07 May 2013

Fertilizers: Organic vs. Inorganic

Do organic amendments (OA) improve yield trends and profitability in intensive rice systems? The following conclusions emerged after analyzing yield trends of 25 long-term experiments (LTE) with rice-rice (R-R) and rice-wheat (R-W) systems across a wide geographical range in Asia.

Regular use of either manure or straw did not improve grain yield trends in R-R and R-W cropping systems.

Overall, yield trends in treatments with application of farmyard manure (FYM) were not statistically different from those treatments with recommended NPK application only, either in single cropping seasons (Figure 1) or the total annual grain production averaged over the entire duration of the LTE.

Figure 1. Relationship of rice and wheat yields between treatments with recommended NPK application (NPK) and treatments receiving organic amendments. Values shown are mean treatment yields of each crop sampled at each site.

Second, use of manure or straw in these cropping systems may be profitable provided the OA are used as a complement to a recommended dose of inorganic NPK.

In other words, OA are not a full substitute for mineral fertilizers and modern, intensive rice cropping should not rely on use of either manure or straw as the primary nutrient source. However, a better assessment of the relative profitability of OA in irrigated rice systems and upland cropping systems is needed to determine the optimal use of potentially scarce supplies of FYM (straw being always available on-farm).

Third, current experimental designs to assess the suitability of OA need to be improved to allow better comparison of the relative agronomic and economic advantages of inorganic and organic fertilizers.

This includes a better understanding of the benefits of OA in terms of improved soil organic matter and soil physical structure. The major shortcoming of current designs is that they do not properly adjust mineral fertilizer rates to account for the macro- and micronutrient input from OA. As a result, the break-even cost estimates in this analysis are in the upper bounds on the true profitability of OA.

Source: Dawe et al. 2003. Field Crops Res. 83:91-213.

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