Modeling the response of crop emergence to sowing depth and soil water deficit in direct-seeded rice

Dry direct seeding of rice in the tropics often suffers from poor crop establishment owing to soil water deficit. A potential solution is sowing deeply to utilize residual soil moisture farther below the surface. We evaluated rice emergence under various sowing depths and soil moisture conditions and tested a model framework of it as a simultaneous function of sowing depth and soil moisture.

We combined data from three field experiments and one growth chamber experiment to collect emergence data for four rice cultivars (Dontokoi, Dular, Rc222 and Rc420). We independently parameterized the relationships between emergence and sowing depth or soil water tension using logistic functions. We expressed the final emergence as the product of the two functions.

Emergence responses to sowing depth and soil moisture fitted the cultivar-specific logistic functions well. For Dular, a cultivar that tolerates deep sowing, emergence was greatest when sown at 4 to 5 cm below the surface under soil water deficit, versus 1 to 2 cm under wet conditions, and our combined model successfully reproduced this result.

Our emergence model framework supports adjustment of sowing depth to account for available soil water, making the model a powerful new tool for drought adaptation in direct-seeded rice.