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Topsoil nitrate was measured every few days under grass, sorghum and bare fallow at two sites in lower Embu District, Kenya. Concentrations were highest soon after the start of rain and within a week most nitrate had been lost. Under bare fallow and sorghum, nitrate concentration increased during dry weather and losses occurred when soil water content exceeded 10%. Modelling of soil water movement indicated that there was no leaching below the rooting zone, so denitri®cation may be the main route for loss of soil nitrogen. Application of nitrogen fertilizer early in the season is not recommended as it would be lost with the native soil nitrate. The use of phosphorus fertilizer at the phosphorus-de®cient site reduced soil nitrate concentration throughout the season and increased nitrogen uptake by sorghum. At the phosphorus-rich site, vigorous sorghum growth kept the soil nitrate concentration low, even though much nitrate was produced and lost in bare fallow. Adequate soil phosphorus is considered essential if the natural nitrogen resources of the soil are to be managed eciently.

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A survey was conducted to determine the occurrence of sweet potato leaf curl virus (SPLCV) infecting sweet potato in Kenya. Vine cuttings of sweet potato were collected in 2011 from fields in Central, Western and Coastal regions of the country. The cuttings were grown in an insect-proof screen-house and scion grafted to Ipomoea setosa indicator plants. DNA was extracted from leaf samples of graftinoculated I. serosa and tested for the presence of SPLCV by polymerase chain reaction (PCR) using degenerate and specific primers. The virus was detected in all the regions with incidence rates of 33, 17.0 and 2.6% from Western, Coastal and Central regions, respectively using specific primers. Coat protein gene AV1 from isolates collected from each region was sequenced and their comparison revealed >88.4% nucleotide identity. Phylogenetic grouping using nucleotide and amino acid sequences showed that four Kenyan isolates (Coast3, Central13, Coast15 and Western14) clustered together, while the other remaining five grouped a long with isolates from different parts of the world. The results reveal that Kenyan SPLCV isolates have a clear diversity and are also closely related with other isolates from different parts of the world. The wide distribution of the virus within the country means that urgent measures are needed to clean the farmers planting material to reduce possible losses incurred due to the presence of the virus.