Cassava (Manihot esculenta Crantz) is a woody root crop which belongs to the family Euphorbiaceae and takes it centre of origin from Brazil, South America. It is widely grown in both tropical and sub-tropical regions of the world mainly for its edible starchy storage roots [1] Cassava is an important source of dietary carbohydrate and provides food for over half a million people in Nigeria [2] Omoregie [3] reported that cassava serves some useful economic purposes; these include:

• garri, the traditional product which is consumed in granule form,

• fufu/akpu which has assumed a national spread in consumption,

• tapioca and usi (starch) which are delicacies among the Urhobos, Itsekiris, and Ijaws of the Niger Delta, materials for industrial and domestic uses,

• the leaves which are used as vegetable and

• chips which are used for livestock feed.

The roots contain about 25 to 35 percent starch, the leaves, although not important in some areas, contain a significant amount of protein and some mineral nutrients [4].Cassava has long played a very important role in ensuring food security, particularly in Sub-Saharan Africa, where food security is a problem. Nigeria currently is positioned as the world largest cassava producer, followed by Thailand, Indonesia and Brazil [4]

Responses of cassava to fertilizer application are very variable under different nutrient and climatic conditions [6]Nair and Sadananadan [7] reported that N and K applications increased the yield obtained by applying 90kgN, 18kg P and 75kgK/ha on Utisol testing low in N and K and medium in P. Highest rate of tuberization and yield response were obtained with a combination of several levels of NPK 15:15:15 at 60, 90 and120 days [8]. Nigerian soils are fragile and inherently infertile [9]. This infertile nature of the soils has led to reduction in yields, and resource – poor farmers are unable to meet national food demands. All that is required is to sustain the fertility of the soils and modify or improve on them [9].

To increase the yield potential of cassava, the crop had been reported to respond to good soil fertility and adequate fertilizer [10] Farmers do not fertilize cassava because they are contented with the minimal yields obtained from it using limited inputs or even from their infertile soils. The indifference towards low productivity can be attributed to the low and unstable prices of cassava tubers. However, fertilizer requirement for optimum yield in cassava is determined by soil fertility status of the farmland, intensity and soil improve the cassava yield. Potassium level in soil stimulates response to N cropping K levels in fertilizers  but excess amount of both nutrients leads to luxuriant growth at the expense of tuber formation [11,12] It is therefore necessary to embark on aggressive cassava production in this agro-ecology to ensure that Nigeria attains sufficiency in cassava production. This can be achieved mainly by cultivating improved and adaptable cassava cultivars. In order for cassava to reach its full production capacity, there is need to address nutrient deficiency. Reports on the nutrient requirements and response of cassava to fertilizer in Delta State, Nigeria are limited. Consequently, there is need to provide information on the missing link for better production of cassava in Delta State. Therefore, this study was conducted to assess the effects of nitrogen and potassium status of selected soils and their effects on the growth, yield and cyanide content of cassava in Delta State, Nigeria.

• Location:

  The study was carried out in Delta State, Nigeria. Delta State which lies in the geographical coordinates of 6° 18' 0" N, 6° 23' 0" E in the Greenwich meridian. Rainfall occurs mainly from April to October. Annual rainfall is usually from 2000mm – 3000mm with an intense sunlight, which lasts for a minimum of 8 hours daily. Temperatures are high for most parts of the year, especially in the months of November to April with a mean monthly temperature of 31°C. The annual range of temperature is thus small, only varying between 30°C and 50°C. Relative humidity varies from 60% during dry season to about 90% in the rainy. The experiment was done at Abraka because N and K were found to be deficient, from soil analysis.

• Land Preparation and Fertilizer Application:

The land used for the experiment was manually cleared, and debris packed without burning. Nitrogen and Potassium were applied at rates of 0, 40, 80 and 120 kgN/ha and 0, 20, 40 and 80 kgK/ha, respectively. The applications were made based on N and K contents of the soil resulting from soil analysis. Nitrogen was applied as urea while Potassium was applied as muriate of potash.

A proven variety of cassava TME (419) obtained from the International Institute for Tropical Agriculture (IITA) Ibadan was used in this study. The design was a 4 x 4 factorial scheme fitted into a randomized complete block design giving sixteen treatment combinations with three replicates. The following treatment combinations were applied N₀K₀kg/ha, N₀K₂₀kg/ha, N₀K₄₀kg/ha, N₀K₈₀kg/ha, N₄₀K₀kg/ha, N₄₀K₂₀kg/ha, N₄₀K₄₀kg/ha, N₄₀K₈₀kg/ha, N₈₀K₀kg/ha, N₈₀K₂₀kg/ha, N₈₀K₄₀kg/ha, N₈₀K₈₀kg/ha, N₁₂₀K₀kg/ha, N₁₂₀K₂₀kg/ha, N₁₂₀K₄₀kg/ha and N₁₂₀K₈₀kg/ha. The soils were mixed with various levels of nutrients using urea to supply nitrogen and muriate of potash as source of potassium at 4 weeks after planting (WAP) by using the side placement method.

