This study confirms the importance of responsible nitrogen, phosphorus and potassium fertilisation and underlines the fact that deficiencies of both nitrogen and potassium must be prevented.
The trial was conducted over four seasons (from 2009/10 to 2012/13) in a full-bearing, seven-year-old drip irrigated Shiraz/101-14 Mgt vineyard established on a sandy (0 – 300 mm soil layer) to sandy clay loam (300 – 600 mm soil layer) soil near Stellenbosch as described in Part 1. Broadcasting 30 kg/ha of phosphorus (P) during April 2011 and 15 kg/ha early May 2012, eventually increased the P level to 104 mg/kg and 56 mg/kg in the 0 – 75 mm and 75 – 150 mm soil layers, respectively. From November 2010 onwards, 30 kg/ha of potassium (K) was broadcasted during full bloom (late November/early December) and during post-harvest (May/June). Twenty eight kg/ha nitrogen (N) was broadcasted annually during June. During the 2010/11 season, 28 kg/ha N was applied during November as well. An additional 28 kg/ha N was applied late April 2011. To avoid excessive vegetative growth, however, no N was applied during November 2012.
Leaf nutrient status
Throughout the study, the level of N in the leaf blades of all the treatments was within the norm for South African vineyards (Table 1). The lower values observed during 2010, but especially during 2011 and 2012, was attributed to the increase in shoot mass observed throughout the study (see Part 1). Despite a luxurious supply of P in the 0 – 150 mm soil layer (see Part 1), the P concentration in the leaf blades and petioles did not exceed the maximum norm for South African vineyards. The level of K in the leaf blades and petioles remained below the generally accepted norm throughout the study. This trend was attributed to the continuous increase in shoot growth measured from year 2009 to 2012 (see Part 1).
Nutrient status of the juice
The level of N in the juice was very low for the duration of the study (Table 2), despite the N applied post-harvest, grapevine flowering (2010) and grapevine full bloom (2011). Nitrogen was not applied during grapevine full bloom in 2012, due to a continuous increase in the vegetative growth of the grapevines. The level of N in the juice indicated that the application of N during the growing season of the grapevines should definitely be considered, as long as the vegetative growth of the grapevines is in balance with the harvest. The P levels in 2011 and 2012 being above the generally accepted norm is an indication that the grapevines benefitted from the luxurious supply in the 0 – 150 mm soil level. The K applied from the grapevine flowering stage in 2010 to grapevine full bloom 2012 did improve the K levels in the must during the 2011 and 2012 harvests. The results indicate that K can be applied every year during post-harvest and grapevine full bloom, without jeopardising the quality of the must.
The above-mentioned results confirm the importance of monitoring the plant nutrient status annually and that the application of K and N at the flowering stage of grapevines growing on sandy soils should be considered favourably where the levels are below the generally accepted norms.
Monitoring the plant nutrient status annually is important. The application of K and N at the flowering stage of grapevines growing on sandy soils should be considered favourably where the levels are below the generally accepted norms. The importance of responsible N, P and K fertilisation was confirmed, which underlines the fact that deficiencies of both N and K must be prevented.
The author thanks the ARC, Winetech and Dried Fruit Technical Services for financial support, the staff of the Soil and Water Science Department at ARC Infruitec-Nietvoorbij for technical support and Blaauwklippen Wine Estate for supplying the trial site and farm support.
– For further information, contact Johan Fourie at FourieJ@arc.agric.za.