Soil nitrate and nitrite in different soil profiles of Peri-Urban and horticultural farm soils of Gaborone as indicators of nitrogen leaching.

Soil nitrite (NO2-) and nitrate (NO3-) along the soil profile can be used as indicators of nitrogen loss by leaching. Preliminary studies have shown that excess usage of fertiliser is common amongst farmers who were aiming to increase crop yield. As such a study to assess levels of nitrite and nitrate in soil profiles in relation to fertiliser use of peri-urban horticultural farms around Gaborone in Botswana was conducted at four farms in Matebele, Glen Valley (NAMPAAD (National Master Plan for Arable Agriculture and Dairy Development)), Kopong and Ramaphatle from 1st July 2013 to 30 May 2014. Soil profiles were sampled for nitrite and nitrate at different depths (0-30cm, 30-50cm and 5080cm).Factors that could alter the leaching ability of soil being soil pH and moisture content were also assessed. Generally in all profiles, moisture content increased with depth from 5.14% (g/g) at 0-30cm to 12.52 % at 50-80cm. Soil pH decreased with depth from 6.9 to 6.0 in Kopong, 7.1 to 6.3 in Matebele and 5.1 to 4.0 in Ramaphatle. Soil nitrite content increased from 0.406µg/g soil to 0.746 µg/g and from 0.009 µg/g soil to 0.018 µg/g soil with depth in Matebele and Kopong respectively and decreased with depth from 0.103 µg/g soil to 0.014 µg/g soil and from 0.06 µg/g soil to 0.03 µg/g soil in Glen Valley and Ramaphatle respectively. Soil nitrate content decreased with depth from 0.005 µg/g soil to 0.003 µg/g soil (Kopong),from 0.012 µg/g soil to 0.007 µg/g soil (Matebele) and from 0.038 µg/g soil to 0.006 µg/g soil in Nampaad respectively with the exception of Ramaphatle where it remained constant down the soil profile (0.01 µg/g soil). Glen Valley soil had higher nitrate content (030cm) of 0.38ug/g soil, which was more than both Matebele (0.012 µg/g soil) and Kopong (0.005 µg/g soil) soils. Higher moisture content at greater soil depths explains the presence of nitrite and nitrate ions owing to the mobility in solution. Field tests have revealed that Glen Valley soil is clayey, as such leaching of nitrite and nitrate is lower due to predominance of micropores compared to sandy loams soils of Kopong and Ramaphatle, thus higher nitrite and nitrate content on the surface soil. The study indicates an increase in soil nitrite and nitrate with depth in soils that have higher moisture content in the lower parts of the soil profile. Thus overwatering in these study farms is resulting in leaching of soil nitrogen and could result in contamination of ground water.