Effects of potato (Solanum tuberosum L.)–Mucuna pruriens intercropping pattern on the agronomic performances of potato and the soil physicochemical properties of the western highlands of Cameroon

2.1 The study site

The experiment was conducted at the teaching and research farm of the Faculty of Agronomy and Agricultural Sciences of the University of Dschang, located at an altitude of 1,396 m a.s.l, between latitudes 5°10′ and 5°38′ North and longitudes 9°50′ and 10°20′ East. Dschang is located on the southwestern slope of the Bamboutos Mountains, dominated by a low plateau that is strongly dissected by small valleys that are sometimes marshy. The climate is characterized by a dry season from mid-November to mid-March and a rainy season from mid-March to mid-November [11]. The vegetation cover was made up of Tithonia diversifolia, Mimosa pudica, Ageratum conyzoides, Cyperus and Bidens pilosa. The sol is predominantly oxisols, it is well drained with a characteristic red color.

2.2 Experimental design and crop husbandry

The experiment was laid out in a randomized complete block design with three replications. Each experimental unit had a surface area of 7.8 m² and a spacing of 0.5 m and 1 m between the experimental units and the blocks, respectively, giving a total surface area of 166.6 m² for the field trial. The planting materials used were Irish potato (Solanum tuberosum L. Var. Desirée) and Mucuna pruriens (var. Utilis). Potato and Mucuna pruriens seeds were purchased from “pepinière semence d’avenir” in Dschang.

The potato and Mucuna pruriens were grown in sole stands and as intercrops. Pre-sprouted tubers were planted at a uniform depth of 10 cm and two Mucuna pruriens seeds were planted per hole. The different treatments that gave rise to the uniform density of 31,250 plants per hectare for potatoes and 62,500 plants per hectare for Mucuna pruriens included the following:

T1: Potato sole stand (0.4 m × 0.8 m);

T2: Mucuna pruriens sole stand (0.4 m × 0.8 m);

T3: Potato and Mucuna pruriens in 1:1 intercropping pattern (0.4 m × 0.8 m);

T4: Potato and Mucuna pruriens in 1:2 intercropping pattern (one row of potato alternate with two rows of Mucuna pruriens with the distance of 0.2 m × 1.6 m for potato and 0.4 m × 0.8 m for Mucuna pruriens;

T5: Potato and Mucuna pruriens in 2:1 intercropping pattern two rows of potato alternate with one row of Mucuna pruriens with the distance of 0.4 m × 0.8 m for potato and 0.2 m × 1.6 m for Mucuna pruriens.

NPK (11:11:22) fertilizer was applied 21 days after planting (DAP) at 200 kg ha⁻¹ [12]. Weeding and mulching of potatoes and staking for Mucuna were carried out manually 21 DAP. The potato was sprayed against the late blight disease using the fungicide Bonsoin (30% Chlorothalonil + 6% Cymoxanil) at 1,500 g ha⁻¹. This treatment was carried out every week beginning from week 5 after sowing following the appearance of the first symptoms of the late blight disease. Tubers were harvested 70 DAP, and Mucuna pruriens harvested 180 DAP.

2.3 Data collection

2.4 Growth and yield variables

Weekly measurements of growth variables (plant height (cm), number of leaves, and collar diameter) on six randomly selected potato plants began at 26 DAP and ended at 63 DAP. Yield variables consisted of potato tuber weight and Mucuna grain weight. Yield (tubers and grains) was recorded in kg ha⁻¹ and converted into potato equivalent yield (PEY) [14], using the following equation:

where PY and MY are potato and Mucuna yield (in kg ha⁻¹), respectively; PP is the potato market price (192 CFA kg⁻¹), and MP is the Mucuna market price (2,500 CFA kg⁻¹).

2.5 Biological efficiency of potato–Mucuna intercropping patterns

The biological efficiency of the potato–Mucuna intercropping patterns was accessed using appropriate indices that included the land equivalent ratio (LER), area time equivalent ratio (ATER), aggressivity (A), competitive ratio (CR), and the relative crowding coefficient (K).

2.6 LER

LER indicates the efficiency of intercropping using environmental resources compared to monocropping [15]. LER was calculated as follows:

where PY and MY are the yield of potato and Mucuna, respectively.

2.7 ATER

It is given by the formula below [16]:

where LER_p = Ypi/Ypp and LER_m = Ymi/Ymp.

LER_p and LER_m represent LER of potato and Mucuna, respectively; Ypi is the intercrop yield of potato; Ymi is the intercrop yield of Mucuna; Ypp and Ymp are potato and Mucuna sole crop yield; t _p and t _m are durations of potato and Mucuna in days, and T is the total duration of the intercropping system in days.

2.8 Aggressivity

Aggressivity (A) is a competitive index, which is a measure of how much the relative yield of one crop component is greater than that of another [17]. Aggressivity is expressed as

where Zpi and Zmi represent the sown proportion of potato and Mucuna in intercropping, respectively.

2.9 Relative crowding coefficient (RCC or K)

It is a competitive index, used as a competitive power coefficient to measure the relative dominance or aggressiveness of either legume on potato or vice versa in an intercropping system. It is calculated following the equations below [18]:

2.10 CR

CR was used to assess the competitive ability of the component crops in an intercropping system. It was calculated using the following formula below [19]:

2.11 Disease incidence of late blight

Disease incidence was evaluated according to James [20]:

2.12 Data analysis

Collected data were processed in Microsoft excel 2010 software. The MINITAP version 17 (INC USA) was used for one-way analysis of variance at a 5% significant level, and means were separated using Tukey’s test.

