Abstract: Background and Objective: White yam is one of the most cultivated and accepted species of yam in Nigeria and its production is constrained by many challenges such as high cost of production and poor yield due to low soil nutrient amongst others. Hence, a study was carried out to evaluate the comparative effect of NPK (15:15:15) and poultry manure on the growth and yield of white yam. Materials and Methods: The experiment was laid out in a randomized complete block design with three replications. The experiment involved seven treatments: 400 kg ha–1 NPK (15:15:15), 200 kg ha–1 NPK+20 t ha–1 poultry manure (PM), 40 t ha–1 PM, 30 t ha–1 PM, 20 t ha–1 PM, 10 t ha–1 PM and control. NPK fertilizer was applied in about 3 drills made around the plant, while the poultry manure was applied on the mound as mulch at 6 weeks after planting. Results: Results indicated that NPK (15: 15: 15) produced the highest dry matter yield of 20.36 t ha–1 compared to poultry manure treatments which had dry matter yield of 14.93-16.42 t ha–1, while control had 15.21 t ha–1. The fresh tuber yield of poultry manure treatments varied from 45.89-57.22 t ha–1, while NPK fertilizer and control had fresh tuber yield of 48.22 and 48.67 t ha–1, respectively. Conclusion: Poultry manure at a rate of 40 t ha–1 effectively increased soil fertility and produced the highest tuber weight and yield.
INTRODUCTION
White yam (Dioscorea rotundata Poir.) is one of the most cultivated and accepted species of yam in Nigeria1. It is an important staple food crop which is grown for its edible tubers2. Yam tubers are eaten fried, boiled, roasted or pounded after boiling. It can also be processed into yam flour which stores well. In West Africa they are major sources of income and have high cultural value. Because of it multi-purpose uses, Africa alone is responsible for over 90% of the world production of yam; Nigeria accounts for over 70% of world production equaling more than 37 million tons3-5. However, yam tuber yield of 12.66 t ha–1 is below6 its yield potential due to constraints such as low soil fertility status and poor cultural practices among other factors7. The declining soil fertility may be due to continuous cropping, leaching, soil erosion, etc. Farmers in most of the developing countries continue to crop such infertile lands that do not guarantee sustainable production without external inputs, particularly the use of external sources of nutrient elements to improve crop yield8. Yam is a long period growing crop and needs a balanced supply of nutrients for higher tuber yield. This necessitates the need for soil fertility maintenance through external sources of nutrients for crop production. This can be effectively achieved through the application of either inorganic or organic fertilizers or a combination of both9,10. Farmers therefore need access to innovation to reduce fertility problems and improve productivity since bush fallowing can no longer sustain the soil fertility due to population pressure.
Inorganic fertilizer (NPK) was advocated in the past years to ameliorate low inherent fertility of soils in the tropics11. Ayoola12 reported that the experimental studies on fertilizer application showed increased crop productivity. It is easier to ensure a balance adequate supply of nutrients by applying mineral fertilizer13.
However, inorganic fertilizer enhances soil acidity, nutrient leaching, nutrient imbalance and degradation of soil physical properties and organic matter14,15. Since yam spends longer time on the field growing and producing tubers, it is therefore necessary to explore alternative means to improve the nutrient status of the soils under cultivation for sustainable production.
Organic fertilizers (poultry manures) are reservoir of nutrients like N, P, K and other macro and micro-nutrients for healthy growth of plants16. Organic fertilizers can sustain cropping system through better nutrients recycling and improvement of soil physical attributes17. Organic fertilizers could also help by reducing soil acidity where necessary18,19. However, they are required in rather large quantities to meet up crop requirements20. According to Ayeni et al.21, the combined application oif organic and inorganic fertilizer gave superior effect in terms of balanced plant nutrient and improved soil fertility.
Therefore, for farmers or researchers to meet the high demands in yam production, efficient innovation needs to be accessed to improve the soil fertility for sustainable growth. Farmers do not have adequate information on the actual amount of poultry manure needed for soil improvement or the combined effect of NPK fertilizer and poultry manure for growth and yield of white yam. Thus, this study examines the comparative effects of NPK (15:15:15) and poultry manure on the growth and yield of white yam in the rainforest agro ecology of Benin City, Nigeria.
