FERTILISER GUIDELINES FOR THE WINE INDUSTRY

MACRO-ELEMENTS P lant material consists of 70-95% water, with oven dry material consisting 90-95% of organic compounds and the rest being made up of mineral compounds from a large number of elements. Of these the essential macro-elements are carbon (C), hydrogen (H) and oxygen (O), derived from air and water, with nitrogen (N), phosphorus (P) and potassium (K), the so-called fertiliser elements, as well as calcium (Ca), magnesium (Mg) and sulphur (S) the other macro elements, which are assimilated naturally by the roots. Nitrogen (N) The role of N was only established at the beginning of the 19th century. It is the basic element for cell division and the development of vegetative organs as it forms part of proteins, nuclein acids, auxins and chlorophyll. It is also stored as reserves in cells in the form of alkaloid granules. Nitrogen is more frequently the growth restricting nutrient, than any of the other essential elements. 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 usually predominantly the additions in the form of fertiliser. Additions from the atmosphere, due to lightning activity, can some times also make a significant contribution. Water-insoluble (organic) N-compounds is by far the largest fraction of the total N-content of soils. This organic N cannot be utilised by plants. However, on account of conversion (or mineralisation) of these compounds to inorganic (plant-available) N-compounds, the organic N content of the soil does give an indication of the “Nsupplying capacity” of a specific soil. The inorganic or mineral fractions of N in a soil, which are significant for vine nutrition, are the ammonium (NH 4 + ), nitrite (NO 2 - ) and nitrate (NO 3 - ) ion contents. Young, acid-loving and marsh plants usually give preference to NH 4 + -ions, while NO 3 - - ions are most prominent for plant nutrition in well aerated soil, but can also be toxic in high concentrations (i.a. growth-arrestment phenomenon). The conversion or mineralisation of organic N-compounds to soluble (inorganic) compounds can be described as follows:

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