Keeping fertigated nitrate-nitrogen in the crop rooting zone is an important management strategy to ensure maximum uptake. Nitrate-nitrogen is very mobile in the soil. It can move with water out of the rooting zone and down through the soil profile.

The timing of the nitrate-nitrogen fertilizer injection during the irrigation event is key to keeping it in the rooting zone and available to plants. Inject it too early and nitrate nitrogen can move beyond the rooting zone; injecting it too late means higher concentrations around emitters and uneven distribution in the root zone.

Californian researchers studied nitrate-nitrogen distribution from subsurface and surface drip systems in different soil textures1. The fertilizer was injected either at the beginning, middle or end of the irrigation event.

Their observations, shown in the figures below, demonstrate three important points to keep in mind when fertigating nitrate-nitrogen:

  1. Soil texture.
  2. Timing of the nitrate-nitrogen fertilizer injection during the irrigation event.
  3. Crop rooting depth at the time of fertigation application

Figure 1: Subsurface drip, sandy loam soil

•= drip line
Soil: sandy loam
Irrigation event: 27 hours
Coloured bar = relative nitrate-nitrogen concentrations (white = little or no nitrogen, blue = low, red = high)

A

Nitrate injected for 2 hours starting one hour after the irrigation event begins

  • little or no nitrate near drip tape (white area around drip tape)
  • band of nitrate around drip line
  • higher nitrate concentrations above drip line than below drip line Nitrate injected for 2 hours ending 1 hour before the end of the irrigation event

B

  • Nitrate accumulates around drip tape
  • Higher nitrate concentrations than when injected near the beginning of the irrigation event, figure A
  • Results were similar in loam and silty loam soils Nitrate injected during the middle half of the irrigation event

C

  • Nitrate spreads throughout most of the wetted area
  • Smaller nitrate leached zone (white area around drip tape) than in Figure A
  • Higher nitrate concentrations occur below drip tape than above, opposite to Figure A due to soil being wetter and reducing upward pull at the time of injection
  • Results were similar in loam and sandy loam soils

Figure 2: Surface drip, loam soil

•= drip line
Soil: sandy loam
Irrigation event: 36 hours
Coloured bar = relative nitrate-nitrogen concentrations (white = little or no nitrogen, blue = low, red = high)

A

Nitrate injected for 2 hours, starting 1 hour after the irrigation event begins

  • Nitrate band about 15-18" from drip tape
  • Nitrate leaching close to drip tape (white area)
  • Similar results in sandy loam Nitrate injected for 2 hours ending 1 hour before the end of the irrigation event

B

  • Relatively high nitrate concentrations in a zone next to the drip tape
  • Similar results in sandy loam Nitrate injected during the middle half of the irrigation event

C

  • More uniform nitrate distribution through soil profile
  • Similar results in sandy loam

Figure 3: Surface drip, silt clay soil

•= drip line
Soil: silt clay
Irrigation event: 36 hours
Coloured bar = relative nitrate-nitrogen concentrations (white = little or no nitrogen, blue = low, red = high)

A

Nitrate injected for 2 hours starting 1 hour after the irrigation event begins

  • Nitrate distributed in horizontal band, extending about 20" from drip tape
  • Slower infiltration rate allowed water to pond and flow downward Nitrate injected for 2 hours ending 1 hour before the end of the irrigation event

B

  • Nitrate accumulates in a horizontal band extending about 23" from drip line
  • Little penetration occurred into the soil profile Nitrate injected during the middle half of the irrigation event

C

  • More even and deeper nitrate accumulation in soil profile

These diagrams show the importance of knowing a crop's rooting depth with respect to nitrate-nitrogen distribution pattern. Maximizing their intersection increases nitrogen uptake and reduces losses below the rooting zone.

For example, injecting nitrogen for 2 hours starting 1 hour after the irrigation event begins (Figure 1 and 2, scenario A) means a considerable amount of nitrogen would be below the rooting zone of newly planted transplants. This nitrogen strategy may fit better for an annual crop production during the mid-season or established shallow rooted perennial crops such as strawberries.

Scenario B in figures 1, 2 and 3 injecting the nitrate fertilizer for 2 hours ending 1 hour before the end of the irrigation event, may be a better strategy for early season fertigation when roots are concentrated near the soil surface.

Injecting during the middle half of the irrigation event as in scenario C, figures 1, 2 and 3, may be applicable to mid-season nitrogen applications to annual crops or established perennial crops such as strawberries and raspberries. Roots have begun to explore a larger soil volume.

1 Fertigation with Microirrigation. 2006. University of California Agriculture and Natural Resources Publication 21620