Abstract: 【Objective】Study the response of maize (Zea may L.) to progressive drought and soil drought degree formulation and the threshold of red soil.【Method】Summer maize was planted in field plot with rainproof shed, and six treatments were disposed in which soil drying form 12 days to 36 days. The soil water contents in 0-60cm layers were measured daily with FDR, and maize plant morphic features and leaf stomatal conductance are also investigated periodically. We checked seed and measured grain yield after harvest.【Result】The plant height, stem diameter, leaf stomatal conductance and root number decreased significantly after different soil drought days, namely, the threshold response presented to special soil drought conditions. Five percent grain yield reduced significantly after soil drought lasted 25 days. In soil drying development, the relationship between the soil layer water storage and the cumulative relative water loss was expressed as a logarithmic linear equation. We transferred the slope to an index, notated as D, which spans in the interval [0, 1], to explain the soil drought degree.【Conclusion】The index D is an indicator both for soil drought degree and crop water stress. Maize yield decrease significantly when red soil drought degree D reached 0.55 in the elongation-heading stage.

Key words: Red soil, Soil drought degree, Maize, Threshold response