Long-term effects of short-fallow frijoltapado on soil phosphorus pools in Costa Rica

Many Costa Rican farmers have shortened the fallow period associated with frijol tapado, a slash/mulch bean (Phaseolus vulgaris L.) system, from 3 or more years to only 9 months. This study determined the long-term effects of continual short-fallow (9 months) frijol tapado on soil phosphorus (P) fractions and assessed the importance of these fractions for crop nutrition. It also examined the effect fallow length had on biomass production and mulch P, when fallow included established, dispersed trees. The study was conducted on three bean sites that had been cropped using short-fallow frijol tapado for more than four decades and on a virgin forest site. Prior to seeding in 1999, one bean site was fallowed for 9 months, another for 21, and the third for 33. There were established, dispersed trees within each. Samples of surface soil (four divisions to 30 cm), mulch cover, and living vegetation were collected before slashing, after seeding, and at harvest. Plant material and soil samples were analyzed for total P; soils were subjected to sequential P extractions. Results demonstrated that, compared to the virgin forest soil, soils which had been cropped with short-fallow frijol tapado for decades were depleted in top- and subsoil stable organic P (Po) and labile inorganic P. The bean soils had not accumulated P in other fractions, indicating that substantial fractional redistribution had not occurred. Of the soil P fractions, NaOH Po made the most important contribution to plant nutrition during the cropping season in the 1- and 2-year fallow bean sites. The 1- and 2-year fallows supplied similar quantities of mulch and mulch P. The dispersed trees probably played a role in this, and lower P concentrations in the older fallows' mulches also helped equalize total mulch P values. Additionally, mulch variability, before and after slashing, in the 2- and 3-year fallow sites obscured what differences there may have been. The exceptionally steep slope of the 3-year fallow site (92%) overwhelmed other effects and prohibited meaningful comparisons with other sites.