Irrigation strategies for vegetables

Before we dig more into different methods of irrigation scheduling, it is very important to understand the basics of irrigation and soil water attributes. Here are some common terms: 

Saturation – The water which readily percolates or drains out from the root zone by gravitational force. Also called gravitational water.

Field capacity – It is the amount of water that remains in the soil after all the excess water at saturation has been drained out. When the soil is allowed to drain for approximately 24 hours after saturation, field capacity is reached. 

Permanent wilting point – When plants uptake all the available water for a given soil, soil dries to a point that it cannot supply any water to keep plants from dying.

Available water holding capacity (AWC) –The amount of water that soil can store to be extracted by the plant. It is the water held between field capacity and permanent wilting point.

Management allowable depletion (MAD) –The soil water content where crops begin to experience water stress. Usually, most of the crop does not experience water stress before 40-60% of AWC has been removed.

Soil water deficit –The amount of water removed by the crop from active rooting depth.

Early season strategies

In the spring, always make sure the soil in the germination and early-growth root zone is moist. If necessary, irrigate to wet this zone.

• As the seedlings germinate and develop, moist soil is necessary for proper root development, as roots will not grow through a dry layer of soil. A dry layer will result in a shallower rooting depth than desirable. 
• Hold back on irrigating during the early vegetative growth stages to promote deeper root growth. This increases the opportunity to store rainfall when it occurs and decreases the risk of leaching valuable nutrients.
• For most crops, the soil water deficit can be as high as 60-65% of the plant-available water (water between field capacity and permanent wilting point) in the early growth stages without affecting the plant development.

Mid-season strategies

As the crop nears its critical growth period or its peak water-use period, reduce the soil water deficit to minimize the risk of not meeting the crop's water needs and causing yield losses.

• For most crops, the soil water deficit limit in the mid-season should be 30-40% of plant-available water.
• During these critical periods of high water use, regularly project the next two to three days of water needs to plan ahead and avoid stressing the crop before it’s irrigated.
• If you use drip tape on all of your fields but only have enough water pressure to irrigate some of your fields at once, project what the water deficit will be over the next few days and use this to determine when to start irrigating.
• Too much water during this period is also potentially harmful and can result in leaching. To reduce the leaching potential of a rainfall event, always consider the weather forecast when scheduling the next irrigation.

Late season strategies

As the crop nears maturity, you can generally increase the soil water deficit (60-65% for most crops) to greater limits without causing stress to the crop.

There are several methods of irrigation scheduling. While irrigating based on the feel and appearance of the soil are commonly used methods, more precise methods such as soil water monitoring or crop water demand calculations will allow you to produce healthier, more consistent crops.

*See Estimating soil moisture by feel and appearance method.

Volumetric water content is the volume of liquid water per volume of soil. It is usually expressed as a percentage. For example, 25% volumetric water content (VWC) means that in one cubic inch of soil, there will be 0.25 cubic inches of water. When this %VWC is multiplied by the desired depth (effective rooting depth measured in inches), you can determine the total amount of water present in the rooting zone.

For irrigation scheduling, the soil water content measured using soil moisture sensors (current soil water content) is compared to the soil water content at field capacity to calculate the total water depletion in the soil profile.

Soil water depletion (D) = soil water content at field capacity – current soil water content

Field capacity can be measured very easily in the field using soil moisture sensors: The VWC measurements provided by the soil moisture sensor after 12-24 hours of heavy irrigation or rain is the field capacity of the soil.

You also can get the field capacity and available water holding capacity (AWC) values from the USDA's Web Soil Survey.

For irrigation scheduling, it is also important to understand the amount of soil water depletion that causes crops to begin experiencing stress. In general, most crops begin to experience stress when soil water depletion is 30-50% of plant-available water. This is called management allowable depletion (MAD) or irrigation trigger point.

MAD can be varied depending upon crop, growth stage, and irrigation system’s pumping capacity and can be determined accurately through trials. For instance, take a note of soil moisture when you notice that your crops started experiencing stress. Irrigation should be started when the soil water depletion is equal to the management allowable depletion.

Most common soil water content or volumetric water content sensors

Electromagnetic sensors

• These sensors indirectly measure VWC based on the dielectric and electric properties of the soil medium (soil bulk permittivity or soil dielectric constant).
• The dielectric constant is a measure of the ability of the substance to store electrical energy.
• Since soil particles, water, and air, all have different dielectric constants, their ability to store or dissipate electrical energy is different.
• This is how it can be correlated to soil water content. Some examples of electromagnetic sensors are shown in the figures below.

Neutron probe

• These sensors measure volumetric water content at various depths with a radiation source.
• The operator must be licensed and receive special training to use.
• The unit is expensive and requires a lot of time to take field readings.

Case study

An example of irrigation scheduling using the volumetric soil water content sensors is shown in the graph.

• The soil water was monitored at a 12-inch soil depth for a period of 40 days. The field capacity (FC) and permanent wilting point (PWP) were 0.20 and 0.07 in³ in^-3, respectively ( in³ in^-3 is the unit of measure for volumetric water sensors).
• Estimated soil available water holding capacity (AWC =FC - PWP) is 0.13 in³ in^-3, and plant available water (PAW) at the 12-inch soil depth is 1.56 inches (PAW = AWC x Depth).
• Irrigation scheduling was set up to maintain PAW at 60% of AWC (i.e., 40% MAD), using the 0.15 in³ in^-3 water (Readily available water (RAW)) as the trigger point to irrigate.
• Whenever the volumetric water content from the soil water sensor in the 12-inch soil profile reaches 0.15 in³ in^-3 or close to it, irrigation of 0.6-inch is required to increase the volumetric water content by 0.05 in³ in^-3 to bring it back to FC (0.2 in/in).

