendation systems; one from Hungary and one from the United Kingdom. The fertiliser recommendation system of the UK does not recommend additional fertiliser use on croplands with highest P supply, which covers regions mostly in Belgium and the Netherlands. According to a Hungarian advisory system there is a need for fertilizer P input in all regions of the EU. We established a P fertiliser need map based on integrating results from the two systems. Based on data from 2009 and 2012, P input demand of croplands in the European Union was estimated to 3,849,873 tons(P2O5)/year. In the meanwhile we found disparities of calculated input need and reported fertiliser statistics both on local (country) scale and on EU level. The first ever uniform topsoil P survey of the EU highlights the contradictions between soil P management of different countries of the Union while also highlights the inconsistencies between reported P fertiliser consumption and advised P doses. Our analysis shows a status of a baseline period of the years 2009 and 2012, while a repeated LUCAS topsoil survey can be a useful tool to monitor future changes of nutrient levels, including P in soils of the EU., JRC.H.5-Land Resources Management