For maximum dry beans yield potential, dry beans will use about 14 to 18 inches of soil moisture during the growing season, depending on the cultivar. With good management, irrigation can supplement rain to provide optimum soil moisture conditions throughout the growing season.
For those years where the growing season-available water (soil moisture plus rain) is less the 14 to 18 inches, each additional inch of water provided by irrigation will result in an additional 150 to 300 pounds of yield, depending on bean class and cultivar.
Daily dry bean water use or evapotranspiration (ET) depends on the stage of growth, local weather conditions, available soil moisture, disease pressure and soil fertility. Plant architecture (Type I, II, III and IV) also will affect the daily dry bean water use. Generally, the larger, bushier dry bean types will use more water than the shorter, narrow types.
The frequency and amount of irrigation depends on the growth stage of the beans (which determines the daily crop water use), the water-holding capacity of the soil in the root zone and the prevailing weather conditions.
Dry Bean Rooting Depth and Water Use
Beans are shallow rooted. Typically, in deep soils, roots grow laterally 8 to 12 inches and downward to a depth of 3 feet or more.
Root distribution is concentrated near the soil surface. About 90% of the roots will be found in the top 2 feet, which is considered the effective rooting depth for irrigation purposes. During the course of a growing season, only about 10% of the water used by the beans will be drawn from the soil below 2 feet.
Average bean water use rates will increase from about 0.05 inch per day soon after emergence to more than 0.25 inch per day during pod development (Figure 3).
The bean water use amounts include the evaporation from the soil surrounding the plants. Water use is a depth measurement because the assumption is that the beans remove soil water from under every square foot of soil surface in the field.
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Water-holding Capacities of Soil
The depth and water-holding capacity of the soil have a great influence on when and how often irrigation is required. Soil texture determines the amount of available water in the root zone (Table 19), although other factors such as organic matter and soil compaction will modify these numbers.
Note that the greater the water-holding capacity of the soil in the root zone, the less frequent the irrigation applications should be.
Knowing the soil texture and water-holding capacity of the dominant soil type in a field and using that information to make irrigation decisions is important. However, if different soil types are in the same field, irrigation scheduling should be adjusted based on the most drought-prone soil type.
Irrigation Water Management
Having a soil profile that is near field capacity at planting, which occurs naturally with normal fall and spring rainfall, together with some winter snowmelt, is highly desirable. Stored soil moisture lower in the root zone serves as a supplement during high water-use periods.
Beans planted on shallow soils (12 to 18 inches of top soil) underlain by coarse sand and gravel will have a reduced root zone. That means less soil moisture is stored and available to the plants. Sometimes, this requires applying less per irrigation (0.75 inch) more frequently, compared with deeper soils with greater water-holding capacity.
During the period prior to flowering and the period after the majority of the pods are full, dry beans are relatively drought-tolerant. They can withstand 50% to 60% soil water depletion without a significant impact on yields (Figure 3).
However, during the flowering and pod-development period, soil moisture levels in the root zone should not be depleted more than 50% (preferably 40%) to achieve maximum yields.
The first irrigation should be applied when the soil moisture is between 50% and 60% depleted after emergence. With normal rainfall, this should be when the beans are almost at flowering. After flowering, irrigate before the soil moisture profile reaches 50% depletion.
Beans will mature properly if ample moisture is available during the vegetative growth stage (pre-flowering) and if the last irrigation occurs when the first pods are filling.
Late-season irrigations can delay maturity. If the beans are starting to dry, irrigation will not be needed because the beans no longer are removing much water from the soil profile.
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Most center pivots should be set to apply from 0.75 to 1 inch of water per revolution, which could take from 2½ to 3½ days. During flowering, monitoring the soil moisture profile frequently is critical because keeping up with the dry bean water use during periods of high temperatures and wind may be difficult.
However, if white mold is a concern with rainfall/irrigation occurring during flowering, another strategy could be to fill the soil profile with water prior to flowering and avoid irrigation until pods are set.
Some tips for good irrigation management of beans:
• Irrigations may need to be scheduled to minimize disease problems rather than maximize yield.
• Maturity may be delayed up to 15 days by letting the soil get too dry after planting, and yield potential also will decrease.
• Avoid excessively dry soil levels during the flowering stage. The shock of watering dry soil can cause flowers to fall off the plant.
• Do not irrigate when lower foliage on the plant is still wet from rainfall or irrigation.
• To avoid aiding the development of white mold, do not use light, frequent irrigations. Set your pivot timer to apply 0.75 to 1 inch each irrigation event. A wet soil surface contributes to white mold development, especially during flowering.
• Late-season irrigations may delay the final maturity date.
Irrigations can be terminated when at least 80% of the pods show yellowing and are mostly ripe. Another indicator is when 50% of the leaves are yellowing on the plant.
Determining when to start and stop an irrigation system is a very important part of irrigation water management. Soil in the root zone is the reservoir that stores water for a crop’s use.
Soil moisture levels in the root zone are the criteria used to determine when to start and stop irrigations. Several soil moisture monitoring tools are available to estimate the soil moisture level at a particular time and location in the field.
Soil moisture estimates can be done several ways. The “soil feel” method is the most widely used. It involves using a soil probe to obtain a soil sample from specific depths in the root zone, and the amount of soil moisture is estimated by squeezing the soil in the palm of your hand. Accuracy of the soil feel method improves with experience.
Soil moisture also can be measured using tensiometers, soil moisture blocks and other devices. When these are used, typically a measurement site may have two sensors, one at 12 inches and one at 18 to 24 inches below the soil surface.
The soil moisture level is estimated by reading a gauge on the device or with a portable meter. These devices only indicate the soil moisture status at that particular location.
Measuring soil moisture for irrigation scheduling requires a high level of dedication by the irrigation manager during the growing season. Informed irrigation decisions require reading soil moisture measurements two or three times per week. Measurements must be taken at several locations in the field and the readings recorded.
Another form of irrigation scheduling requires daily bean water-use estimates. This method, sometimes called the “crop water use replacement method,” is based on obtaining daily estimates of bean water use and measuring rainfall amounts.
Irrigations are scheduled to replace the amount of soil moisture used by the beans minus the amount of rain received since the last irrigation.
Along with rainfall measurement, these parameters help predict the time and amount of water needed to replenish the root zone to maintain proper soil moisture levels.
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