An experimental irrigation tool for creating a water gradient across soil depths under terminated rainfall conditions

An experimental irrigation tool to research the response of plants to soil drying from deep to shallow soil layers observed under limited rainfall conditions was tested. Water was supplied from the tips of 4-mm-diameter tubes measuring 0.1, 0.3, 0.6, and 0.85 m in the length inserted into the soils. Each tube was set from shallowest to deepest at intervals of 1.5–3.0 m in the field. Control by the irrigation system was evaluated in rice and spring wheat fields under rain-protected shelters, where two cultivars with different root depths were planted. The irrigation tool created a series of different soil water profiles in which wet soil layers decreased with increasing irrigation depth due to the inserted tube lengths in both fields. In fine sandy soils more than sand, continuous decreases in soil water content were well simulated. Leaf conductance, leaf green color (SPAD value), and aboveground plant dry weight in both fields decreased with increasing irrigation depth in shallow root cultivars to a greater extent than that in deep root cultivars. The lower reducing effect of deep irrigation on plant dry weight in wheat compared to that in rice suggested that the irrigation depth based on tube length should be changed depending on the root distribution traits in the tested crops. These results indicated that the irrigation tool can conveniently simulate a diverse and continuous soil water content profile closer to that of an actual water gradient under limited rainfall conditions and contribute to studies on crop responses to different soil water absorption traits.