Fallow enrichment with pada (Macaranga denticulata (Bl.) Muell. Arg.) trees in rotational shifting cultivation in northern Thailand

Shifting cultivators in Thailand widely attribute the maintainance of crop productivity to pada (Macaranga denticulata (Bl.) Muell. Arg.), rotation cycles having become much shorter than the customary 10-20 years. This paper examines the use of pada in a 7-year rotation on an acid soil with low available soil P (2-4 mg kg-1 by Bray II). Dense pada patches in 7-year-old fallow averaged 43 tons ha-1 of above ground biomass, 20% more than sparse patches. The biomass in dense pada contained disproportionately more P, K, Ca and Mg (34%, 92%, 80% and 107% more, respectively) than in sparse pada patches. Slashing and burning 7-year-old fallow with dense pada produced a subsequent rice yield that was three time that with sparse pada. Rice grown after dense pada had been slashed and burned after three years yielded less than one third of that after a full 7-year rotation. It is, as yet, unclear how rice yield in dense pada patches is enhanced in the full 7-year rotation. Nutrient concentrations in the mature rice were generally either the same or higher in the sparse than dense pada patches. In dense pada patches rice accumulated twice to four times as much nutrients as in sparse pada patches, and a much larger fraction of the nutrients was stored in the fallow. Uptake of nutrients in the sparse pada patches may have been limited by some factor that either governs availability of the nutrients released by burning or depressing rice growth and so its nutrient demand.