In order to comprehensively study the root spatial distribution in soil, a monolith method (Böhm, 1979) was used to harvest roots on 93 DAS, on which date the maize plants were at the grain-filling stage and had the largest root system as shown in our preliminary experiment. Soil cubes with 5 cm sides (125 cm3) were dug one by one in a soil volume of 60 cm × 50 cm and a depth of 55 cm (two plants, with a distance of 33 cm between plants). The total number of the monoliths for the two plants was 2640. Each soil block was placed in a separate plastic bag and marked with the spatial coordinates.