The size of each plot  measured 5m x 4m (20m²) with spacing of 1m between plots, and 1.5m between replicates, giving a plant population of 20 plants per plot (10,000 plants/ha). There were, thus, a total of 48 plots (16 x 3). The experimental area was 1701m² (0.17 ha). Weeding was carried out manually by hoeing at 3 WAP and repeated at 8 WAP for the second operation. Subsequent weed control was by rouging at 14 and 20 weeks after planting (WAP). Data collection was carried out at 2, 4, 6, 8 and 10 months after planting (MAP). Four plants from the centre were randomly selected per plot, tagged and sampled as necessary. Assessments of the following parameters were done:

• Plant Height:

A tape rule was used to measure the height of plants from the soil surface to the top on the main branch. The stem girth was determined by using a string around the plant at the first internode and later spread on a ruler. The measurements were expressed in cm.

• Fresh Tuber Weight:

Four mature tagged cassava plants at the centre rows (net plot) were selected from each plot for harvesting and weighing.

• Data Analysis:

All data of parameters were subjected to the appropriate analysis of variance (ANOVA) and Correlation procedures as described by Steel and Torrie, 1980.

• Soil Analysis Result:

The texture of the soils varied from sand to loamy sand. Soil samples for N0K80, N₄₀K0, N₄₀K_, N₄₀K₆₀ N₈₀K₆₀ and N₈₀K₈₀ were found to be sandy at the top soil while N₀K₀, N₀K₂₀, N₀K₆₀, N₈₀K₂₀, N₁₂₀K₀, N₁₂₀K₂₀, N₁₂₀K₆₀ and N₁₂₀K₈₀ were found to be loamy sandy texture (Table 1).

Total N content of the soils at all the location ranged from 0.03% in N₀K₀ to 0.17% in soil for N₀K₀ and N₁₂₀K₀ (Table 2) at the surface using a critical level of 0.15% (FDALR, 2004). The soils at the location had a moderate total nitrogen contents.

Available phosphorus for the locations ranged from 3.23mg/kg at the surface (Table 2). This ranged from low to high. Most of the soils of the various location treatments were low in phosphorus with the exception of soils of N₀K₆₀ that increased in phosphorus content. N₀K₆₀ had high phosphorus content while N₀K₀ where low in available phosphorus when compared to the established critical value of 17mg/kg [14]

Table 1: Physical Properties of Soils (0 - 15cm) after 1^st cropping

                                 S = Sand; LS = Loamy Sa

Table 2: Chemical Properties of Soils (0 -15cm) after 1^st cropping and application of N and K

Av. P =       Available phosphorus

EC     =       Electrical conductivity

ECEC=       Effective cation exchange capacity

OM    =       Organic matter

T        =       Trace

• Plant Height:

The effect of N fertilizer application on plant height of cassava at Abraka soils are shown in Table 3. The highest mean plant height of 213.2cm was obtained from120kg/ha at 10^th MAP. This was not significantly different from the mean plant heights obtained from the other treatments while the effect of K fertilizer application on plant height of 207.0cm obtained from 80kgK/ha was highest and not significant at 4, 6, 8 and 10 MAP. The application of120kg/N and 80kg/K gave the highest mean plant height when compared to other treatment combinations. N and K applications generally resulted in increased plant height even though this trend was not consistent during some of the months of planting. Nitrogen has been reported to favour plan height of cassava especially at the initial stage of growth in respect to vegetative character and tuber yield and this is in consonance with previous findings of Utomakili and Enobakhare [14,15] 

Table 3: Effect of Nitrogen and Potassium fertilizer on mean plant (cm) of cassava (TME 419) varieties at Abraka

Figures in the column and rows followed by the same letter are not significantly different at 5% level of probability for each location.

NS = Not significant. Fresh Tuber Weight:

The highest fresh weight of 20.5t/ha were obtained from the application of 120kgN/ha +20kgK/ha. The application of N resulted in different yields among the treatments. A fresh tuber weight of 16.37t/ha was obtained from the application of 20kg/ha. The application of K resulted in significant difference among the treatments. The interaction between N and K had significant effect on fresh tuber weight in Delta State (Table 4).  The application of 120kgN/ha +20 gave the best cassava tuber weight. This suggests that high level of N with a corresponding relatively high level of K interacted effectively, to enhance cassava performance. According to Ayoola (2006), K in the soil is readily released for cassava plant. Odeline [16] found out that 60kgK/ha and 120KgN/ha gave highest cassava yield among the rates he considered in his study. He also observed that N requirement of cassava can only reduce when P is applied in adequate amount. The influence of higher potassium fertilization in increasing cassava yield was earlier reported by [17,18] in soils with medium or low K status.

Table 4: Effect of nitrogen and potassium fertilizer on cassava fresh tuber weight (t/ha)

Figures in the column and rows followed by the same letter are not significantly different at 5% level of probability for each location. NS = Not significant.

 Hydrogen Cyanide (HCN) Content of Tubers:

The highest HCN content of 22.5mg/kg was obtained from the application of 80kgN/ha + 20kgK/ha, while the lowest value were 9.3mg/kg for the applications of 40kgN/ha + 80kgK/ha. However, higher application rates of N reduced HCN contents. In the same vein, HCN content was raised from 16.0mg/kg in the control to 22.5mg/kg when K was applied at 80kg/ha.

