Home About Us Our Services Publications Testing Submissions Contact Us Links

The Loam Ranger - Why perform a grape petiole analysis?

Dear Loam Ranger,

I’m a grape grower. One of my neighbours says I should get my soil tested, but the other says I should get the grape leaves tested. Who’s right?

Whether tissue or soil testing is more useful in grapevine management is always the subject of argument. But these arguments can miss the point: that soil testing and petiole testing are used for different management purposes.

Soil testing

Soil testing is most useful for determining the soil pH, salinity (EC), cation exchange capacity and possibly phosphorus (P) status before the growing season starts. But soil testing cannot predict the availability of some nutrients to the plant, notably nitrogen (N), sulphur (S) and trace elements. This is because of deficiencies in the extraction methods used, the soil moisture regime, the exact location of the soil sampling and the wide volume of soil available to the plant. N availability is particularly difficult to predict from soil tests on account of the cycling of N in the soil and the unpredictability of N mineralisation rates.

Testing soils is a bit like testing people’s food in order to reach a diagnosis on a given problem. Plant tissue testing is more akin to taking a blood specimen.

Petiole testing

Tissue testing can be a more reliable indicator of the nutritional status of a plant at a critical stage in the crop cycle. The petiole is the most favoured tissue to sample in grapes because it is considered to be more sensitive to nutritional status and because so much background data and correlations with field trials have been collected, mostly from California. Where there is a correlation with field trials, it is possible to make reliable recommendations from petiole tests.

Fertilisers for Wine Grapes gives a general guide to the interpretation of petiole test information. Before making recommendations from the results (i.e. whether we need to use fertilisers or not to correct an apparent problem), we must consider the cultivar and rootstock. Semillon, for example, will always show lower nutrient status than Sauvignon Blanc in the same soil under the same fertiliser regime; but this does not necessarily mean that the Semillon must be fertilised more. Grower records are therefore essential to fine-tuning the test results and recommendations.

The most important results of tissue testing are those for P and N. Both nutrients are critical to the fermentation and wine quality determinants. For example, if the grape juice contains inadequate concentrations of inorganic N to satisfy the requirements of the yeast, the yeast will break down proteins in the juice to get N and so adversely affect flavour by releasing S as hydrogen sulphide (rotten egg gas). The rate of fermentation is also dictated by the concentration of N in the grape must. Early petiole testing helps manage this final N content.

Assessing the vine nutrient status, especially N and P levels, at the full-flowering stage will allow a grower time to receive the test results and make any necessary adjustments to improve the current year’s harvest. Note that N applied to the vines between fruit set and veraison will have the greatest impact on the N levels in the juice at the time of harvest.

Petiole analysis is useful in other situations too:

• In trouble-shooting exercises
• To assess the effects of altering management practices
• To assess the effectiveness of a nutritional program

Collecting samples

To collect enough samples for a tissue analysis, take around 75–100 petioles from the basal blade opposite the second bunch from within one cultivar and soil type. The petioles of the youngest mature blade are also used, but there is less data available to make reliable recommendations. You may wish to separate samples from vines showing good vigour from those showing poor. Remove the leaf blade and place the petioles in a clean paper bag for transport to the laboratory.

Always keep records of petiole testing together with those from soil testing and management practices for future reference.

Further reading

Fertilisers for Wine Grapes, B.H. Goldspink et al. 1997, Agriculture Western Australia.

Making Good Wine, B. Rankine 1991, Pan Macmillan Publishers Australia.

Using Soil and Petiole Testing, S.W. Leake 2000, extract from Proceedings of the Hunter Valley Vineyard Association Autumn Seminar 2000.

Viticulture Vol. 2, B.G. Coombe and P.R. Dry 1998, Winetitles, Adelaide.