Healthy soil and smart irrigation secure sustainable production
According to Nemlab’s director Sheila Storey, healthy soil remains key to sustainable grape farming. “It is the foundation for everything.” By Jorisna Bonthuys
Sheila Storey’s presentation focused on a holistic approach to promoting soil health. Healthy soil delivers many ecosystem services that ultimately benefit humans, she said.
Soil health consists of three components: a chemical, physical and biological component. “To reach soil health, all three components must function, and they’ve got to work together,” Storey said.
The remainder of the session was dedicated to exploring how these three components can be optimised.
Read MoreUncovering the benefits of cover crops
In his presentation, Ivan Jansen van Rensburg (an agronomist at Barenbrug South Africa Seeds) explained how cover crops contribute to soil health.
These crops provide the soil with organic carbon, increase microbial activity, support nitrogen fixation and help with carbon sequestration. Cover crops can also help farmers retain soil moisture in their vineyards.
Furthermore, these crops can help suppress weed growth, prevent erosion and assist with the biological control of some insect and mite pests – they have proven helpful in attracting natural predators (of harmful soil organisms like certain nematodes), as well as insects that are beneficial for pest control.
“Currently, there are four management approaches available for dealing with damaging nematodes, namely biofumigation, planting cover crops that are not hosts for plant-parasitic nematodes, increasing cover crop diversity and rotation, and using chemical and other control measures,” Jansen van Rensburg said.
Several factors play a role in cover crop selection, including rainfall and irrigation, soil type, costs, equipment and growers’ goals regarding weed suppression and nitrogen fixation. “These factors will determine the success of all the different cover crop species that you’re considering,” he said.
There is no blanket recipe for choosing cover crops. However, diversifying your cover crop mix helps create better soil structure and healthier soil. Growers should select the most appropriate cover crop or crop mixture for their specific circumstances, Jansen van Rensburg advised.
For work rows, there are different cover crop options available. Some of the annual winter options include grasses and cereals (e.g. forage rye, forage barley and triticale), legumes (e.g. vetch, forage peas and clover), and brassica and forbes (e.g. radishes, mustard and phacelia). There are also perennial options available.
“Many producers follow a ‘mow-and-blow’ practice throughout the season (… whereby) you mine your work row for nutrients that you then apply on the ‘bankie’ (vine row), so that it is more easily accessible to the vine roots,” Jansen van Rensburg explained. Using cover crops on the vine rows in this manner is becoming increasingly popular.
Optimal irrigation management
Dr Philip Myburgh and Dr Carolyn Howell, researchers at ARC Infruitec-Nietvoorbij, focused their presentation on optimal irrigation management for sustainable yields in grape production.
Howell leads a SATI-funded project aimed at developing guidelines for the irrigation of table grapes. This is being done by determining irrigation refill lines for a variety of cultivars based on their midday stem water potential threshold.
The table grape industry has many water-related challenges, including rising irrigation water tariffs and pressure to reduce its ‘water footprint’, Myburgh said. In addition, there is growing competition among water users due to the expansion of urban areas.
Water resources are limited in the majority of grape-growing regions, and inconsistent rainfall causes periodic droughts. This scenario may worsen if climate change reduces rainfall and increases air temperature in the near future.
The water-holding capacity of the soil in which table grapes are grown is often low, particularly in the case of shallow, gravelly, stony and sandy soils. Physical limitations on root depth also play a role, including the natural compactness of the soil, shallow weathered rock, cemented pans and inverted texture layers.
Myburgh and Howell selected a variety of different grape cultivars grown in commercial vineyards on four farms in the Northern Paarl region. On each trial site, there is an ‘experimental plot’ inside a ‘commercial block’. On each of the sites, the researchers measured soil water content and midday stem water potential as the soils started to dry out after bud break. They also determined the relationship between midday leaf water potential and soil water content in the case of the different cultivars. Lastly, they compared seasonal vineyard water use in the experimental and commercial sites.
Stem water potential displays an inverse relationship with soil water content as soils dry out. If the soil is very wet or near-saturated, the stem water potential drops very low. If the soil is extremely dry, the opposite occurs. Soil water content at -0.8 MPa is considered ideal for table grape production. However, if stem water potential drops below -1.2 MPa, it causes negative stress factors.
On average, the seasonal vineyard water use was 700 mm in the commercial blocks. However, vines growing in the experimental plots where less irrigation was applied based on their midday water potential measurements, used 28% less water.
Comparisons between the experimental plots and commercial blocks in terms of pre-harvest evapotranspiration and midday thresholds showed very little difference between the sites. However, during the postharvest period, the researchers documented larger differences. The commercial blocks used 7% less water during this period than in the pre-harvest period and the experimental blocks used 57% less.
Grapes grown in the experimental and commercial sites differed little in terms of berry mass and diameter. There were, however, slightly more berries in bunches from the experimental plots, where bunch mass was also somewhat higher.
Next, the researchers compared the yield per grapevine in the experimental plots to that in the blocks. Howell reported that the number of 4.5 kg cartons p/ha produced on the experimental plots was more than the number produced in the commercial blocks. The sugar level of the grapes from the experimental plots and those from the commercial blocks proved identical, while the total titratable acidity of grapes from the experimental plots was slightly higher.
With regards to berry quality, the commercial blocks and experimental plots yielded similar numbers of cracked, blushed and pricked berries. The researchers observed more loose berries in the case of bunches from the commercial blocks, although the incidence of this was still very low at less than five berries per box.
Irrigation in the experimental plots according to a pre-harvesting stem water potential threshold of -0.8 MPa, had no negative effects in comparison to irrigation in the control blocks, irrespective of the cultivar studied. These results indicate that it is possible to save water significantly when using grapevine water potential to schedule irrigation. By doing so, the latter can be applied more accurately and water use can be reduced, Myburgh said.
“Deep, well-developed root systems are essential to stretching the intervals between irrigations,” Howell added. The researchers found that deep root systems (of up to 1 m) require half the water that shallow root systems of 40 cm require (i.e. 345 mm compared to 764 mm water).
The researchers conceded that such water savings might not be possible where rain is low in the postharvest period or in summer rainfall regions. The water savings achieved in the postharvest period did not have any negative carry-over effects in the first two years. This trend will be verified in the third season.
Another notable finding of the study was that midday stem water potential correlated well with soil water content. “This means we can calibrate any soil water content probe against stem water potential as long as the probe ‘sees’ the soil water content correctly,” Howell concluded.
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