Irrigation Scheduling Strategy
Soil moisture after planting and before flower initiation can generally be depleted to 50 to 60 percent of the total available water holding capacity within the root zone before an irrigation is necessary. Keeping it wetter may discourage adequate root development. Dry beans have a normal effective rooting depth of two feet, but on many irrigated sandy soils in Minnesota, the rooting depth may only be 12 to 15 inches. It is estimated that 85 percent of the soil water used by the bean plant is taken from the top two-thirds of the rooting profile. This rooting depth is generally reached between 30 and 35 days after emergence.
Typically, the first irrigation will be needed about 5 to 10 days before flower initiation. At this time the soil profile should be completely recharged and then kept between field capacity and 40 to 50 percent depletion (60 to 70 cbs. of soil tension) until upper pod filling.
Additional irrigation during this period should be applied only when the soil dries to 40 to 50 percent depletion (60 to 70 cbs. of tension in top 12 to 15 inches of loamy sand or sandy loam soils). When the average of the sensors near the starting point in the field reaches the desired level (60 to 70 cbs.) then irrigation should be initiated and enough water applied to replenish the soil profile.
If the soil is a very light texture, it may be necessary to start irrigator at a lower soil tension to avoid stressing the farthest end of the irrigated field. Initiating more frequent irrigations will maintain a too wet soil surface and wet foliage, which could cause greater incidences of white mold development.
When bean pods are nearly filled, soil water can be allowed to dry to 50 to 60 percent depletion before another irrigation is needed. Irrigation can typically be stopped when 50 percent of the leaves are yellowing on the plants. Once bean seeds have begun to dry in the pod, soil water is no longer taken up by the plant.
If the plants have a shallow root system because of root rot or other rooting barriers, the suggested irrigation scheduling strategy should be modified to encourage more frequent and lighter irrigation to meet the crop water needs.
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