MATERIALS AND METHODS
Experimental site: The experiment was conducted during the growing period of 2017, at the Teaching and Research Farm of the Faculty of Agriculture, University of Benin, Benin City (06°20‘E, 5°39’E; 78 m asl) in the Rainforest of Nigeria. The soils are underlain by sands, clayey sands and discontinuous clay sequences of Benin Formation of the Niger delta Basin classified as ultisols22. Rainfall is of high intensity and bimodal, beginning in March/April and ending in October/November with a dry little spell in August usually referred to as “August Break”. About 2025 mm of precipitation falls annually in Benin City with an average annual temperature of 26.1°C23. The zone has a total growing period of 211-270 days24. Prior to experimentation, the trial field was under fallow for a year and was dominated by weeds such as Minosa pudica (sensitive plant) and Panicum maximum (guinea grass).
Experimental design and treatments: The experiment was laid out in a randomized complete block design with three replications. The treatments were randomly assigned to the experimental units. The treatment had seven levels [400 kg ha–1 NPK (15:15:15), 200 kg ha–1 NPK (15:15:15)+ 20 t ha–1 poultry (PM), 10 t ha–1 PM, 20 t ha–1 PM, 30 t ha–1 PM, 40 t ha–1 PM and control (zero application). Each block had a dimension of 8 m×7 m and was demarcated into 7 experimental units. Each experimental unit in a block consists of a single row plot of 6m long and spaced at 1m interval. The blocks were separated by 1.5 m.
Cultural practices: At the onset of the growing period, the experimental site was cleared of vegetation using cutlass and the debris were removed. The mounds were made using hoe in each experimental units. The seed yams/sets of ‘Abakaliki’ cultivar (white guinea yam) were used for the trial. They were weighed before planting and the average weight were recorded (about 0.61 kg). The yam setts were sown on the 23rd of April, 2017 at a spacing of 1 m×1 m. Each treatment was a 6 m row plot and yam mounds were made at interval of 1 m within the row. Staking of yam shoot were done when the yam shoot were about 1 m long and were unable to support themselves, using a single erect stake (Wooden bamboo) of about 6 m in length per plant. Weeding was done manually with the use of hoe and cutlass throughout the experiment. At about 6 weeks after planting the NPK (15:15:15) and poultry manure were applied on the mounds. The NPK fertilizer was applied in about 3 drills made around the plant while the poultry manure was applied on mound as mulch. No pesticide was used, throughout the experiment. Harvesting was done on the 10th of November, 2017 when the plants have attained maturity.
Data collection: Data were collected from two representative samples in each treatment row in a block. At 12 weeks after planting, the number of vines, length of 10th internode, stem girth, height at first branching and length of petiole were determined. The number of vines growing from each mound was determined by counting the number of shoots or vines that developed from the seed yam. The length of the 10th internode of the two representative samples was determined by measuring the distance between the two nodes using a measuring tape. The stem girth of the two representative samples was determined by placing a rope around the girth and the dimension was read using a measuring tape. Measurement from the ground level to the first branch on the vine using a measuring tape was done and recorded as the height at first branching. At 20 weeks after planting, the length of the petiole was measured and the leaf area was determined by measuring the length of the leaf (L) and widest portion (B) and multiplying the product of L and B by 0.6425. On harvesting, the middle 3 m portion were harvested, weighed and tubers length and girth were measured. The tuber number and number of vines at harvesting were also taken and tuber per vine estimated. Fresh tuber yield was estimated in t ha–1 and dry matter yield in t ha–1 was determined by oven drying a fresh small sample to a constant weight for 24 hrs at 70°C.
Laboratory analysis: A composite soil sample (0-30 cm depth) was taken from the site using a soil auger prior to land preparation and at harvest for the routine soil physical and chemical properties, using standard laboratory procedures26.
The poultry manure was milled with a bench hammer mill to obtain a uniform sample. The milled material was dried for 1 hr in an oven to drive off residual moisture and cooled in a desiccator. The poultry manure was then analyzed for chemical properties (Organic carbon, total nitrogen, available phosphorus, calcium, magnesium, potassium, bulk density and pH).
Statistical analysis: All data were subjected to statistical analysis using SAS27 to conduct analysis of variance (ANOVA) and the means were separated using Student-Newman-Keuls Test for multiple mean comparisons.
