Soil fertility status of 20 seed production areas of Tectona grandis Linn. f. in Karnataka, India

The seed production area (SPA) is an improved plantation managed for production of quality seeds for a large-scale plantation programme. The soil nutrient is one of several factors affecting seed production among SPAs. The status of soil nutrients and their effect on seed production are poorly understood. Hence, the present study was undertaken in 20 seed production areas located in different seed zones of Karnataka, South India. Results showed that there was a greater variation among SPAs in various soil properties like soil pH, organic carbon, available NPK (nitrogen, phosphorus and potassium). Some of the studied parameters recorded significant variations among three different depths: 0-20 (top), 20-40 (middle) and 40-60 cm (bottom). For instance, organic carbon and available potassium showed significant variations at different depths, where the highest content was recorded in the top layer, followed by middle and bottom layers. Considering associations between soil properties and tree growth, organic carbon was positively associated with dbh (diameter at breast height; r = 0.500), stem roundness (r = 0.351) and stem volume (r = 0.250). Similarly, available nitrogen positively influenced the stem volume (r = 0.250). Though the fruit yield varied among SPAs, none of the studied soil parameters showed a significant influence on fruit yield indicating that some other factors like genetic ones, phenology, rainfall overlapping with peak flowering might control it. Data on site quality showed that all existing SPAs studied were growing in poor site conditions, however, this could be one of the factors affecting overall seed yield among SPAs. Hence, it is recommended to undertake a few important silvicultural interventions like application of fertilizer/organic manure, soil working, spraying of floral hormone and others to improve the existing seed production level.