Project number: WW EM 17-01
Project title: Improving poor performing grapevines by means of root pruning or mulching
Project leader: Emma Moffat
Institution
Stellenbosch University. Department of Viticulture and Oenology
Team members
Myburgh, P A
Lategan, E L
Howell, C L
Setati, E
Wilding, M
Project completed: 2020
EXECUTIVE SUMMARY
Objectives and rationale
Trial 1. The effect of compost mulch thickness on soil water conservation and grapevine performance – The effects of different compost mulch thicknesses on soil water content, the rate of water infiltration into the soil, grapevine water constraints and grapevine performance were determined for two seasons.
Trial 2. The effects of tillage and compost amendment on soil conditions and grapevine performance – Effects of compost incorporation using two different tillage methods, i.e. root pruning (RP+comp) and furrow plough (FP+comp), were compared to root pruning without compost (RP) and a control. i.e. no tillage or compost. The effects of these treatments on soil conditions, grapevine performance and cover crop growth were examined.
Methods
Trial 1 – Compost mulch layers of 2 cm, 4 cm, 8cm and 16 cm were applied to the grapevine row in September 2016 and compared to a control treatment without any mulch. Soil water content was measured at four depths: 0-30 cm, 30-60 cm, 60-90 cm, and 90-120 cm on the grapevine row. The water infiltration rate was also measured on the grapevine row. Grapevine stem water potential (ѰS), vegetive growth, yield and its components were determined during two seasons.
Trial 2 – Poor rainfall in April and May 2015 resulted in the treatment applications being delayed until early Spring of 2015 when soil moisture conditions were suitable for tillage. Soil water content (SWC), water infiltration rate and soil penetration resistance were measured in 2016 and 2017. Grapevine stem water potential (ѰS), vegetative growth, yield and its components, and juice and wine characteristics were determined. Dry matter production and chemical status of the cover crop were determined at bud break. The effect of the various tillage actions and compost application on the root systems was determined in October 2018.
Key Results
Trial 1 – Mulch thickness did not affect SWC at all four depths on the grapevine row. Despite no differences in SWC of the different treatments following the winter rain in June 2016, there was a tendency for the soils under the two thicker mulches, i.e. 8 cm (T4) and 16 cm (T5), to be drier than those under the control (T1) and thinner mulches of T2 and T3. Measurements taken before and immediately after a major rain event confirmed that as the thickness of the mulch on the grapevine row increased, the effectivity of the rainfall event decreased, i.e., the depth of penetration into the soil was reduced. However, saturated hydraulic conductivity (infiltration) increased with mulch thickness up to 16 cm. Mulching did not affect the grapevine ѰS compared to bare soil on the grapevine row. During the first season, vegetative growth, as measured by cane mass, was highest in grapevines mulched to 16 cm and lowest in the control. The yield was also higher under the two thicker mulches than the control. During the second season, vegetative growth and yield did not respond to mulch thickness. Juice and wine quality did not respond to mulch thickness during both seasons.
Trial 2 – Compost incorporation in all rows increased the water infiltration rate compared to the control. The incorporation of compost resulted in the lowest penetration resistance. However, this effect was only ca. 15 cm and 45 cm deep for the furrow plough and root pruning treatments, respectively. Deeper than 45 cm, the penetration resistance of all treatments was comparable to the control. The soil of the control had the highest penetration resistance to a depth of 20 cm. It exceeded the 2000 kPa level where root growth is inhibited at a depth of 12 cm. Soil water content in the work rows was relatively homogenous when the penetration resistance was measured, except for slightly wetter conditions near the surface in the case FP+comp. The SWC on the grapevine row and grapevine ѰS were unaffected by any treatments. All treatments, except root pruning without compost (RP), produced higher yields than the control. The furrow plough plus compost (FP+comp) and root pruning (RP) stimulated more and deeper root formation than the control in the 0 to 60 cm layer. In contrast, root pruning plus compost (RP+comp) stimulated more root formation than the other treatments. Root pruning, i.e. with or without compost, had more roots in the 0 to 30 cm layer than in the 30 to 60 cm layer. Deeper than 30 cm, the tillage and compost treatments had no effect on root density compared to the control. However, the incorporation of compost tended to induce more roots below 30 cm depth.
Conclusions
Trial 1 – Under relatively dry conditions, there was no positive effect of mulching up to a height of 16 cm on the grapevine row, grapevine water status and soil water status over two seasons. The thicker mulches appeared to intercept the rain, preventing downward water movement to the soil until adequate rain fell by September 2016 to saturate the soil. The infiltration rate, however, responded positively to mulch thickness. Under normal rainfall conditions, or in the case of heavier rainfall events, increased infiltration rates could lead to higher SWC under the thicker mulches. Vegetative growth and grapevine yield responded positively to mulch thickness during the first season only. Mulching on the grapevine row did not harm juice and wine quality. The diminished effect on yield and vegetative growth after one season may be due to the weathering of the mulch.
Trial 2 – The furrow plough and root pruning treatments with compost (FP+comp and RP+comp) created more favourable soil physical conditions in the work row. Root pruning decreased soil strength deeper than the furrow plough. The infiltration rate was higher where compost was incorporated. The combination of these effects positively affected cover crop growth, vegetative growth and yield. Although the furrow plough and root pruning with compost in every row produced higher yields, it would appear that the treatments in which the compost was incorporated in alternate rows were more profitable over the five years.
Take home message for the industry
Where compost is available to the grower, it may be more effective to address poor grapevine performance with compost incorporation rather than an organic surface mulch on the grapevine row, particularly in the case of sloped or terraced vineyards in heavier soils. In dryland vineyards, or where water resources are limited, mulching up to 16 cm on the grapevine row is expected to buffer the grapevine against drought conditions or reduce yield losses during dry seasons.