FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

FIGURE 4: Dense canopy, due to excessive N fertilisation (Photo: P. Raath).

Occurrence of N in soil: The N in the soil is a function of the amount and nature of the organic material in it, the circumstances that control the mineralisation thereof and binding from the atmosphere, as well as additions in the form of fertiliser. Additions from the atmosphere, due to lightning activity, can sometimes also make a significant contribution. Water insoluble (organic) N compounds constitute the largest fraction of the total N content of soils, by far. This organic N cannot be utilised by plants. However, because these compounds can be converted by means of mineralisation to inorganic (plant available) N compounds, the organic N content of the soil does give an indication of the “N supplying capacity” of a specific soil. The rate and level of mineralisation of organic material are determined by various factors, with the most prominent being the texture of the soil, the pH, water supply, aeration and temperature. The inorganic or mineral fractions of the N in a soil, which are important for grapevine nutrition, are the ammonium (NH 4 + ), nitrite (NO 2 – ) and nitrate (NO 3 – ) ion contents. The conversion or mineralisation of organic N compounds to soluble (inorganic) compounds can be described as follows:

Organic N j NH 4

+ j NO 2

– j NO 3 –

a b c where a is a slow process (ammonification) executed by non-specialised organisms. The process whereby NH 4 + is oxidized to NO 3 – is called nitrification and entails

42 | MAINTENANCE FERTILISATION