Among horticultural plants, grapevines are acknowledged as having less exacting nutritional requirements. They can be grown on a wide range of soil types, but nutritional problems show most commonly on lighter textured soils. Most established vineyards in New Zealand are on medium to heavier textured soil, with the exception of Marlborough. Nutrient disorders are often suspected where in fact, the causes of poor vine growth result from other factors such as waterlogging, phylloxera, nematodes, drought and herbicide injury.

New Zealand vineyards are commonly located on fertile soils, and normally annual fertiliser requirements are not necessary. As a general rule annual fertiliser applications can be regarded as an unnecessary expense unless indicated by deficiency symptoms, soil tests or preferably, by plant analysis.

PLANT ANALYSIS
Australian and Californian studies have shown that petioles best reflect the nutritional status of grapevines. A typical sample should consist of 30 large or 50 small petioles taken at flowering from the leaf opposite the basal cluster. The petioles should also come from leaves exposed on the outside of the vine. For monitoring the nutrient status of vines later in the season, whole leaf blades (without the petioles) can be taken from mid shoot leaves during fruit ripening.

Tentative critical nutrient standards based mainly on overseas research are listed for petioles and leaf blades in Tables 1 and 2 respectively. These values are presently being evaluated for New Zealand conditions. Sprays containing Mn, Zn, or Cu can cause high levels if the sampled tissues are not washed in water. Varieties are known to differ in nitrate and Zn levels, so the standard concentrations listed in Tables 1 and 2 may not apply in all situations.

SOIL TESTING
There is a lack of definitive soil test levels for grapevines in New Zealand, but the following values have been found as a useful guide in Hawke's Bay and Poverty Bay. A different relationship can be expected for soils in other districts where the Mg and Ca status of soils will be much lower. The recommended MAF quick test soil levels are:

Boron deficiency can be expected with a soil test value of less than 0.5 ppm B, and toxicity with more than 2.5 ppm B in hot water extracts. In acid soils, Al, Mn and possibly Cu toxicities may be expected. Aluminium toxicity can be expected with a soil pH of 5.0 or less, and when the exchangeable Al is more than 50 mg/kg (N KCl extract). Manganese toxicity is expected with a soil pH of 5.0 or less, and reducible Mn levels (extracted in N ammonium acetate at pH 7) of more than 100 mg/kg. Copper toxicity is expected with a soil pH of less than 6 and more than 100 mg/kg in clay soils (extracted in N ammonium acetate of pH 7).

Making allowances for fertiliser inefficiency and root and shoot growth, the following inputs would be required in kg/ha: N, 66; P, 20; K, 122.

While the above calculations could lead to annual maintenance fertiliser applications, it is recommended that the grower follows the recommendation given in Table 4 and makes whatever applications are needed, only when indicated by plant analysis.

Growers are cautioned against annual applications of N as this may produce undesirable vigour problems. Commonly, New Zealand vineyards are excessively vigorous, and annual N applications will make this problem worse. In fact, in many situations generous N applications may be the cause of this problem. Growers are recommended to monitor nutrient levels in the vines before using N. Excessive shoot growth, particularly late in the season will reduce crop sugar levels.

The rates of N listed in Table 4 are based on studies in California where soils are commonly of lighter texture than those in New Zealand. It is stressed that growers should consider rates of about half the values given unless nutrient tests indicate severe deficiency levels. Where grassing down is used, an extra 20-30 kg N/ha could be applied if no clovers are present. Where the vineyard shows only limited weak areas, spot treatment rather than blanket treatment should be considered. For young vines grown on soils other than the silts and loams, N should be applied using split applications during early spring and mid summer.

To correct severe deficiencies of K in California, rates containing up to 1000 kg K/ha have been found to be necessary. New Zealand growers are advised to use these high rates only on trial plots initially as large, single applications of potash could induce Mg and Ca deficiencies.

Applications of K can be made in late autumn or early winter to heavy soils, or early spring for light textured soils.

Where the presence of a micronutrient deficiency has been confirmed, foliar rather than soil application of fertilisers is commonly used to correct the imbalance (Table 5). To avoid possible toxicity problems arising though, care should be taken when using this method of application.

Where deficiencies involving N, Mg and B are indicated, either solid or foliar fertilisers can be applied (Tables 4 and 5). Boron is normally applied as a mixed fertiliser (eg, borated superphosphate), but growers are cautioned against repeated annual dressings. Applications are required only every 2-8 years. Apply fertiliser to the soil in autumn or early winter.

