Banana in South Africa: net profit increased by up to 19% with KNO3 application

Net profit increased by up to 19% by applying _KNO3 compared to other granular K sources containing KCl or _K2SO4 in a scientific trial investigating Williams banana yields in South Africa.

Large differences in the yield of the Williams banana plant were found in a scientific trial, testing the response to fertilization with prilled potassium nitrate (KNO₃(as Qrop® KS), potassium sulfate (K2SO₄) or potassium chloride (KCl). Average banana yield (total hand weight) across two varied rates of N and K was best for Qrop® KS at 35.4 MT/ha, and substantially lower for _K2SO4 (32.8 MT/ha, -7%) and KCl (28.5 MT/ha, -20%). The application of Qrop® KS resulted in the best performance in commercial yield and fruit quality parameters. Fruit weight and quality parameters were reduced when KCl was applied: a statistically significant difference. The application of Qrop® KS proved to be a profitable investment for the grower.

A total revenue increase of US$ 1,291/ha for Qrop® KS over SOP gives a three-fold return on the additional investment of US$ 385/ha, and a total revenue increase of US$ 3,423/ha for Qrop® KS over KCl gives a four-fold return on the additional investment of US$ 795/ha.

The highest yield of 40 MT/ha was recorded for Qrop® KS at the highest N and K rates. N applied at 300 kg N/ha and K applied at 700 kg K/ha (840 kg _K2O/ha) resulted in 24% and 19% higher marketable yields relative to less N (200 kg N/ha) or less K (450 kg K/ha, 540 kg _K2O/ha).

Table 1. The three variable factors in the trial. K sources were applied to supply the total of the two different K rates, total N was balanced with CAN (calcium ammonium nitrate, 28%N) and SFS (simple superphosphate, 15% P) is constant for all treatments. The amount of nutrients/ha for treatments with variations in different K sources is given as an example of how the K source affects the ratio of _N-NO3:N-NH4, S and Cl in this table only for treatments with high N and K rates.

The trial was established in the Lowveld region of South Africa in collaboration with AgNova (Pty) Ltd, in a randomized complete block factorial design, in which three sources of K, two rates of N and two rates of K were varied, in total 12 treatment combinations with 20 replicates each. This allowed showing that the differences between Qrop® KS and KCl were statistically significant at the 5% level for all measurements except for bunch weight, which, however, showed a strong decrease when using KCl. It could be concluded that chloride uptake had a negative impact on plant growth and thus on total yield. Alternatively or additionally, potassium nitrate applied with Qrop® KS may have resulted in more efficient nutrition and thus plant health due to its nitrate supply capacity to promote the uptake of the other cationic plant nutrients that constitute most of the nutrient salts required by plants.

Figure 1. Preparation of the planting area after cross harrowing and disk plowing.

Figure 2. Micro sprinklers installed, drip irrigation lines, the photograph is taken at the time of the second N and K fertilizer application in June 2019.

Study design

Details of the three variable factors of the trial are given in Table 1. Phosphate was applied at planting. 31.5 kg P/ha (72 kg _P2O5/ha) was applied with simple superphosphate (15% P). Nitrogen was applied as CAN (28% N) in treatments with _K2SO4 and KCl. For Qrop® KS treatments the amount of nitrogen provided as CAN was reduced by the amount of _N-NO3 applied with potassium nitrate. All treatment lots received a foliar spray application of a combination of micronutrients in the spring.

Per treatment, 20 individual plants were assigned as replicates in a completely randomized block design. Plants were planted in virgin soil in a decompacted and leveled site the year prior to planting (Figure 1). The soil was of the Hutton series with pHKCl 5.6 and CEC 4.2 cmol kg-1 soil with 71.2 % ^Ca2+ - 23.6% ^Mg2+ - 2.7% ^K+ and 2.1% ^Na+. The P content was 3 mg/kg (Bray1). Williams banana plants (tissue grown) were planted in early January 2019 and harvest started on June 25, 2020 (Figures 2-5). The planting frame was 3m x 2m (1,667 plants/ha).

In February, June and December, plants were provided with N and K in an application of granular fertilizer at the base of the plant, and water was supplied through micro irrigation in weeks of low rainfall (<20 mm/week) between April 2019-March 2020. To assess bunch productivity, the following yield parameters were recorded: total bunch weight (including rachis), number of hands per bunch, weight of each hand, total weight of hands, length of the two central fingers on each hand, and thickness (circumference) of the two fingers picked on each hand.

Table 2. Yield parameter, average of main effects by factor. Data were analyzed with an appropriate statistical method: ANOVA for main effects and interactions, followed by 95% separation of the mean MDS for the average of the factors.Averages followed by the same letters in the same factor are not statistically different at p<0.05.

Figure 3. Early March, cluster development a few weeks after calving.

Figure 4. Harvest time. Cluster harvest begins on June 25, 2020.

Table 3. Economic analysis of returns in this trial. The analyses deviate from the average effect on yield of different K sources over varying N and K rates.

Figure 5. Ready for bunch quality evaluations. Bunches were cut 20 cm above the most distal hand.

Total marketable yield, and the relative difference compared to Qrop® KS of Williams banana plants fertilized with potassium nitrate (Qrop® KS ), potassium sulfate (_K2SO4) or potassium chloride (KCl) as potassium source.

Outstanding results

The detailed results of the bunch analysis are in Table 2 and the marketable yield of the K sources compared to potassium nitrate in Qrop® KS is detailed in Figure 6. Thus, larger bunches have more hands, and heavier hands carry more, longer and thicker fingers. Greater bunch weight means an increase in all attributes. No interaction was found between the three factors (K source, N rate or K rate).

The use of Qrop® KS as a source of K resulted in higher plant productivity in total marketable hand weight with a higher return on investment for the grower. Increased levels of N and K boosted productivity, which is probably related to the high demand for both nutrients of the banana crop as well as the infertile state of the soil in which the trial plants were planted. The application of KCl reduced the total weight of the hands by about 20% compared to bunches produced by plants fertilized with _KNO3 for K, and 13% compared to bunches produced by plants fertilized with K2SO4 for K.

Based on fertilizer costs in 2020, the difference in cost (K + CAN source) between the programs with different K sources was: US$ 795/ha between Qrop® KS and KCl and US$ 385/ha between Qrop® KS and K2SO4. At US$ 8.82/ 18kg box, the total increased revenue per hectare per crop cycle for the producer is US$ 3,423 for Qrop® KS over KCl and US$ 1,291 for Qrop® KS over K2SO4 per hectare, which more than offsets the additional cost of Qrop® KS (Table 3).