RESULTS
Chemical composition of poultry manure and soil physical and chemical properties: The results of the chemical composition of the poultry manure are presented in Table 1. The pH of the poultry manure was neutral and had a very low C:N ratio (2.25). The poultry manure contained some appreciable amount of plant nutrients. Before planting, the soil at the trial site was sandy loam and was moderately acidic (Table 2). The soil had the following properties; organic carbon of 0.96 %, pH 5.92, total N content 0.06 %, available phosphorus 0.70 mg kg–1, calcium content 1.28 Cmol kg–1 and potassium content 0.07 Cmol kg–1. The poultry manure and NPK fertilizer applied to the yam mound had effect on the physical and chemical properties of the soil (Table 2).
Effect of poultry manure and NPK (15:15:15) on soil physico-chemical properties: Treatments influenced the pH of the soil. The highest soil pH of 6.26 was obtained at 10 t ha–1 PM. There was no significant difference between the combined effect of 200 kg ha–1 NPK+20 t ha–1 PM (5.28) and control (5.34) which had the lowest soil pH. All the treatments had no significant effects on the soil texture (sand, silt and clay) except when 400 kg ha–1 NPK was applied which reduced the silt and clay content of the soil. Likewise low clay content was obtained in the control. Treatments increased the soil organic carbon significantly. The highest organic carbon content of 1.52 % was obtained from the application of 40 t ha–1 PM. The combination of 200 kg ha–1 NPK+20 t ha–1 PM, 400 kg ha–1 NPK, 10 t ha–1 PM and control had comparable but low organic carbon content when compared with other PM treatments. The nitrogen concentration of the soil was increased by the treatments. The highest nitrogen concentration was obtained from the application of 400 kg ha–1 NPK, followed by the application of 40 t ha–1 PM.
| Table 1: Analysis of poultry manure | |||||||
| (Cmol kg–1) | |||||||
pH |
Bulk density (g cc–1) |
Org C (%) |
Total N (%) |
Aval. P (mg kg–1) |
Ca |
Mg |
K |
7.19 |
0.63 |
8.29 |
3.68 |
42.46 |
11.05 |
3.19 |
0.64 |
| Table 2: Effect of poultry manure and NPK (15:15:15) on soil physic-chemical properties | |||||||||||
| Exchangeable bases (Cmol kg–1) | |||||||||||
| Treatments | pH |
Sand |
Silt |
Clay |
Org carbon (%) |
N (%) |
P (mg kg–1) |
Ca |
Mg |
K |
Na |
| Soil sample before planting | 5.92 |
82.00 |
4 |
14 |
0.96 |
0.06 |
0.7 |
1.28 |
0.2 |
0.07 |
0.04 |
| NPK (15:15:15) 400 kg ha–1 | 5.50c |
84.00a |
3.00b |
13.00b |
1.24c |
0.17a |
4.20e |
1.12c |
0.50d |
0.16a |
0.093ab |
| NPK (15:15:15) 200 kg ha–1+ | 5.28d |
82.00a |
4.00a |
14.00a |
1.24c |
0.08d |
39.00c |
1.00d |
0.30e |
0.12d |
0.11e |
| PM 20 t ha–1 | |||||||||||
| PM 40 t ha–1 | 5.50c |
82.33a |
4.00a |
14.00a |
1.52a |
0.13b |
94.00a |
1.28b |
0.64b |
0.12b |
0.08bc |
| PM 30 t ha–1 | 5.97b |
82.00a |
4.00a |
14.00a |
1.39b |
0.11c |
70.00b |
1.32b |
0.55c |
0.12b |
0.10ab |
| PM 20 t ha–1 | 5.57c |
58.00a |
4.00a |
14.00a |
1.38b |
0.09d |
38.00c |
1.04d |
0.30e |
0.10bc |
0.12a |
| PM 10 t ha–1 | 6.26a |
82.00a |
4.00a |
13.67a |
1.24c |
0.08d |
20.00d |
1.68a |
0.24f |
0.08c |
0.12a |
| Control | 5.34d |
83.00a |
4.00a |
13.00b |
1.20c |
0.07d |
1.78e |
1.12c |
0.80a |
0.08c |
0.06c |
Means with the same letters or alphabets are not significantly different. PM: Poultry manure |
|||||||||||
However, the combination of 200 kg ha–1 NPK+ 20 t ha–1 PM, 20 t ha–1 PM, 10 t ha–1 PM and control had similar but low nitrogen concentration. Available phosphorus was significantly increased after the growing season with the application of PM and NPK fertilizer (Table 2). The highest value of 94 mg kg–1 was obtained from the application of 40 t ha–1 PM, followed by the application of 30 t ha–1 PM (70 mg kg–1) when compared with other treatments. However, lower available P concentration were obtained from the application of 400 kg ha–1 NPK and control. Exchangeable Ca concentration was not significantly influenced by all the treatments. However, the highest value of 1.68 Cmol kg–1 was obtained from 10 t ha–1 PM. The combined application of 200 kg ha–1 NPK+20 t ha–1 PM had the lowest Ca content. There was no effect in Ca content of the soil when 40 and 30 t ha–1 PM was applied. Exchangeable Mg concentration was significantly increased by the treatments (Table 2). The highest exchangeable Mg concentration was obtained from control when compared with the other treatments. All the treatments had significant effects on exchangeable K concentration. The highest exchangeable K concentration was obtained from 400 kg ha–1 NPK fertilizer. There was no difference in Na content when 10, 20 and 30 t ha–1 PM and 400 kg ha–1 NPK was applied which was higher than other treatments (Table 2).