A guide to liming rate is given below. A target of pH range of 5.8-6.8 is recommended with a minimum of 5.5. Note that for a subsoil pH problem, surface broadcasting is not effective in overcoming the problem. Lime must be incorporated deeply and be well mixed into the soil.

To raise the pH by 0.5 units for a 20 cm layer of soil requires the following amounts of ground limestome (85% CaCO₃):

On acid northern gumland clays rates of lime of 7.5 to 10 tonne/ha are not uncommon and may need to be repeated to raise the pH to desirable levels.

Symptoms associated with N deficiency include foliage which is pale green to yellow-green. Typically, young leaves near the shoot tips are yellow and internodes are short. Yields can be greatly reduced with severe deficiency. Nitrogen deficiency is more common on light textured soils, where soil organic matter is low, or where the vineyard interrow area is grassed down. Such grass swards can utilise up to 70 kg N/ha.

Potassium deficiency symptoms become obvious in early summer at the leaf margin and progress into the area between the main veins. Leaves are characteristically shiny. The yellow leaf areas can turn bronze or red (for coloured fruit varieties) and marginal burning and leaf curling also occurs. Alternatively, K deficiency may show up as black leaf, where blue-black flecks appear in mid season on the upper leaf surface. This symptom commonly occurs on American (Labrusca) grapes. With severe deficiency, shoot growth is reduced and leaves may drop early. The lower part of the bunch stem may collapse, causing berry raisining. Potassium deficiency can be confused with leaf roll virus symptoms.

Magnesium deficiency results in chlorosis (yellowing) of margins of basal leaves in mid season. The chlorosis moves inward between primary and secondary veins, and may become creamy white in colour for white varieties. Leaf margin burn may subsequently develop. For red fruited varieties, a red interveinal colouring develops. Magnesium deficiency is commonly found on sandy soils, especially where heavy K applications have been made. It is also commonly found on vines grafted to certain phylloxera resistant rootstocks such as SO4.

Foliar symptoms of B deficiency appear in early summer. Young leaves show a mottled fading between the veins which can develop as a severe interveinal chlorosis. When severe, older leaves will show interveinal necrosis. The shoot tip commonly dies, and lateral growth develops. Tendrils and internodes near the shoot tip show black bands when held up against the light. Root extension is reduced, and the tips are swollen and stubby. Fruit set is much reduced and small seedless berries are commonly found along with normal sized ones. Boron deficiency effects on leaf deformation and fruit set can be confused with similar 'fan leaf' virus symptoms. Similarly, longitudinal cracks in the shoots can be confused between acute B deficiency and 'corky bark' virus. Boron deficiency is commonly found on sandy, gravelly soils particularly those with low pH. Temporary deficiency is often associated with drought.

As with other B sensitive plants, the margin between B sufficiency and toxicity is very narrow for grapes. Extreme care should therefore be taken when applying B fertilisers. The first sign of B toxicity is dark brown to black spots around the inside of the leaf margin which can also develop inwards towards the centre of the leaf between the veins. Young leaves typically show cupping. When severe, defoliation of all but the youngest leaves is seen. Boron toxicity can be due to irrigation water with a high B concentration or excessive application of B fertiliser. The latter has occurred on the sandy and gravelly soils in Marlborough.

Manganese toxicity is to be expected on acid soils, especially where waterlogging is a frequent occurrence. Among the most consistent symptoms of high Mn content of leaves is the development of black stripes along the conducting tissues (shoots and petioles); the leaf is rolled, marginal necrosis is common and leaf fall is frequent. Yield can be severely reduced.

There are no characteristic foliar symptoms associated with Al toxicity, although root growth is restricted and young plants may die. Both Mn toxicity and Al toxicity can be overcome by raising the soil pH above 6.0.

Methods and rates suitable for correcting specific nutrient deficiencies in grapevines are outlined in Tables 4 and 5.

Christensen, P., Kasimatis, A. and Jensen, F. (1978). Grapevine Nutrition and Fertilisation in the San Joaquin Valley. University of California, Priced publication 4087, USA. pp.40.

Delas, J. (1984). Les toxicities metalliques dans les sols acides. Le Progres Agricole et Viticole 4: 96-101.

Winkler, A.J., Cook, J.A., Kliewer, W.M. and Lider, L.A. (1974). General Viticulture. University of California Press, USA. pp.710.