Nitrogen Forms and Nutrient Availability
Nitrogen form is important to nitrogen-availability. Organic manures, while helping to improve soil structure and fertility, may be unpredictable in the way they release nitrogen.
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 (<18 oC) and higher (>35 oC) soil temperatures.
Ammonium – with its positive charge – is better retained in the soil, and as well as increasing long-term soil long-term soil supplies. It can be transformed to nitrate for root uptake. Small amounts of ammonium are absorbed, but the majority is converted to nitrate for uptake.
Both uptake of ammonium and nitrification reduce 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 aluminium at toxic levels. On high pH soils, ammonia can also be released to toxix levels, again significantly reducing vine performance.
Application of nitrate nitrogen at key periods such as fruit development, can ensure immediate availability and stronger growth. It can also help improve uptake of other nutrients.
