Viticulture: Soils

25 October, 2006

We walk on it, drive on it, and dig it but what is it?

Earth

45%

Mineral, originally from weathered rock broken down by elements

Water

25%

It dissolves & carries nutrients & minerals to the plants in the soil.

Field capacity – full of water

Wilting point – all available water is gone

Amount of water soil can hold depends on soil type.

Air

25%

Essential for root growth, water & nutrient uptake & biological activity

Anaerobic – lack of oxygen (water logged)

Roots & micro organisms respire, they need oxygen for photosynthesis & cell division

Organic matter

1-5%

Food source for all soil micro organisms.

Grapevines can be grown in a wide range of soil types. However different management techniques need to be applied depending on soil type in order to produce high quality fruit.

Soil texture

Basically, soil is made up of different percentages of sand, silt & clay particles.

Sand – 2 – 0.02 mm

Silt - 0.02 – 0.002 mm

Clay – < 0.002 mm

The combination of the above determines whether a soil is described as sand, silt, clay or a combination like sandy loam or sandy clay.

Knowing the soil’s texture is important in planning block sizes especially in relation to the water holding capacity of the soil.

Sandy soils will have a low water & nutrient holding capacity, which increases as the silt and/or clay content increases.

In clay soils, water logging might become a problem as can fixing of nutrients such as Phosphorus.

Soil structure

Whilst soil texture gives us information on the type of soil, soil structure tells us how these particles are held together.

Soil texture can be determined by looking at samples removed by an auger whereas soil structure is normally determined by observing the profile of the sides of a soil pit.

Soils with good structure have open pore spaces between 0.5 & 2 mm which allows for good movement of air and water and for good root penetration.

Soil should consist of a range of small, medium & large pores. The small pores retain water-containing nutrients required for plant growth.

Roots grow by forcing themselves through smaller pore spaces or following existing pore spaces.

Cultivation of wet soils and excessive tractor movements in the vineyard will have an effect on soil structure. A compacted area develops under the wheel tracks and also under the cultivation depth.

Chance of compaction is greater with movements during the winter when soils are wet then during the summer when soils are dry.

To maintain structure, it is important to monitor physical and chemical changes in the soil.

Some basic guidelines:

 

• Do not cultivate when soils are too wet
• Avoid mixing sub surface clay with top soil when cultivating
• Use herbicides for weed control
• Increase organic matter with cover crops
• Monitor soil acidification

 

Water logging

Water logging causes the roots of the vines to be weakened which pre-disposes them to be attacked by root rotting organisms.

Grapevines can tolerate some excess water in the winter period but basically from bud burst onwards this should be avoided.

Soil colour is a good indicator for the soil’s drainage status.

As the colour moves from black to brown to yellow to grey to blue/green grey it becomes more poorly drained.

Chemical monitoring of vineyard soils

Soil sampling

Soil sampling has become an integral part of vineyard management however, poor sampling procedures can lead to the wrong interpretation of the problem.

Soil must be sampled from set depths. Grapevines utilize a large depth of soil and have feeder roots that are generally found between 10 and 40 cm in depth.

Sampling to only 10-15 cm will not provide sufficient information about the nutrient status of the soil. To sample efficiently to the depths required, it is necessary to use a soil auger.

By using a soil auger, different sections of any required depths can be sampled. The use of a spade is not recommended due to mixing soils from different depths and therefore getting an unbiased sample. Recommended depths for sampling can vary depending on soils and location but generally a topsoil sample at 15 cm and sub surface sample at around 40 cm will give good information.

Soil sampling is a difficult practice at the best of times. Without the correct sampling tools and sampling procedures it can be a costly, almost meaningless exercise.