During the 40-day period in the example, over-irrigation was avoided by irrigating only when it was required. However, heavy rainfall events increased the water content at certain times, which is unavoidable. In the above example, the irrigation water application was accomplished manually.

There are automated systems available that can irrigate based on water status.

Soil water tension indicates the amount of energy required by plant roots to extract water from soil particles. As water is removed from the soil, soil tension increases. Soil tension is expressed in centibars or bars of atmospheric pressure. When the soil is full of water, soil water tension is close to zero.

For example, in coarse-textured soils at 25-45 centibars, 50% of AWC is depleted, so in these soils, crops should be irrigated before the sensor indicates 25-45 cb.

However, soil tension measurements are soil specific and can be inaccurate, so depending on your crop and soil observations, soil tension limits should be refined.

• At the earliest indication of water stress, use your tensiometer to measure the soil tension and note the reading.
• In the future, make sure that you irrigate before it reaches that point.
• You can track your water movement by taking measurements right after an irrigation event.
• After irrigating, if your deepest sensor gives a zero reading, that means you might have irrigated more than required.
• If it shows no change from your reading prior to irrigating, you did not irrigate enough.

Laboratory-developed charts like the soil water deficit table can also be used to convert the tension readings from sensors to soil water deficit values.

The most common soil tension sensors

Tensiometer:

• These sensors are made from a porous ceramic tip sealed to the base of a water-filled plastic tube, sealed at the top with a removable airtight cap.
• A vacuum gauge connected to the tube measures the soil tension.

Electrical resistance sensors:

• These sensors indirectly estimate soil tension by measuring the electrical resistance between two wire grids embedded in a block of gypsum, plaster, or a special material that maintains its moisture content in equilibrium with adjacent soil.
• The electrical resistance within the block varies with soil water content.
• A manufacturer's calibration curve converts the reading to soil tension. Use the table below to estimate the soil water deficit for the specific soil.

*Soil deficit at 1500 cbs is equal to total available soil water capacity.

See Irrigation management strategies.

Sensor installation and placement

Sensors should be placed at several different depths and locations in the field. Typically, sensors are placed in pairs at one-third and two-thirds the depth of the crop root zone and at two or more locations in the field, preferably in the representative soil type away from high points, depressions and slopes.

If you have multiple crops, consider monitoring and managing each of your primary crops separately. For growers who have many crops (5+ crop families), options could include: monitoring each crop family, monitoring your most valuable crops, or monitoring groups of crops with similar rooting depths or water requirements. 

Some fields contain both heavy and light-textured soils. In those fields, it is recommended that each soil type be monitored and managed separately for irrigation. Field mapping technologies can be used to identify different soil, such as electromagnetic conductivity (EM) mapping. By identifying different soils (different water holding capacities), management zones can be created that can be managed separately.

High tunnels should be monitored separately from fields since evapotranspiration dynamics differ substantially between covered and uncovered environments. 

Where to place sensors

• Place stationary sensors between plants within a crop row at their desired depths.
• Flag the sensors so field equipment operators can see where they are and prevent damage to them.
• Make sure that the sensor is in direct contact with soil and there is minimal soil disturbance during installation.

How to install sensors

The method of installation is dependent upon the design of the sensor. Follow the installation instructions given by the manufacturer. In general, soil moisture sensor installation is done in one of two ways: 

• By digging a hole or a trench and installing sensors horizontally at different depths or
• By using an auger or soil sampling probe to bore a hole and install sensors vertically.

Care must be taken when drilling the hole. Do not install the sensor in an oversize hole as it may cause voids and air gaps. To prevent air gaps, some users use a mixture of soil and water (soil slurry) during the installation; however, in many cases, the structure of the slurry does not match the surrounding soil, which may adversely affect the sensor reading.

Other recommendations

• Use data loggers to store and log the data. This will help in data interpretation and quick decision making.
• Read the sensors every two to three days.
• Flag the location of the sensor for easy accessibility.
• Always irrigate to replenish the soil water to less than field capacity so that there is some room for potential rainfall.
• Consider your pumping capacity and application efficiency for scheduling irrigation.

Crop water demand

• The crop water demand method, also known as the crop evapotranspiration (ETc) method, consists of applying the daily ETc corresponding to the crop growth stage.
• This method uses the average daily ETc of previous years to determine the average volume of water required for each day.
• The calendar-based schedule also can be developed based on the ETc of the previous day, where the irrigation manager calculates the ETc from the previous day to determine the crop water requirements of the actual day.

Water budgeting method

• This method is the combination of ETc and the soil water monitoring method and is the most accurate of all methods.
• Irrigation events using this method are adjusted based on the ETc for each stage of crop development (crop water demand method).
• It is corrected using the dynamic water balance required by the soil water status method.
• The checkbook spreadsheet is one tool that can be used for water budgeting. 

See more information about the ETc and water budget method.