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Cover Crop Management and the Performance of Grapevines in the Olifants River Valley
INTRODUCTION
The maintenance and improvement of soil quality is a critically important factor in the process of establishing an environment for future generations within which sustainable agriculture may be practised (Reeves,1997). Intensive mechanical cultivation causes a decrease in the organic matter content and aggregate stability of the top soil layer, which promotes crust formation, resulting in water runoff and erosion during rainfall or irrigation (Laker, 1990). A mulch, on the other hand, prevents crust formation (Radcliffe et al, 1988) and significantly reduces water runoff and erosion (Louw & Bennie, 1992). Moreover, a mulch limited evaporation from the soil surface in a flood irrigated vineyard in Oudtshoorn (Van Huyssteen et al., 1984). Clean cultivation should, therefore, not be applied in intensively irrigated vineyards.
The grapevine industries endeavour to limit the use of agricultural chemicals to the essential minimum. In the Olifants River valley, the total area under grapevines amounts to 9 350 ha (Anonymous, 2001). Approximately 60% of these vineyards are situated on soils adjacent to the flood-plain of the Olifants river, consisting mostly of Karoo soils and aeloic sands. On such soils, with a low organic matter and clay content, inorganic N are readily leached. A cover crop supplies nitrogen to the vine mainly in the form of complex organic compounds. In this form the nitrogen does not leach so easily from the topsoil, but slowly become available to the plant's roots throughout the season (Moulds, 1986). Leguminous cover crops can fix a considerable amount of N per annum, depending on seeding date and growth period (Harris, 1986; Schultz et al., 1999).
Van Huyssteen & Weber (1980) found that cover crop management under dryland conditions resulted in an increase in grape production, compared to mechanical cultivation or full surface chemical control. It was, therefore, important to determine the impact of the various soil cultivation practices on the performance of young grapevines under intensive irrigation.
The purpose of this study was to determine the effect of grain and nitrogen fixing broadleaf species, as well as the management thereof, on the soil, grapevine performance and water consumption of grapevines established on light textured soils in the Olifants River valley.
MATERIAL AND METHODS
Saia oats and grazing vetch (Table 1) were sown the second week of April in a Sauvignon blanc/Ramsey vineyard established on a sandy soil (Table 2) on the Nietvoorbij experiment farm at Lutzville. Seed bed preparation was done with a disc harrow approximately six weeks before sowing. Just before sowing, the crust on the soil surface was broken with a ghrop and after sowing the seeds were covered using a ghrop. Saia oats were sown at a sowing density of 100 kg/ha and grazing vetch at 50 kg/ha. During seed bed preparation, superphosphate was applied at 150 kg/ha (29 kg P). The vines received 60 kg/ha KCl (30 kg K) and 100 kg/ha LAN (28 kg N) post-harvest, as well as 60 kg/ha KCl and 150 kg/ha LAN (42 kg N) during spring. Saia oats received 50 kg/ha LAN (14 kg N) at the two to four leaf stage. One treatment of each species did not receive any fertiliser from the 1996/97 season onwards, while the spring application of N in the treatment in which grazing vetch was controlled chemically before budbreak (BB) and received the standard amount of fertiliser (SF), was halved as from the 1995/96 season. An irrigation of 18 mm was applied by means of 5 mm per hour micro-sprinklers, with a delivery of 5 mm per hour, the first four weeks after sowing. Thereafter the irrigation schedule was extended to once every three weeks. If rainfall occurred in this period, the amount of water applied was adjusted downward accordingly. The BB treatments were sown annually, while the treatments that were left to die back naturally and irrigated compensatorily (AB), were sown every second year only.
The organic matter content of the soil, the nitrate content in the leaf stems, the total N in the must and the crop mass were determined according to standard ARC Infruitec-Nietvoorbij methods. The cumulative profit was calculated at an income of R1 500 per ton of grapes. The water requirement of the various treatments was determined with tensiometers and the vineyard irrigated accordingly. The seasonal water consumption was monitored with the aid of water meters.
RESULTS AND DISCUSSION
Nitrogen status of the vineyard
Grazing vetch (nitrogen fixing broadleaf species) displayed the ability to make a contribution to the nitrogen status of the grapevines during the course of the growing season (Table 1). The nitrate in the leaf stems of grazing vetch (BB, SF) already pointed towards an oversupply of N (more than 1 000 mg/kg, Conradie, 1994) to the Sauvignon blanc/Ramsey vines in the 1995/96 season. Consequently, the spring application of N in this treatment was withdrawn as from the 1996/97 season, without a detrimental effect on the nitrogen status of the vines. The nitrate in the leaf stems of the grazing vetch treatment that received no fertiliser as from the 1996/97 season (NF), was still at an acceptable level in the following growing season (more than 700 mg/kg, Conradie, 1994). Although the nitrate in the leaf stems of this treatment was low in the 1997/98 season, it normalised in subsequent seasons. The shoot growth, however, declined annually from the 1996/97 season onwards (Table 3), indicating that the cover crop was unable to supply in the nutritional need of the vines. Despite this, the nitrogen level in the must, as measured from the 1996/97 season (first season in full production) up to and including the 2000/2001 season, exceeded 400 mg/l (Fig. 1), considered by Jiranek et al. (1995) to be the minimum amount necessary for normal fermentation. In the case of grazing vetch (BB) the shoot growth in the 2000/2001 season also indicated that the cover crop could not supply sufficiently in the grapevine's need for N (Table 3). This may be ascribed to the cover crop's poor performance (a dry matter production (DMP) of only 0.82 tons/ha in that particular season). In the case of grazing vetch (SF,) that was allowed to die back naturally (AB), the nitrate in the leaf stems also increased drastically in the 1997/98 season. This indicates that even with this management practice, nitrogen, derived from the cover crop, becomes available to the grapevines early in the growing season.