The interaction between N and K fertilizers increased HCN level at lower application rates but the reverse was the case at higher rates.

High levels of nitrogen in the soil tends to increase the level of glucoside in cassava plant within a particular cultivar, the exact level glucoside can vary slightly, depending on environmental factors [19]

Table 4: Effect of nitrogen and potassium fertilizer on level of hydrogen cyanide (HCN) content in tubers.

Figures in the column and rows followed by the same letter are not significantly different at 5% level of probability for each location. NS = Not significant. 

The determination of the effects of nitrogen and potassium status of selected soils and their effects on the growth, yield and cyanide content of cassava (Manihot esculenta Crantz) variety was the principal objective of this study. The study was conducted in Delta State. Soil analysis to determine the levels of plant nutrients in the soils was carried out. The determination of the effects of nitrogen and potassium status on the growth, yield and cyanide content was determined. However, the application of 120kgN/ha +20 (20.5) gave the best yield while the highest HCN content of 22.5mg/kg was obtained from the application of 80kgN/ha + 20kgK/ha in Delta State, Nigeria.

The authors declare that they have no conflict of interest

No funding sources

The study was approved by the University of Delta Agbor

1. Henry, C. (2006). Cassava Improvement in Sub-Saharan African and North Eastern Brazil. Proceedings of the First International Meeting on Cassava Breeding. Biotechnology and Ecology, held in Brasilia, 11-15 November, 102-108.

2. Bassey, E. E. (2016). Field Evaluation of Cassava Genotytpe for Tuber Bulking, Early Maturity and Optimum Harvest time in Uyo,South Eastern Nigeria. Journal of Agricultural Science, 1(15), 83-88.

3. Omoregie, A. U. (2005). The export drive for cassava: implications for the Nigeria Economy. Proceedings of the 39^th Annual Conference of the Agricultural Society of Nigeria. 37-39.

4. Azaino, E. (2008). Business opportunities in the cassava value chain in Nigeria. In A paper presented at Agricultural Product Workshop Organized by Uptonville Foundation by Rivers State Sustainable Development Agency. Port Harcourt.

5. Food & Agricultural Organization FAOSTAT. (2016). Cassava Quantity (tones) for all countries, Rome, pp.2-9.

6. Leakey, C. L. A., & Wills, J. B. (1977). Food crops of the lowland tropics. Oxford University Press.

7. Nair, S. M. & Sadananadan, J. E. (1985). Late Cassava Development. Canadian Journal of Botany, 30,110-1120.

8. CIAT. (1985). Cassava Program Annual Report for 1995. Cali, Colombia: CIAT.

9. Agboola, A. A., & Unamma, R. P. A. (1991). Maintenance of soil fertility under traditional farming systems. In Proceedings of National Organic Fertilizer Seminar, 26-27.

10. Gomez, J. C., Howeler, R. H., & Webber, E. J. (1980). Cassava production in low fertility soils. In: Toro MJC, Graham M (eds.) Cassava Cultural Practices. Bowker Publication Co Ltd, Epping, U. K.

11. Madhava, R. D., Satyanarayana, M., & Venkatasubba, R. K. (1986). Nitrogen and potassium requirements of cassava. Journal of Research, 2, 157-161.

12. Onwueme, I. C., & Charles, W. B. (1994). Tropical root and tuber crops: production, perspectives and future prospects (No. 126). Food & Agriculture Org..

13. Agboola, A. A., & Corey, R. B. (1993). Soil Test Calibration for NPK for Maize in Soils Derived From Metamorphic and Igneous Rock of Western Nigeria. Journal of west African Science Association,17, 93-100.

14. Utomakili, V. B., & Enobakhare, D. A. (1995). On farm cassava variety performance study in Bendel State Nigeria. Nigeria Agricultural Journal, 27, 87-90. 

15. Agba, O. A., Ogar, E. A., & Odey, S. O. (2005). Efficacy of nitrogen on the growth and yield of maize (Zea mays L.) in Obubra, Cross River State. Journal of Agriculture, Forestry and the Social Sciences, 3(1), 35-40.

16. Odedina, S. A. (2005). Evaluation of NPK rate combinations for maize production in humid zones of Nigeria. Nigeria Journal of Soil Science, 15 (2), 111-115.

17. Enwezor, W. U., Udo, E. J., Usorah, N. J., Ayotade, A., Adepeu, J. A., Chude, V. O., & Holland, N. V. (1989). Fertilizer use and management practices for crops in Nigeria. Fertilizer Procurement and Distribution Division, Federal Ministry of Agriculture. Wastes Resources and Rural Development, Lagos, 1- 24.

18. Odiete, I., Ejekwolu, C. C., Adamu, J. U., & Adegboye, M. A. (2006). Different rates of P. fertilization on some yield indices of Irish Potato Nicola Spp. under a typical Haplustalfs at Kuru, Jos. An International Journal for Agriculture Research, 2(1), 72-77.

19. IITA International Institute for Tropical Agriculture. (1990).Annual Report.