Handbook for Irrigation of Wine Grapes in South Africa

Chapter 7

7.2.5 PULSE IRRIGATION When more than one short irrigation is applied per day, it is referred to as pulse irrigation. When applying pulse irrigation, only an extremely small volume of the soil is wetted. These small wetted volumes might be insufficient if heat waves last for a couple of days. Pulse irrigation also creates a huge risk of severe grapevine water constraints if unforeseen failures in the water supply occur. The latter may be caused by dripper clogging, breakages in the irrigation system, or unreliable electricity sources, particularly where the water needs to be pumped as opposed to gravity feed. A further concern is that evaporation losses from the wetted soil will be almost continuous. Phase 1 evaporation, i.e. high water losses, will prevail during, as well as between the irrigation pulses. This implies that the irrigation water use efficiency will be unacceptably low. The fact that the irrigation system needs to remain filled between irrigation pulses, also poses practical, as well as financial challenges. Considering the foregoing, pulse irrigation is not recommended for wine grape vineyards. In certain cases, the reason for pulse irrigation is to apply water and fertilizers according to the crops’ daily requirements. The irrigation consists of a number of carefully timed irrigation pulses with nutrient enriched water, usually applied by means of drippers. The objective of this strategy is to minimize water and nutrient constraints in the grapevine. Due to its similarity to the hydroponic systems used in glass houses or plastic tunnels, this concept is referred to as an “open” hydroponic system. A field trial with table grapes in the Agter-Paarl region showed that irrigation with nutrient enriched water held no advantage in terms of yield over (i) low frequency fertigation where the same amounts of fertilizers were either applied in three increments during the season or (ii) weekly fertigation applied from two weeks after budbreak until ten weeks after harvest, except during berry ripening. The only advantage of the OHS strategy seemed to be notably less berry crack following rainfall during ripening, compared to the conventional fertigation strategies. It must be noted that the field trial was carried out with Dan-Ben-Hannah (Black Emperor) which is highly susceptible to berry crack, compared to other cultivars. A matter of concern is that the pulse irrigation required ca. 490 mm irrigation, compared to ca. 260 mm for the weekly drip irrigation (Myburgh & Howell, 2012a). This illustrates the huge evaporation losses due to frequent wetting of the soil. As a result of the short drip irrigation pulses during daytime, root systems had adapted to the small wetted volumes (Fig. 7.14). Such limited root systems increase the risk of serious damage to yield and quality if breakdowns in the water supply occur. A further problem is that the soil may become acidic directly beneath the drippers (Fig. 7.15A). Excess nutrients could cause salt accumulation around the small wetted volume, i.e. even in non-saline soil (Fig. 7.15B). Although nutrition was applied according to the relatively conservative norms of Conradie (1994), it appeared to be slightly excessive under the prevailing conditions. The acidity and salinity, respectively, below and around the root system may restrict root

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