The nitrate levels in the leaf stems (Table 1), N content in the must (Fig.1) and shoot growth (Tabel 3) of Saia oats (NF), all indicate that the grapevines were undersupplied with regard to N in subsequent seasons.
Grape production
The average grape production of all the treatments in which a cover crop was established exceeded that of the mechanically cultivated control by between 0,72 and 2,72 tons/ha (Fig. 2). Even so, no difference in grape or wine quality was observed between treatments (data not shown). It appears, therefore, that the fertiliser applied during spring and the additional irrigation were sufficient to prevent competition between the growing cover crop and the grapevines. The higher production of Saia oats (AB, SF) compared to Saia oats (BB, SF), can only be explained in the light of the higher DMP and the accompanying better quality cover crop that was obtained in the case of the former (data not shown). A similar result was obtained with grazing vetch also not to the same extent. This can possibly be ascribed to the additional nitrogen that was supplied to the vine by the cover crop in the growing season (Table 1). There is no explanation for the relatively high average grape production of the grazing vetch (BB, NF) treatment. Whether this trend will continue, will have to be seen in future.
Economy
The average input cost of Saia oats' AB, SF and BB, NF treatments, as well as those of grazing vetch (BB, NF), was lower than that of the mechanically cultivated control (Fig. 3). The average input cost of grazing vetch (BB) was the highest and R162 more than that of the control. The cumulative profit of the cover crop treatments was consistently more (between R7 515 and R28 740) than that of the mechanically cultivated control (Fig. 4). It is therefore financially beneficial to apply cover crop management in the Olifants River valley.
Water and soil
Due to the low winter rainfall in the region it is necessary to irrigate the cover crops in winter. Consequently the savings in water consumption due to cover crop management are not big (Fig. 5). The average seasonal water consumption of the treatments in which the cover crops were chemically controlled before budbreak, were, with the exception of Saia oats (BB, NF), between 8 and 74 mm water per hectare per annum less than that of the mechanically cultivated control. However, if the cover crops were not controlled chemically before budbreak, the water consumption was between 11 mm and 97 mm higher than that of the control.
Five years after the different treatments were implemented, the organic matter content of the top 0-150 mm soil layer of the grazing vetch treatments was higher than the initial amount (Fig. 6). In order to obtain the same effect with Saia oats, it seems necessary to establish the cover crop annually, fertilise during establishment and apply chemical control before budbreak, so that sufficient dry matter may be produced annually. In the case of the mechanically cultivated control, the organic matter content decreased by 26%. In the medium term, therefore, cover crop management appears to improve soil fertility.
SUMMARY
It is beneficial to apply cover crop management in the Olifants River valley for the following reasons:
LITERATURE
ANONYMOUS, 2001. South African Wine Industry Statistics nr. 25. SAWIS, PO Box 238, Paarl.
CONRADIE, W.J., 1994. Wingerdbemesting. Handleiding van die werksessie oor wingerdbemesting, Nietvoorbij, 30 September, ARC Research Institute for Fruit, Vine and Wine, Private Bag X5026, Stellenbosch, 7600 R.S.A.
HARRIS, R.E.,1986. Cover crops for the Western Cape orchards. Decid. Fruit Grower, 36, 359-362.
JIRANEK, V., LANGRIDGE, P. & HENSCHKE, P.A., 1995. Amino acid and ammonium utilization by Saccharomyces cerevisiae wine yeasts from a chemically defined medium. Am. J. Enol. Vitic. 46, 75-83.
LAKER, M.C., 1990. Die invloed van Landbou wanpraktyke op grondagteruitgang en omgewingsbestuur. Plantvoedsel 2, 4-6.
LOUW, P.J.E. & BENNIE, A.T.P., 1992. Water runoff and soil erosion in vineyard soils. Austr. Grapegrower & Winemaker Annual Technical Issue, 110-113.
MOULDS, G.A., 1986. Cover crops are very beneficial. Austr. Grapegrower & Winemaker 267, 12.
RADCLIFFE, D.E., TOLLNER, E.W., HARGROVE, W.L., CLARK, R.L. & COLABI, M.H., 1988. Effect of tillage practice on infiltration and soil strength of a Typic Hapludult soil after ten years. Soil Sci. Soc. Am. J. 52, 798-804.
REEVES, D.W., 1997. The role of soil organic matter in maintaining soil quality in continuous cropping systems. Soil & Tillage Res. 43, 131-167.
SCHULTZ, S., KEATINGE, J.D.H. & WELLS, G.J., 1999. Productivity and residual effects of legumes in rice-base cropping systems in a warm-temperate environment I. Legume biomass production and N fixation. Field Crop Res. 61, 23-35.
VAN HUYSSTEEN, L. & WEBER, H.W., 1980. The effect of selected minimum and conventional tillage practices in vineyard cultivation on vine performance. S. Afr. J. Enol. Vitic. 1, 77-83.
VAN HUYSSTEEN, L., VAN ZYL, J.L., & KOEN, A.P., 1984. The effect of cover crop management on soil conditions and weed control in a Colombar vineyard in Oudtshoorn. S. Afr. J. Enol. Vitic. 5, 7-17.
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