June 26, 2009

With corn in much of Nebraska currently in the V8 to V12 growth stages and rapidly growing, crop water use and nitrogen (N) uptake are increasing rapidly as well. Many irrigated growers are starting to closely monitor crop water use and soil moisture conditions in order to anticipate when to begin irrigation.

Figure 1. Corn plants need increasing levels of nitrogen from the VE to R6 growth stages. It will use at least 50% of its total nitrogen need before irrigation even starts.

Fertigation is an efficient method of supplying part of the N needed for the crop through the irrigation system, near the time of maximum nitrogen uptake. Figure 1 illustrates the typical pattern of N uptake by a corn crop. While this pattern and amounts of uptake will vary slightly with hybrid, the most rapid period of N uptake is between V8 and VT growth stages. During this time a steady supply of N is critical to insure optimum yield.

In most growing seasons and for most soils in Nebraska, there is adequate soil water and precipitation so that irrigation isn't necessary until late June or early July, when much of the crop may be in the V14 to R1 growth stages. Thus, don't rely on fertigation to provide all the crop's nitrogen need — the growing crop will need significant N before irrigation is likely to begin. On average the crop might take up as much as 50% of its total N before irrigation starts (Figure 1).

Timing Fertigation

Nitrogen fertigation for corn generally should begin with the first irrigation and be complete by the R1 to R2 growth stages. Application rates of 20-30 lb N/acre per irrigation event are recommended. However, higher rates of up to 50 lb N/acre per irrigation are unlikely to cause crop damage because the fertilizer is diluted in water. Producers should insure that water is applied uniformly, without runoff, to insure even distribution of N. Generally, fertigation is not recommended for furrow irrigation because of the likelihood of uneven water application.

Urea-ammonium nitrate (UAN) solution is the most common N source for fertigation, though ammonium thiosulfate (ATS) also can be used, particularly in sandy soils likely to benefit from sulfur (S) fertigation. Anhydrous ammonia is not recommended for fertigation in Nebraska, as there is potential for precipitates to form and plug nozzles when ammonia is injected into water with dissolved calcium and magnesium.

Disadvantages of Fertigation

While fertigation is efficient because N is applied just when the crop needs it, there are potential disadvantages as well.

• Nitrogen solution fertilizers are generally more expensive per pound of N than anhydrous ammonia or urea.
• The irrigation system must be set up with appropriate check valves, injection pumps, and fertilizer tanks.
• The operator must be certified for chemigation.
• Rainy weather can delay the need for irrigation, thus delaying N application during critical growth stages as well.

As-needed Fertigation

While most growers use fertigation to apply a pre-determined amount of N based on recommendations, fertigation does allow N application on an as-needed basis, depending on crop N status. A chlorophyll meter, aerial imagery, or crop canopy sensors can be used to assess crop N status in-season. For more information on these methods, see

• Using a Chlorophyll Meter to Improve N Management (NebGuide G1632)
• Site-Specific Nitrogen Management for Corn (Extension Circular EC163)

Richard Ferguson
Extension Soils Specialist