Fertigation is the process in which soluble nutrient fertilizers are applied to crops after being blended with irrigation water. Vegetables crops produced in the Lower Colorado River Region are typically irrigated with sprinkler and furrow irrigation systems. Sprinkler irrigation systems are used during the germination and stand establishment phase of most vegetable crops grown in the region. All subsequent irrigations are applied with furrows. There is a limited acreage of vegetable crops grown in the region where season-long sprinkler irrigation is practiced, and an even smaller area irrigated with drip systems. This page will address aspects of fertigation under each irrigation system.

Furrow irrigation systems

All lettuce types grown in the region, broccoli, cauliflower, and celery are irrigated by furrow following early season use of sprinkler systems for stand establishment purposes. With sound management, the furrow irrigation systems in the region can attain acceptably high levels of efficiency (8, 88). Factors contributing to higher furrow irrigation efficiency levels in the region are discussed elsewhere in greater detail (88). However, a summary is provided here: precision land leveling of crop fields to the desired slope (often furrows have a zero-degree slope), low infiltration rate soils, compacted furrow beds (leading to further reduction in infiltration rates), relatively large inlet flow rates, and short furrow lengths (seldom exceed 600 ft). The net effect being faster advance rates, and hence, uniform irrigations (8). Uniform irrigations, in turn, can result in uniform fertilizer applications provided the fertilizer solution is well mixed, and the metering device is accurate. Uniform irrigations can lead to higher application efficiencies provided the duration of irrigation application is timed correctly.   

During thinning and cultivation operations, the sidedress method is used for most in-season nitrogen (N) applications in furrow irrigated systems. However, there are occasions where supplemental N application could be needed later in the season. The need for applying N fertilizer later in the cropping season can arise from greater-than-anticipated N leaching of fertilizer applied earlier in the season by above-average rainfall, less than ideal irrigation management (such as longer irrigation durations than required and longer than optimum irrigation intervals resulting in the development of preferential flow paths at the time of irrigation), and/or production on relatively coarse textured soils than are typical for furrow systems in the region. Under conditions where late season supplemental N applications are required, crop size could interfere with tractor-applied sidedress operations. Therefore, fertigation is the preferred option. 

During N fertilizer fertigation, a feed tank containing a liquid fertilizer solution  is typically placed in an elevated platform, often a tractor trailer. The fertilizer solution is then metered into a field supply canal through a hose which is connected to a box fitted with a float valve arrangement that allows injection of the fertilizer solution into the canal at a near constant rate at the inlet end of the field. The N fertilizer injection/delivery rate is calibrated based on anticipated time to complete the advance in furrows, the furrow plan view (furrow spacing multiplied by furrow length), the N fertilizer application rate, and the number of furrows irrigated at a time. As noted above, on most of the loam to clay loam soils used for vegetable production in the region, the lower infiltration rates coupled with the relatively high inflow rates and short furrow lengths lead to short advance times, and hence, to uniform irrigations/fertigation (8).

Decisions concerning the need for additional N fertilization, and the amount to be applied, later in the season should be based on pre-plant soil nitrate-N testing. For N applications later in the season, the lower threshold soil nitrate-N should be 20 ppm. The N fertilizer requirement can be calculated as the difference between the minimum threshold soil nitrate-N and the measured soil test nitrate-N level. Approximately 4 lbs of N/acre needs to be applied to raise the soil test nitrate-N level by 1 ppm.

Sprinkler Systems

Solid-set sprinklers are almost universally used in the stand establishment phase of furrow irrigated crops. Early season sprinkler irrigation can then be used to apply N fertilizers, when soil test nitrate-N levels are low and pre-plant fertilization is insufficient.  

Fertigation management for crops that are irrigated with sprinkler systems throughout the cropping season, consisting mainly of baby spinach and baby lettuce, is addressed in section 3.5. However, there is a smaller acreage of other crops including romaine hearts, Boston and leaf lettuce, bok choy, and Napa, grown on wider 80-84 inch beds under season-long sprinkler irrigation. For these crops, all in-season N requirements are applied through fertigation with sprinkler systems. As a result, N application rates can be better matched with N requirements throughout the season. 

Seasonal water requirements are typically applied in four to six events suitably distributed across the season, and potentially each irrigation represents an opportunity for fertilizer application. Generally, about 25% of the N fertilizer requirement should be applied over the first third of the cropping season, 35% during the middle third, and 40% of the total seasonal N requirement needs to be applied during the final third of the season. The actual rate applied in each fertigation event should be guided by in-season nitrate-N testing. For applications later in the season, the minimum threshold soil nitrate-N level for should be 20 ppm. The N fertilizer requirement is equal to the difference between the minimum threshold soil nitrate-N and the measured soil test nitrate-N level. Approximately 4 lbs of N/acre needs to be applied to raise the soil test nitrate-N level by 1 ppm.

The Arizona Leafy Greens marketing agreement (ALGMA) requires the treatment of all overhead irrigation water applied within 21 days prior to harvest (89). The water treatments include oxidants such as hypochlorite and peracetic acid. However, because many fertilizers are not compatible with these water treatment chemistries, the ALGMA allows for a window for fertigation to take place preceding water treatment but with specific protocols for testing irrigation water and crop product if needed. When the sprinkler fertigation occurs 21 days prior to harvest, these protocols have to be followed for compliance with the ALGMA.

Subsurface Drip Irrigation

Cantaloupes and watermelons account for most of the acreage irrigated with subsurface drip systems (Section 3.11), although very small acreages of other vegetables are grown under drip irrigation. The reasons for the rather low acreage of drip irrigation systems in vegetable cropping systems in Arizona are summarized elsewhere (9,88). Results of evaluation of drip irrigation systems under Arizona conditions where vegetables are grown on fine textured soils, mainly in the Yuma area, showed that they generally do not have sustained economic or water conservation advantages over furrow irrigation systems. Other concerns with the use of drip systems in the region include non-uniform plant emergence due to uneven soil moisture distribution, salinity problems associated with shallow salt leaching depth, and the unsuitability of the system for the complex crop rotations practiced in Yuma area which requires reconfiguring the field prior to planting each of the different crops in the rotation (88). However, drip irrigation may offer advantages if the vegetable crop industry expands to areas where coarse textured soils, which are challenging to irrigate efficiently with furrow systems, predominate. Drip irrigation could also potentially help improve efficiency levels in other parts of Arizona where soil texture, field length, and available flow limit attainable levels of efficiency with furrow irrigation methods. 

In areas where drip irrigation can be used, N fertigation can be implemented in a manner that better matches crop uptake through the season (90, 91). The actual rate of application should be guided by in-season nitrate-N testing. Later in the season, the minimum threshold soil nitrate-N concentration should be 20 ppm nitrate-N. The N fertilizer requirement is equal to the difference between the minimum threshold soil nitrate-N and the measured soil test nitrate-N level. Approximately 4 lbs of N/acre needs to be applied to raise the soil test nitrate-N level by 1 ppm. Alternatively, one might use plant N tissue testing outlined guidance. Although the author did not find plant tissue testing to be a sufficient prognostic guide in the typical sidedress N systems, it has been reported to work well under buried drip production systems (77).