Abstract:
Cassava (Manihot esculenta Crantz) is a substantial contributor to the food and industrial crops sectors in most African, Asian and Latin American countries. However, cassava production in Africa has been threatened by the shortage of improved cultivars and a decrease in soil fertility linked with continuous cultivation, while inadequately applying macronutrients; nitrogen (N), phosphorus (P) and potassium (K). The objectives of this study were to: (i) Assess the growth and yield performance of two local cassava cultivars (P4/10 and MSAF-1) in Mbombela, Mpumalanga, South Africa (SA), (ii) Examine the effects of different combinations of NPK on cassava growth and yield, and (iii) Evaluate the utility of SPAD-meter for assessing N status in cassava plants. The P4/10 and MSAF-1 cultivars growth and yield parameters were evaluated in a field experiment. MSAF-2 cultivar was planted in a greenhouse to optimize N and K applied at 0, 100, 150 and 200 kg ha-1 and P at 0, 15, 20 and 30 kg ha-1. MSAF-2 was also planted in a greenhouse to assess N status of the plants fertilized at N rates of 0, 100, 150 and 200 kg ha-1. SPAD-meter readings collected from five different leaf positions (Lf1 – Lf5) located at the top of the main stem. The results showed that MSAF-1 growth and yield parameters were significantly higher (p < 0.05) than those of P4/10 over time. The differences between the parameters were attributed to genotypic variation since the cultivars were grown under the same conditions. The three-way interaction (N × P × K) had a significant effect (p < 0.05) on the growth and yield over time. Both growth and yield parameters increased with increase in NPK application rates. Growth parameters were promoted by the different NPK combinations, but the best combination for cassava output was 200 kg ha-1 N, 30 kg ha-1 P and 150 kg ha-1 K. SPAD readings were found to be significantly influenced (p < 0.05) by N application rates and leaf position over time. The N application rates and leaf N concentration levels at 98 days after planting (DAT) were positively correlated with the lower leaves (Lf4 and Lf5) together with Lf3, but negatively correlated with uppermost leaves (Lf1 and Lf2). However, the lower leaves readings were almost similar but higher than Lf3 and therefore their average was used to assess nitrogen sufficiency index (NSI). The highest mean NSI values were obtained when 200 kg ha-1 N was applied (NSI = 1.04) compared to 0.99 and 0.88 when 100 and 0 kg ha-1 N was applied.
