Evaluating root systems in profile pits

On 12 September 2019, SATI and the ARC with the involvement of soil scientists from institutions/companies who support the table grape industry presented a
“Root Safari”. The purpose of the Root Safari was to see what root systems must look like where less irrigation will be applied, and what type of soil preparation is required to establish such a root system in a specific soil type. The information provided in this article is the take- home message from the Root Safari.

ARC Infruitec-Nietvoorbij is currently working on a SATI project to determine guidelines for irrigation of table grapes, according to midday stem water potential measurements. With this approach, irrigation is scheduled according to the grapevine’s water status. The primary objective of using this approach is to save water without compromising on yield or quality. The project is being carried out with 10 cultivars on four different farms in Noorder-Paarl. One of the most important criteria for efficient water use by the grapevine is the establishment of a well-developed and well-structured root system. In addition to anchoring the grapevine, ensuring nutrient uptake, storing reserves and producing hormones, the grapevine root system is also responsible for water uptake. The more effectively the root system is able to fill the available soil volume, the better grapevines can withstand dry/drought conditions. Conversely, single, deep roots do not make a significant contribution to absorption of water and water will be wasted if irrigation is applied to depths where these single deep roots occur. Therefore, water application should focus on where the greatest portion of roots is located.

Figure 1

Meeting these four criteria enables an efficient root system (refer to Figure 1):

1.  LATERAL ROOT DISTRIBUTION: Roots should extend to the middle of the work row (A), in full surface irrigation and for strip wetting (e.g. drip irrigation). Importantly, the width of the wetted soil volume is as indicated by B in Figure 1, and roots should be developed accordingly.
2. DEPTH OF DISTRIBUTION: Roots should extend to at least 60 cm deep, if good quality irrigation water is abundant and 80 cm if water is limited.
3. NUMBER OF FINE ROOTS (THINNER THAN 2 MM Ø): The more the better, for optimal water and nutrient absorption. All fine roots are feeder roots, regardless of the depth at which they occur. Ideally, fine roots must be distributed uniformly, not clustered in layers.
4. QUALITY OF FINE ROOTS: Health roots are brown, while dead or unhealthy ones are black or slimy.

TIPS TO CORRECT ROOTS/ENCOURAGE GOOD DEVELOPMENT:

• Loosen soil between tracks if some roots occur in the middle of the work row.
Use a ripper with mechanical action from PTO (e.g. wiggle plough).

• If wheel tracks restrict roots – loosen the soil between the wheel tracks followed by ripping on wheel tracks, using a two-time ripper.

• Do corrective measures during the postharvest period, when wheel traffic is low.

• Install cut-off drains if waterlogging causes dead or unhealthy roots, due to lateral subsurface water flow into the vineyard.

• Irrigate less, but more frequently if roots are shallower than expected, and vice versa if roots are deeper than expected.