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Brian Chatterton
I can remember visiting the south of Spain in the 1960s and olive monoculture seemed to be well entrenched even then. I was horrified to see the trees on little moulds of earth. This was the result of soil erosion over years and years of cultivation. The land between the trees had been cultivated to death and washed away with every heavy rain.
We have been managing our olives with a legume and grass cover for the last 27 years and I was pleased to read some interesting research carried out in Tuscany that confirmed the merit of the system. It reversed the commonly held belief that cultivation is important to conserve moisture for the trees rather than consumed by the weeds. The research showed that after many years of cultivation the rainfall penetrated at a miserable rate of less than 5 mm per hour while the figure for a legume and grass cover was around 40 mm per hour. The significance of this is that cultivation might conserve the moisture for the tress rather than the weeds but if the rain does not soak in there is no moisture to conserve for either.
Of course moisture is not the only story. The legumes add fertility to the soil which is released slowly to the trees.

Brian Chatterton. Italy.