FERTILISER GUIDELINES FOR THE WINE INDUSTRY

Organic N ───> NH 4

+ ───> NO 2

- ───> NO 3 -

a

b

c

where a is a slow process (ammonification) executed by nonspecialised or ganisms, with b a rapid process (nitrification), which is executed by specialised, autotrophic Nitosomonas bacteria, and c a very rapid process (nitrification) which is executed by specialised, autotrophic bacteria, Nitrobacter . Various factors determine the rate and level of mineralisation of organic ma terial in soil, with the most prominent being the organic material content of the soil, the composition thereof, its pH, water supply, aeration and temperature. Gains of N in the soil can possibly occur via photochemical reaction, non symbiotic Nfixing by Azotobacter in particular, Clostridium and the blue-green algae Nostoc and Calotrix , but especially through symbiotic fixing by Rhizobium root nodule bacteria in legumes, and also by some non-leguminous plants. Amounts of N fixed by legumes range from 5476 kg/ha -1 for peas, beans and vetch, to 158-188 kg/ha -1 for clovers. Additions from the atmosphere through rain range from 1-62 kg/ha -1 year -1 . The most important addition is however, through fertilisation in the form of fertiliser and/or organic fertilisers. Losses of N from the soil occur via plant removal and leaching, which can be of the same order, as well as through denitrification. The latter takes place where there is organic material in the soil, as well as water logged conditions: NO 2 - , NO 3 - → N 2 , N 2 O Low soil pH is favourable for the formation of N 2 O (laughing gas), which can potentially contribute to global warming, while at pHs close to neutral, losses mainly occur in the non-harmful N 2 form. Nitrogen has a positive effect on growth, but excessive applications can lead to a decline in carbohydrate contents. Carbohydrates from photosynthesis are then used for the production of protoplasm and are consequently not available for accumulation, like sugar in the case of grapes, which leads to a delay in ripening. At high levels of N-supply, cells tend to be larger, which can have a negative effect on water holding capacity and disease resistance. An indirect result is that larger leaves and longer shoots can assist canopy density and

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