Impacts of soil management practices on soil fertility in potato-based cropping systems in the Bolivian Andean highlands

The central Andean highland plateau region (Altiplano) in Bolivia is a semi-arid region primarily inhabited by indigenous agricultural communities that have mixed livestock and potato-based cropping systems. The objectives of this research were to: (1) determine community perceptions of the effects of climate change on the soil quality of soil resources based on local soil classification systems; (2) assess the effects of changes in the crop rotation and fallow length on soil fertility and soil organic C and N; (3) to evaluate the relative first-year and subsequent residual effects of different strategies to improve SOM and increase soil fertility, including use of traditional and new organic and inorganic soil amendments, and (4) to assess a rapid and low-cost field test to improve N fertility management. Four representative communities in the Umala Municipality were selected for this research based on their relative elevation above sea level in this mountainous region. Two communities (i.e., San Jose de Llanga and San Juan Circa) were located at relatively low elevation and two (i.e., Kellhuiri and Vinto Coopani) from relatively high elevations. A household baseline survey and participatory workshops were conducted in each community to assess soil management practices, the indigenous soil classification system, and perceptions on climate change and its effects on soil management practices. Based on results of this survey, a study was conducted to assess the effects of cropping and decreasing lengths of fallow periods on soil organic C, total N and other soil properties in the crop rotation in order to determine if increased soil degradation had occurred due to the decreasing fallow length. On-farm trials were conducted for three growing seasons from 2006 to 2009 in each community to determine the agronomic effects for growth of potato (Solanum tuberosum L.) in the first year and quinoa (Chenopodium quinoa Willd.) in the second year of an unfertilized control and separate and combined applications of local and alternative organic sources (i.e., composted cow and sheep manure, household compost and Biofert, a solid biofertilizer), and inorganic fertilizer (diammonium phosphate+urea applied at 80 kg N ha-1, 120 kg P2O5 ha-1, and 0 kg K2O ha -1). The manure treatments were applied at a rate of 10 Mg ha -1 (wet weight basis with an average dry matter (DM) of 73%), the compost treatment was applied at a rate of 5 Mg ha-1 (wet weight basis with a DM of 59%), and the Biofert treatment was combined with the manure and compost and applied at an application rate of 0.2 Mg ha-1 (wet weight basis with a DM of 88%). An additional study examined the use of a rapid and relatively low cost field test (i.e., the Cardy nitrate meter) to determine N status of potato by analysing petiole sap nitrate at blooming time. Results of the survey indicated that farmers have well-developed indigenous soil classification systems largely based on differences in soil texture, color, the presence of stones, depth of the plow layer and crop productivity. A general change perceived by community members in most cropped lands was the increase in soil erosion and the consequent reduction in soil productivity. These changes were primarily attributed to increased frequency and intensity of wind, seasonal concentration of rainfall in fewer months and an increase in air temperature. Farmers also perceived a decrease in the length of the fallow period over the past 25 years, and this length reduction was considered one factor that contributes to reduction in soil quality and other soil characteristics. An investigation into the effects of cropping and fallow on soil fertility restoration indicated that cropping generally decreased total and active soil organic C, and total, inorganic and active soil N. In contrast, the practice of fallowing restored total and active soil organic C and total and active soil N more rapidly in the upper communities than was observed in the lower communities. This difference was mainly attributed to differences in soil properties and land management in cropped fields at both elevations. (Abstract shortened by UMI.)...