Mulch treatments on nursery rootstock

9 June, 2009

The use of mulch in vineyards has become more common in recent years thanks to increased awareness of potential benefits and a desire to make better use of winery and vineyard waste products such as grape marc and prunings. Mulching grapevines can produce undesirable results if the wrong materials are used in the wrong situation. Introduction of disease, nutrient imbalance, increased frost risk, and effects on grape composition are amongst common concerns.  Several studies have reported benefits from mulching which include improved soil water retention during dry summers, weed suppression, increased soil nutrient availability and increases in yeast available nitrogen in grape juice.                          

A trial was undertaken at Corbans Viticulture to assess the effect of mulch treatments on 1YO 3309 rootstock vines.  The aim of this trial was to assess the suitability of several different mulching materials on young grapevine plantings. A number of measurements and observations were recorded to assess if and how the mulches affected vine and soil health. However, not all measurements could be evaluated for the purpose of this report, due to foliar damage caused earlier in the season. The results obtained highlight some key findings.           

Methods and materials

Mulch treatments were applied in February 2008. In the treatments where two materials were combined, each material had been composted or aged separately then combined on the trial site. Grape marc was collected from the 2007 vintage and had been composted over nine months.  Vine prunings (cane) were collected during the 2007 season, mulched using a wood chipping machine and aged in piles that went through a thermal composting cycle. Temperatures of 55 degrees C have been found to reduce plant pathogen numbers and kill some weed seed (Ingham 2004). Paper mulches manufactured by EcoCover Ltd were supplied as one meter wide rolls and were secured with biodegradable pins (except where additional mulch material was applied to cover the paper). Two paper mulches were included in the trial; a standard layered paper product (paper I) and a paper manufactured with fertilizer (paper II).  Wood chip supplied by a local arborist had been aged for 12 months.  Woodchip breaks down very slowly due to the high lignin content and can rob soil of nitrogen when applied to bare soil, especially if used when fresh.

Mulch treatments:

1. Marc plus grape cane
2. Wood chip plus marc
3. Paper I plus grape cane
4. Paper I plus wood chip
5. Paper II plus grape cane
6. Paper II control
7. Weed mat 
8. Wood chip control
9. Grape cane control
10. Paper mulch control
11. Wood chip plus grape cane
12. Negative control – no mulch

Results and discussion

Soil food web

Two mulch treatments, marc plus grape cane (1) and grape cane control (9) were compared with the negative control treatment (12). Both mulch treatments show a significant shift toward fungal dominance, which is desirable for grapevines, trees and most perennial plants.  Fungal dominated soils typically maintain a lower pH than bacterial dominated soils and fungal predators release nitrogen as ammonium, the preferred nitrogen source for vines and trees. Marc plus cane mulch (1) had the best ratio of fungal to bacterial biomass and the grape cane control (9) treatment was close to the desirable range. Grape cane control (9) had the best hypal (fungal body) diameter and fungal diversity, which indicates disease suppressive fungi populations are present. All three treatments have low active fungi and active bacteria counts indicating that both fungal and bacterial foods were low at the time of sampling. Whilst some active bacterial populations are desirable for grapevines, fungal foods would be considered the most important addition to this site.  Active bacteria to active fungi ratios indicate that all of the soils sampled will revert to bacterial dominance without the addition of fungal foods.  The very low active bacteria count for the marc plus grape cane (1) treatment was not expected. It is possible that bacteria populations bloomed with the addition of the mulch and then dropped sharply without recovering when the food source was exhausted.        

Soil dry weights did not change in either mulch treatment over the trial period.   

 Table1.  Soil Food Web test results excluding ratio comparisons

Soil temperature

Soil temperature for all mulch treatments was measured three times during the year at midday.  There was no significant difference in soil temperature between the treatments in March.  Mulch treatments on average were 0.25 degrees C warmer during May and 2.0 degrees cooler during September.  The moderating effect of mulches on soil temperature is likely to be more noticeable when comparing differences between night-time and day-time temperatures.  Whilst midday soil temperatures under mulch were cooler during September, it is likely that evening temperatures where higher due to the insulating effect of the mulch. 

Bud burst         

Mulch treatments had little effect on bud burst, with the exception of the marc plus grape cane (1) treatment which appeared to advance bud burst.   

Table 2. Bud burst comparison at time of monitoring

Weed suppression

Effective weed suppression was provided by most mulch treatments throughout the first season with one application of herbicide in June 2008.  By February 2009 weeds were well established in most mulched rows, more so where creeping grasses such as couch and kikuyu could easily spread from the inter-row sward.  Non-creeping weeds were better controlled with only isolated plants becoming established within mulched rows.  Control rows without mulch only had partial weed cover during the dry summer months but quickly became covered during autumn and again in spring without any herbicide application.  Auckland’s temperate climate means that weed control pressure is greater than in many wine growing regions where weed suppression from mulching may extend beyond one season.  Paper mulch alone had broken down by September 2008 but provided effective weed suppression when combined with either the cane or woodchip.         

Discussion

The use of the mulch treatments on this site did not induce any disease or nutrition problems in the first year after application. To reduce any potential disease transmission from the mulch, vine canopies should be trained or pruned to ensure foliage is not in direct contact with the mulch.  This is more easily achieved in a vineyard where grafted vines can be trained to a standard height wire. It is recommended that composted mulches should be used where possible, although some vineyards have been applying marc directly into the vineyard with positive results.

Whilst soil testing in this trial was limited, these results show that mulches including woody or high carbon materials can encourage desirable changes to soil biology with the potential to improve soil nutrient availability and disease resistance overtime. Greater benefit may be gained from mulches with increased diversity as indicated by the comparison of the cane only treatment with the cane plus marc treatment. Diversity of compounds within mulch will support a greater diversity of fungi and bacteria. Results also suggest that it is important to supply foods to maintain beneficial soil fungi and bacteria in addition to mulch. Humic and fulvic acid products, seaweed products and sugars such as molasses are often used for this purpose and are best applied at times of peak nutrient demand.

The mulch treatments reduced the number of herbicide applications required on the trial site from three applications to one during the first season.  This represents a cost saving along with the potential to reduce fertiliser requirements over a number of years as nutrient retention and availability improves. Even with below average rainfall during summer, signs of reduced water stress from mulching were not obvious on these vines.

Whilst Corbans Viticulture makes every effort to ensure the accuracy of information within this report, we accept no responsibility for information which may later prove to be misrepresented or inaccurate, or reliance placed on that information by readers.