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The Loam Ranger - Soil for winegrapes

Dear Loam Ranger,

What do I need to know about the soil in my planned vineyard before I plant the vines?

Site selection

Soil physical and chemical characteristics

Before planning a vineyard, organise a site soil survey. Analysis of horizon depth, soil texture, structure, density and chemical composition will support informed decisions about vineyard design and management required to produce consistent grape quality and quantities.

As a general rule, the topsoil should be friable and at least 400 mm deep to provide adequate room for a good root system to develop. Problems of perched water table and waterlogging can occur on many of our duplex soil profiles where an otherwise suitable topsoil overlies a heavy clay subsoil. Such soils need to be hilled up or to have systematic drainage installed. Good structure in a heavy topsoil will ensure adequate drainage and root system aeration. This may be improved with the addition of lime or gypsum, dependent on soil test results. Establishing the right physical characteristics of the soil before planting is critical, as making improvements once the vineyard is planted is difficult.

Organise a full soil chemical analysis as well. This will identify nutritional deficiencies.

Almost no natural soil will satisfy the ideal conditions required for high-quality wine grape production (Table 1). Therefore, the soil will almost always have to be modified before the grapes are planted. This is relatively easy to do before planting, but can be far more expensive to do after planting.

Table 1. The ideal vineyard soil properties.

Water

Water management is critical for vine uniformity and sustained high yields of superior quality. Water analysis is a valuable tool in selecting a site and developing an irrigation management plan.

The electrical conductivity (EC = salinity) is the most critical factor in determining water quality and the type of irrigation schedule to implement. Avoid water with high pH, EC, sodium content and nitrogen concentration.

Soil preparation

Aim to grow at least one green manure crop before planting the vineyard. This will increase organic matter and plant nutrient levels.

The site soil survey and soil chemical analysis will indicate the necessary changes to achieve the optimum soil conditions before planting. Such things as lime, gypsum and fertilisers are generally required, and should be applied before deep ripping to incorporate them thoroughly throughout the topsoil. Normally, on a new soil, high levels of phosphorus should also be applied.

Once the necessary ameliorants are spread, deep-rip the vine rows to a depth of 500 mm. This action is best performed when the subsoil is relatively dry to increase the shattering effect. It may need to be followed with a couple of passes with discs to break up the soil. If the topsoil is shallow, some hilling up on the vine rows may be needed to deepen the available rooting profile. If indications of periodic waterlogging are present, both hilling up and the installation of systematic drainage are advised.

After the soil preparation, install the trellising and irrigation systems. When the threat of heavy frosts has passed, plant the vines.

Annual soil management

It is usual to plan the fertiliser program for the next season during the winter dormant period. Organise a topsoil test before planting the mid-row cover crop. This test will indicate impending deficiencies that will need correcting for the next growing season, and will determine whether additional supplements are required to sustain the cover crop.

Winter-planted cover crops offer several benefits. They:

• increase and maintain a high soil organic matter content
• stimulate soil organism activity
• improve soil structure
• insulate the soil from large temperature fluctuations
• use excess fertiliser not taken up by the vines
• suppress weed growth
• prevent erosion
• fix atmospheric nitrogen if legumes are grown.

Cut the cover crop and side-cast it onto the vine rows 6–8 weeks before spring to form a mulch layer 50 to 100 mm thick. This is far more beneficial and sustainable than repeated herbicide applications to maintain a bare soil surface. Implement any recommendations that came out of soil tests at this stage.

A petiole analysis when the vines are in full flower can be used to determine the nutrient concentrations in the foliage, especially nitrogen. Doing this test now will allow enough time to apply appropriate N sources before veraison and thus ensure high-quality grapes.

Table 2. Annual test regime for wine grapes.

Nitrogen calendar

Nitrogen is the most important nutrient in managing vines. Excess N will result in excessive vegetative growth at the expense of flowers and fruit, but inadequate N affects fermentation time and wine quality.

Budburst to flowering

Stored N provides only 20% of the N needed for budburst and extension for flowering. The remaining 80% must be supplied from the soil. Therefore, work in or side-cast the cover crop 6–8 weeks before budburst. In a highly fertile soil with clover in the cover crop, this may provide enough N from the decomposing mulch. Otherwise, apply additional surface dressing according to past experience, crop appearance and testing. Remember to allow time for the N to be washed down through the soil profile to the roots. Do not apply N too close to flowering, as bunch shatter can result.

Four to six weeks before budburst, apply any other nutrients indicated by the winter soil test, such as phosphorus, potassium and trace elements.

Flowering to veraison

N supply between fruit set and veraison has the greatest effect on the final N status of the bunches. Except on the most N-fertile soils, extra N applications during this period will return the greatest benefits.

Base the need for any additional N strictly on plant petiole testing, as it is difficult to predict the N-supplying ability of soil.

Veraison to harvest

There is little advantage in adding N during this period.

Harvest to dormancy

During this period, N in the leaves is relocated to the stems and roots for the following season. Provided that enough N was supplied earlier in the season, there is no benefit in terms of wine quality from applying N after harvest, despite the additional burst of root growth it promotes.

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