Growth: Growth characteristics of white yam as influenced by poultry manure and NPK fertilizer is presented in Table 3. There was no significant difference among the treatments for internode length, stem girth at 12 and 20 Weeks after Planting (WAP), height at first branching, leaf area, length at first internode and number of vines. However, 400 kg ha–1 NPK had the highest internode length, closely followed by the combination of 200 kg ha–1 NPK+20 t ha–1 PM and the least value was obtained from control. Highest height at first branching was obtained when 200 kg ha–1 NPK+20 t ha–1 PM was applied, while control had the lowest height at first branching. Poultry manure at 20 t ha–1 had the highest leaf area, while control had the lowest leaf area. Although, 20 t ha–1 PM had the shortest length at first internode, control had the longest length at first internode. For all the treatments, stem girth increased as the plant grows. At 12 WAP, 400 kg ha–1 NPK had the widest stem girth, while at 20 WAP control had the widest stem girth. However, 30 t ha–1 PM had the narrowest stem girth both at 12 and 20 WAP. The combined effect of 200 kg ha–1 NPK+20 t ha–1 PM had the longest leaf petiole, while 40 t ha–1 PM had the lowest leaf petiole. The highest number of vines was obtained from 30 t ha–1 PM and the lowest was obtained from 20 t ha–1 PM (Table 3).
Tuber yield: Tuber characteristics and yield of white yam as influenced by poultry manure and NPK is presented in Table 4. There was no significant difference among the treatments for number of vines at harvest, number of tubers, tuber per vine, tuber length, tuber girth, tuber field weight, fresh tuber yield and dry matter yield. However, high number of tubers was obtained from 40 t ha–1 PM, while the lowest number was obtained from 10 t ha–1 PM. High tubers per vine and long tubers were obtained when 40 t ha–1 PM was applied compared to other treatments. Also, fresh tuber yield were highest for 40 t ha–1 PM (57.22 t ha–1) and was followed by 20 t ha–1 PM (47.22 t ha–1), but somehow lower than control (48.67 t ha–1).
