Potential utility of a six-month long fallow period and the application of slash
biomass from densely planted fallow species to increase soil phosphorus availability and
bean yield was evaluated on a phosphorus-limited Andisol in Costa Rica. The Mexican
Tithonia diversifolia genotype in monocultural fallow performed best for nutrient and
biomass accumulation, even though the nutrient concentrations in biomass were similar
among fallow species and for natural regeneration. In fact, phosphorus concentration in
Tithonia genotypes did not exceed 6 milligram of phosphorus per gram of biomass even
at high phosphorus availability (1 mM of phosphorus). Maximum biomass accumulation
occurs without excessive P concentration in the plant. In Tithonia genotypes, biomass
accumulation was positively correlated with root length density but not with leaf
phosphorus concentration. Higher root length density increases plant capacity to
maximize phosphorus acquisition, but root feedback mechanisms maintain an optimal
level of phosphorus within the plant. The six-month fallow period and the applied slash
biomass of fallow species had no consistent and significant effect toward conversion of
less available soil phosphorus fractions into more readily available phosphorus fractions.
However, bean cultivar yield was higher after the fallow period with the Costa Rican
Tithonia genotype in monoculture and the application of slash biomass from fallow
species; but nevertheless the application of slash biomass only increased potassium
concentration in bean tissue. Additionally, the application of slash biomass did not
increase the number of adventitious roots, but increased the number of root nodules in
bean cultivars.