Citrus utilizes nitrogen mostly in the nitrate form and maximum uptake occurs from N applications made during the periods of active growth.
Highest yields have been recorded where nitrogen is applied as calcium nitrate rather than as ammonium nitrate.
Ammonium nitrate can also encourage soil conditions that favor citrus blight and root diseases resulting in reduced three health.
Care needs to be taken to ensure that as much of this nitrate is utilized and not leached during active periods of growth and finds its way into tree reserves to drive forward yield.
Nitrogen form is also important when fertigating because the nutrients are concentrated in the wetting zone and reactions occur faster than with broadcast, dry applied fertilizers:
- Urea is highly susceptible to leaching and needs to be transformed to ammonium and then nitrate prior to root uptake – a process that can take more than one week, especially at lower (<180) and higher (>350) soil temperatures.
- Ammonium – with its positive charge – is better retained in the soil, and as well as increasing long term soil supplies, it can be transformed to nitrate for root uptake. However, both uptake and transformation reduces soil pH. In soils with a low buffering capacity (sands or low CEC soils), this decrease in pH can be by one or more units. In low pH soils, this can result in the release of aluminum at toxic levels. On high pH soils, ammonia can also be released to toxic levels, again significantly reducing tree performance.