Nutrient imbalances in degraded soils of North Vietnam
There is large field-to-field variability in terms of crops, cropping practices, fertilizer use, and soil fertility status Asia’s irrigated and favorable rice systems where farms are small and mechanization is limited. And there is the more permanent variability in soil properties in relation to the parent material affecting soil nutrient supplies and other factors of relevance for crop production. Consequently, significant spatial variability in crop nutrient needs can be expected, here defined as a crop’s demand for the external supply of inorganic fertilizer nutrients to achieve high and sustainable yield. With this project, we hope to contribute to the discussion on how to deal with the spatial variability of crop nutrients needs in the calculation of site-specific fertilizer recommendations using a case study with small-scale rice farming in North Vietnam as an example.

Past research on degraded soil in North Vietnam indicated imbalances of soil Ca, K, and Mg. Yield response of rice and maize to fertilizer application of N, P and K are limited by these imbalances, but data on the magnitude and spatial distribution of imbalances are scarce. A detailed soil survey in three key production areas covering about 103,000 ha of degraded soil cropped to rice and maize will be used to develop new, more balanced fertilizer programs. The study will also be used to develop more cost effective soil survey strategies to be employed in other areas with degraded soil, where similar imbalances are suspected. Vietnam has several hundred thousand hectares of degraded soil.

Figure 1. Soil exchangeable K (cmol_c kg^-1) on 14,000 ha of degraded soil near Vinh Phuc. A response to fertilizer K is expected for yellowish and red areas representing levels of < 0.15 cmol_c exchangeable kg^-1 soil.