1.4.3.b Which rootstocks are best adapted to the plot’s soil type?

What is the active limestone content in my soil / CPI of the soil?

Limestone resistance is the most important factor when selecting a rootstock. A lack of resistance on calcareous soil leads to the poor assimilation of iron and possible chlorosis. It is essential to have a rootstock perfectly suited to limestone levels in the soil.
The Chlorine Potency Index (CPI) and/or the active limestone indicated in the soil analysis is critical and therefore limits the list of possibilities very quickly. If there is a lot of active limestone, Fercal is a good solution.

Are there any factors which make my soil completely unsuitable for some rootstocks?

The soil and/or climate (drought, humidity, clay, stones, etc.) may present obstacles which narrow the range of possible rootstocks. Unsuitable ones are eliminated:

• Soil water regime: some rootstocks are more or less resistant to humidity or drought. It is therefore necessary to adapt our choices accordingly. Resistance to humidity/drought is the second most important criterion after active limestone content. Our line of questionning should be identical to that for grape varietal selection (see what is the water regime of my soil?)

• Salinity: the soil’s salinity could disrupt the plant’s development so it would be best to use a tolerant rootstock. However, these cases are quite rare.

• Compactness and depth of the soil: these are elements to be taken into account for the development of the roots. For example, on a superficial soil, it is better to take a more vigorous rootstock.

• The acidity of the soil can alter the biological behaviour of the soil and impact the assimilation of certain minerals.

How much of a priority is parasite resistance?

Some rootstocks have varying degrees of phylloxera resistance. This factor must be taken into account when choosing a rootstock, but in most cases they are sufficiently resistant.

The same is true for nematodes responsible for Grapevine fanleaf disease (Management of wood diseases and viruses). Different rootstock species do not have the same resistance levels. It is common practice to wait a long time before replanting to eliminate the nematode population. Even if these parasites are not present, it is better to use resistant rootstocks on soils that are favourable to their development. Consequently, when replanting on a virus-affected plot, a highly resistant rootstock is recommended. Némadex for example is a possible solution but it is not adapted to all soil types. Moreover, it only delays the disease outbreak. As a precaution, nematode count analysis should be repeated before planting.

Is there a risk of incompatibility with the grape variety?

Incompatibility between rootstalk and a vine can occasionally occur and this can cause the vine to decline. This is not a forgone conclusion for some certain variety/rootstock pairings, but the risk is there. For example, Kober 5BB is incompatible with, among others, Cabernet Sauvignon and Cabernet Franc. The Pinot Noir/161-49C combination is also known to have problems with dieback.

What should the rootstalk give to the graft?

The rootstock will impart its vigour to the graft so the choice of rootstock should be based on expected production levels. Vigour can be categorized from weak to strong. For higher-quality wines, a lower yield is recommended, therefore lower vigour. The only exception is very poor or superficial soils where a rootstock with a strong vigour is necessary to allow the graft to develop despite the extreme conditions. Arid, poor and superficial soils will work well with a vigorous rootstock like 140-Richter.

If the rootstock bestows early-ripening qualites on the graft, this can give more flexibility regarding harvest dates. This can be particularly helpful in light of current climate change.

How much magnesium and potassium do the selected varietals need?

Certain grape varieties need more or less potassium and magnesium to ensure their proper development. Pinot noir and Cabernet-Sauvignon have a high magnesium requirement whereas Merlot has a high potassium requirement.
Some rootstocks tend to absorb one of these elements more than the other one, regardless of the K/Mg ratio of the soil. 420A and 3309C absorb magnesium well, whereas 101-14 and 5BB absorb potassium better.