Optimisation of nitrogen and potassium nutrition for selected new potato (Solanum tuberosum L.) cultivars

Dissertation (MSc (Agric))--University of Pretoria, 2019.

There are many older potato cultivars in South Africa such as BP1, Up-to-date and Avalanche. These older varieties of potatoes have been studied intensively under South African conditions such that nutrient requirements for such cultivars are well known. However, new cultivars are developed in order to improve yield, increase pest resistance and improve tuber quality, and as well as to meet the industrial market for processing. These newer cultivars are either developed locally or imported into South Africa. Nutrient requirements for these new cultivars have to be known under South African conditions, since nutrient requirements may vary among cultivars and even across regions. Knowledge of the nutritional requirements of such cultivars, especially nitrogen and potassium, which are taken up in largest quantities, are essential to fine- tune production management and improve yield and quality of tubers. Literature suggests that study attempts should not only focus on levels of N and K, but to also investigate N-K interaction (N:K ratios), since yield response to K is related to N status in the soil. For the aforementioned reasons, the study was conducted to evaluate the response of two newer potato cultivars to nitrogen (N) and potassium (K) levels in the South African environment. The studied cultivars were two foreign cultivars, namely Innovator and Lanorma. Two trials were conducted, first, a pot trial and then a field trial as a follow up. The pot trial was conducted between October 2015 to March 2016, while the field trial was conducted from September 2016 to February 2017. The two cultivars were evaluated at four levels of N and four levels of K for the pot trial, which then gave 16 N x K treatment combinations and 16 N:K ratios. The levels of N were 180, 230, 280 and 330 kg.ha-1 and K levels 160, 230, 300 and 370 kg.ha-1. For the field trial, treatments were reduced to three levels of N and K each, i.e. 160, 230 and 300 kg.ha-1 for both N and K, giving seven different N:K ratios. Destructive growth analyses were performed once during the growing season for the pot trial, while for the field trial destructive growth analyses were done four times during the growing season. During each harvest, plant height, dry leaf mass, dry stem mass, stolon length, dry tuber mass and tuber number were recorded. At final harvest, yield, specific gravity, chip colour score and dry matter content were recorded for statistical analysis. SAS was employed in order to perform an analysis of variance and means were separated using the LSD test at 95% probability level. Growth analyses results for both trials showed that Lanorma outweighed Innovator in terms of dry leaf and stem mass. Lanorma was also taller than Innovator and had longer stolons than Innovator. On the other hand, Innovator had a higher total dry tuber mass than that of Lanorma, although Lanorma had more tubers per plant. At final harvest for the pot trial, yield for Innovator was significantly higher than that of Lanorma. Yield for both cultivars were significantly influenced by N:K levels and ratios. N:K ratios ranging between 0.62 to 1.22 showed a tendency of better yield than yields outside that ratio range, provided none of the two nutrients were insufficient. Field trial yield was also influenced by the N:K ratio, similarly to the pot trial. In contrary to the pot trial, Lanorma had a significantly higher yield than Innovator in the field trial. Tuber specific gravity (SG) was also influenced by N:K ratio for both cultivars. For both pot and field trials, N:K ratios around 1.1 or less proved to have better SGs for both cultivars in most cases. Innovator, a processing cultivar, had higher SG values as compared to Lanorma. Yield and tuber size generally increased with increase in N. Yield also increased with increase in K up to 230 kg.ha-1 in most cases, whereafter it then remained constant with further K increase. For the specific conditions, it is recommended that N and K levels be kept at around 230 kg.ha-1 for both cultivars for optimal yield. However, if the priority is to improve tuber quality for processing, then a fertilizer treatment combination of 160 kg.ha-1 N and 300 kg.ha-1 K would be the best option due to lighter chip colour and higher specific gravity associated with that treatment combination.

Plant Production and Soil Science

MSc (Agric)

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