| Table 3: Effect of different levels of poultry manure and NPK (15:15:15) fertilizer on growth characteristics and yield of D. rotundata | ||||||||
| Treatment | Inter node length (cm) |
Stem girth (cm) 12 WAP |
Height at 1st branching (cm) |
Leaf area (cm2) |
Length at 1st inter node (cm) |
Stem girth (cm) 20 WAP |
Length of petiole (cm) |
Number of vines |
| NPK (15:15:15) 400 kg ha–1 | 29.22 |
2.08 |
12.33 |
65.88 |
4.80 |
3.27 |
7.47 |
3.33 |
| NPK (15:15:15) 200 kg ha–1+ | 28.50 |
2.00 |
18.33 |
73.41 |
4.917 |
3.00 |
8.02 |
3.67 |
| PM 20 t ha–1 | ||||||||
| PM 40 t ha–1 | 23.45 |
1.63 |
13.62 |
71.20 |
5.18 |
3.35 |
6.43 |
3.33 |
| PM 30 t ha–1 | 26.27 |
1.60 |
14.82 |
65.63 |
4.15 |
2.72 |
7.17 |
5.33 |
| PM 20 t ha–1 | 20.90 |
2.02 |
9.23 |
82.26 |
3.70 |
3.18 |
7.28 |
3.00 |
| PM 10 t ha–1 | 22.32 |
1.92 |
12.67 |
76.96 |
5.17 |
2.717 |
7.00 |
3.33 |
| Control | 20.7 |
1.68 |
8.12 |
63.93 |
5.35 |
3.667 |
6.95 |
4.33 |
WAP: Weeks after planting, PM: Poultry manure |
||||||||
| Table 4: Effect of different levels of poultry manure and NPK (15:15:15) fertilizer on tuber characteristics and yield of D. rotundata | ||||||||
| Treatment | No. of vines at harvest |
No. of tubers |
Tuber per vine (kg) |
Tuber length (cm) |
Tuber girth (cm) |
Tuber field weight (kg) |
Fresh tuber yield (t ha–1) |
Dry matter yield (kg ha–1) |
| NPK (15:15:15) 400 kg ha–1 | 3.25 |
5.33 |
1.64 |
37.33 |
38.37 |
14.47 |
48.22 |
20.36 |
| NPK (15:15:15) 200 kg ha–1+ | 3.47 |
5.00 |
1.44 |
33.03 |
41.32 |
11.87 |
39.56 |
10.8 |
| PM 20 t ha–1 | ||||||||
| PM 40 t ha–1 | 3.35 |
6.33 |
1.89 |
37.67 |
40.35 |
17.17 |
57.22 |
16.42 |
| PM 30 t ha–1 | 5.04 |
6.00 |
1.19 |
35.00 |
41.50 |
13.93 |
46.44 |
14.94 |
| PM 20 t ha–1 | 3.00 |
5.33 |
1.78 |
37.5 |
41.35 |
14.17 |
47.22 |
15.12 |
| PM 10 t ha–1 | 3.18 |
4.67 |
1.47 |
36.00 |
40.75 |
13.77 |
45.89 |
15.18 |
| Control | 4.33 |
5.67 |
1.31 |
36.00 |
39.67 |
14.6 |
48.67 |
15.21 |
PM: Poultry manure |
||||||||
The most slender yam tuber was obtained when 400 kg ha–1 NPK was applied. However, the highest dry matter yield was obtained when 400 kg ha–1 NPK was applied, while 200 kg ha–1 NPK+20 t ha–1 PM had the lowest dry matter yield (Table 4).
DISCUSSION
The soil of the experimental site was sandy loam and slightly acidic with low organic carbon, nitrogen concentration, available P, exchangeable K, Ca and Mg. This suggests that the soil was low in fertility. The soils are underlain by sands, clayey sands and discontinuous clay sequences of Benin Formation of the Niger delta Basin classified as ultisols22. This characteristic feature implied that ultisols are known to be in low nutrient status28. The nature of the soil cannot enhance full yield potential capacity of yam production in the agro ecology.
The poultry manure contained appreciable amount of plant nutrient for sustainable crop production. The application of poultry manure to soils with low fertility status enhanced favourable yield and growth characteristics of white yam, which could be due to their rich nutrient composition. Therefore, poultry manure as an organic fertilizer is a better enhancer of soil thereby improving crop growth and yield. This finding is in agreement with the observation of Ibeawuchi et al.29 who reported that poultry manure has high organic matter percentage, with increase in the other soil chemical components. It is an indication that poultry manure has high potential of gradual nutrient release to the soil that can help to improve the fertility of a degraded soil; thereby sustaining yield under continuous cropping system. The acidic nature of the soil was effectively reduced when 10 t ha–1 PM was applied. This implies that small amount of poultry manure is better used to reduce soil acidity. This finding is in agreement with Moyin-Jesu and Adeofun19 who reported that organic fertilizers help in reducing soil acidity. The highest organic carbon content obtained from the application of 40 t ha–1 PM indicated that increased quantity of poultry manure supplies more organic carbon to the soil thereby improving the soil. The higher the organic manure, the higher the organic carbon. Nwaoguala and Law-Ogbomo7 reported that high organic carbon improves the structure of the soil, allowing better penetration of air, water and plant roots. High quantity of NPK fertilizer supplies greater amount of Nitrogen concentration and potassium to the soil. However, it does not stay long in the soil due to soil erosion, leaching, volatilization, etc. The application of high poultry manure (30 and 40 t ha–1 PM) tends to improve the soil N, Avail. P and K content for a longer period of time for crop utilization. This is corroborated with the finding of Duncan30 who reported that organic fertilizer increased N, P and K contents of the soil.
