Soil C–N–P status and pea yield under organic and conventional farming in a long-term field experiment

Field pea (Pisum sativum L.) is a key grain legume in cool-temperate organic and conventional systems, but its performance depends strongly on soil organic matter and C–N–P balance. We used a long-term experiment on a Stagnic Luvisol at Eerika, Estonia, to compare organic (Org 0–II) and conventional (Con 0–III) systems with contrasting histories of winter cover crops, composted cattle manure and mineral fertilisation. Pea yield (standardised to 140 g kg–1 grain moisture) from 2012–2016 was related to topsoil (0–20 cm) sampled in mid-April 2013–2017 for soil organic carbon (SOC), total nitrogen (TN) and plant-available phosphorus (P-AL); C/N, C/P and N/P ratios were calculated. Organic systems had 11.7% higher SOC and 23.8% higher TN than conventional systems, while P-AL differed little. Organic treatments showed lower C/N and higher N/P, consistent with N-richer organic matter and potentially stronger P-related constraints. Pea yields were broadly similar (Org 1.89–2.85 vs Con 1.80–2.79 t ha–1). Yield–soil relationships were stronger in the conventional system, especially for SOC, TN and P-AL, and C/N and N/P provided additional indicators of nutrient-related risks. Mean C/N and N/P were 12.0 and 12.5 in organic treatments versus 13.2 and 10.8 in conventional systems.