The Nelson Mandela long-term comparative organic farming systems research trials: baseline study and trial design.

The Nelson Mandela long-term comparative organic farming systems research trials (the Mandela Trials) were planned and established in 2014 with peer review from the three sites where organic comparative research has been ongoing for the past 20-30 years (Switzerland, Denmark and Pennsylvania in the USA). Organic and conventional farming systems, as well as monocrop and rotational vegetable production, were compared; crops grown were cabbage (monocrop and rotational), with sweet potatoes and cowpeas as the rotated crops. Owing to variation of soil fertility on the research trial site, an indicator crop (Caliente mustard (Brassica juncea)) was planted on the long-term research trial site before any treatments commenced, in order to quantify variation in soil fertility and plant growth. Variation between replications accounted for much of the variability, showing a fertility gradient across the site. Correlations were found in soils between soil pH and available soil P, and between available soil P and leaf P content in the crop. Replication 4 was found to have lower yields and soil fertility than the other three replications, especially plots 37, 38, 39 and 40, which were also found to have some building rubble below the 15 cm of topsoil. Soils were generally acidic and low in available soil P, due largely to high levels of exchangeable Al. It was decided that all plots would receive 1 t/ha of dolomitic lime each year for the first 3 years of the research trial, and that although all four replications would be measured and analysed, Replication 4 might produce atypical results, in which case results from this replication might have to be regarded as outliers in future analysis.