During the growth period, the measured attributes had consistent pattern for all treatments. That is, application of poultry manure and NPK fertilizer increased the fertility of the soil. The long internode and increased height at first branching obtained using NPK fertilizers suggested that N is released faster using inorganic fertilizer due to quick mobilization which is needed for growth. Nitrogen is a constituent of protein and protoplasm; it vigorously induces the vegetative development of the plant31. However, that of control was the opposite. This may be because of the poor intrinsic soil fertility. Reduction in stem girth as at 20 WAP may be due to loss in soil nutrient over time. High number of vines for 30 t ha–1 PM and low number of vines for 20 t ha–1 PM suggested that at higher amount of PM the vine length is increased while at low amount the vine length is shortened. This may be the reason for high leaf area for 20 t ha–1 PM; the nutrient needed for longer vines may have been diverted for wider leaves production.
The no significant difference in tuber characteristics suggested that the treatments and control had appreciable nutrient for sustainable yield in the season. However, the high number of tubers, long tuber length, high tubers per vine, tuber field weight and fresh tuber yield obtained from 40 t ha–1 PM implied that, in improving the fertility status of the soil for a longer time, high amount of organic fertilizer is required for efficient soil amendment. This may be in response to the high NPK content of the soil by Poultry manure. This findings concurred with Nyathi and Campbell20 who reported that organic manure are required in rather large quantities to meet up crop requirements. Slender yam tubers obtained when 400 kg ha–1 NPK was applied further buttressed that NPK chemical fertilizers are easily mobilized and therefore quickly removed from the soil as at when needed which resulted in poor yielding. However, NPK (15: 15: 15) produced the highest dry matter yield of 20.36 t ha–1 compared to poultry manure treatments (14.93-16.42 t ha–1) and control (15.21 t ha–1).
Poultry manure is a cheaper alternative to inorganic fertilizers and is a more concentrated source of crop nutrients, especially N, P, K and calcium. Being naturally organic, it does not need composting and can be applied directly to the fields from the farm. It has additional benefits of improving the physical and biological attributes of the soil and can therefore sustain crop production in tropical soils32,16. In this study, the poultry manure was applied on the yam mounds as mulch at 6 weeks after planting. This method could help to reduce the labour cost of application of the poultry manure. Babasola et al.33 reported the challenges affecting the use of organic manure among farmers in the following order:
| • | High difficulty in the transportation of organic fertilizer from the source to the point of use |
| • | High labour cost of application |
| • | Difficulty in the use of organic fertilizer |
However, the drawback with the method of application used in this study is that the poultry manure was not mixed with or covered with soil for easier contact with the plant roots and may be prone to being washed away from the yam mound during rainfall. Moreover, the poultry manure was not given some time to mineralize before planting. In further study, effort should be made to compare the method of application used in this study with the method of working the poultry manure into the soil/mound about 3 weeks before planting in order for the poultry manure to mineralize being a slow release fertilizer.
CONCLUSION
The results of the study showed that the soils of the experimental site were slightly acidic and inherently low in essential plant nutrients and this was identified as one of the main constraints to yam production in West Africa. The application of poultry manure at a rate of 40 t ha–1 effectively increased soil fertility and produced the highest tuber weight and yield. Compared with the mineral fertilizer, the application of poultry manure did not result in higher soil N levels. Consequently, its use poses no potential ground or surface water contamination hazard.
SIGNIFICANCE STATEMENT
This study discovered the potential of poultry manure as a good alternative to mineral fertilizer for the growth of white yam. Its benefits include a comparable fresh tuber and dry matter yield, a better nutrient balance and a possibility of improved residual N effect. This study will help the researchers and growers to improve the declining yield of yam through better management of tropical soils which have been rendered infertile as a result of continuous cropping and use of mineral fertilizer. Thus, a new theory on poultry manure as a possible replacement for mineral fertilizer may be arrived at.