3.1 Effects of intercropping pattern on potato growth variables

The number of leaves, plant height, and collar diameter showed no significant difference (P > 0.05) for the different intercropping patterns studied (Table 1), indicating that the growth of potatoes was not influenced by intercropping patterns.

3.2 Intercropping pattern versus yield and potato equivalent yield (PEY)

Yield (tuber and grain) and potato equivalent yield showed a significant difference (P < 0.05) among intercropping patterns (Table 2). The highest potato yield (24,913 kg ha⁻¹) was recorded by 1:1 intercropping pattern compared to the other treatments in order potato sole stand (23,438 kg ha⁻¹), 2:1 intercropping pattern (21,207 kg ha⁻¹) and 1:2 intercropping pattern (17,066 kg ha⁻¹). Furthermore, potato yield in 1:1 intercropping pattern was not statistically different (P > 0.05) from the yield of the sole stand. Also, results showed that the grain yield of Mucuna (4,347 kg ha⁻¹) was significantly higher in the 1:1 intercropping pattern. However, this was comparable to the grain yield of the sole stand of Mucuna and the 2:1 intercropping pattern (Table 2); 1:2 intercropping pattern produced the lowest grain yield of Mucuna (2,568 kg ha⁻¹). PEY was highest in intercropping patterns of 1:1 (81,513 kg ha⁻¹), 2:1 (71,798 kg ha⁻¹), and 1:2 (50,500 kg ha⁻¹) as compared to the potato sole stand (23,438 kg ha⁻¹). However, PEY in the 1:2 and 2:1 intercropping patterns were not statistically different.

3.3 Biological efficiency of potato–Mucuna intercropping patterns

3.4 Competitive intensity on potato–Mucuna intercropping pattern

A significant effect of the intercropping pattern was observed on potato aggressivity (Ap) and CR (P < 0.05). Ap and CR values were positive in 1:2 (1.20 and 1.78, respectively) and negative in 1:1 (−0.76 and −0.60) and 2:1 (−1.63 and −1.74) intercropping pattern (Table 4), indicating that potato was a dominated species in 1:2 but dominated Mucuna in 1:1 and 2:1 intercropping pattern.

Intercropping patterns did not have a significant effect on the relative crowding coefficient (K) (P > 0.05). However, in all intercropping patterns, K-values were greater than one which indicates a yield advantage (Table 4).

3.5 Soil physicochemical properties as influenced by potato–Mucuna intercropping patterns

There was no significant effect of intercropping patterns on soil physical properties (P > 0.05). Nevertheless, compared to the initial physical properties (Table 6), the sand content was found to decrease to 1.51%; silt and clay increase by 9.83 and 5.26%, respectively.

For soil chemical properties, pH, N (%) and OC did not vary significantly (P > 0.05) with intercropping patterns but intercropping patterns showed a significant effect (P < 0.05) on CEC and C/N (Table 6). Compared to initial soil chemical properties, there was an increase of 36.66%, 11.93%, 1.81% of N (%), OC (%), and pH, respectively, after the cropping period. C/N was decreasing for all intercropping patterns compared to potato sole crop and initial C/N at the following rate: 26.31% (1:1), 10.52% (1:2), and 5.26% (2:1). The CEC was increasing in 2:1 (36.12 meq/100 g) and decreasing in 1:1 (27.28 meq/100 g) intercropping patterns.

3.6 Relationship between plant traits and soil physical and chemical properties

Physicochemicals properties did not have a significant correlation (Table 7) with growth variables (P > 0.05). There was a positive correlation between growth variables and sand (%) and negative correlations with growth variables and clay (%) after harvest. Only the number of leaves gave a positive correlation with silt% (r = 0.148). Concerning chemical properties, there was a positive correlation between N (%), OC (%), C/N, and CEC with growth variables. However, pH had a positive correlation (r = 0.025) only with the plant height. Mucuna pruriens yield was not significantly correlated with physical properties. However, it was negatively correlated with sand (%) (r = −0.485) and silt (%) (r = −0.246) while positively correlated with clay (%) (r = 0.412). There was a significant negative correlation between Mucuna pruriens yield with C/N (P = 0.017; r = −0.606) and CEC (P = 0.003; r = −0.718) while positively correlated with pH, N (%) and OC (%). Potato yield and PEY were not significantly (P > 0.05) correlated with physicochemical properties. Amongst physical properties, only sand (%) (r = 0.492) was positively correlated with potato yield, while PEY was positively correlated with sand (%) (r = 0.062) and clay (%) (r = 0.023). Potato yield was positively correlated with chemical properties, while PEY was negatively correlated with C/N (r = −0.253) and CEC (r = −0.363).

4.1 Effect of intercropping pattern on potato growth variables

The results of the analysis of variance showed that there was no significant difference (Table 1) between treatments for the number of leaves, plant height, and collar diameter. This may be due to the difference in growth duration between potato and Mucuna. Potato had a fast growth rate compared to Mucuna and the fact that maximum requirements for growth resources occur at different times might have reduced competition for growth resources in each intercropping pattern. Undie et al. [21] and Muoneke et al. [22] had a similar finding in which sole maize and intercropped with soybean did not have significant differences in terms of plant height. Also, Thobatsi [23] found that maize intercropped with cowpea did not have any effect on maize plant height.

4.2 Effects of intercropping pattern on tubers, Mucuna grain, and potato equivalent yield (PEY)

Potato and Mucuna yields varied significantly with intercropping patterns (Table 2). Generally, the yield was higher in the 1:1 intercropping pattern than the other treatments but not significantly different with the potato sole stand. Similar potato yield in a 1:1 intercropping pattern and potato sole stand (Table 2) would imply a positive interaction between the two crops, which could be explained by temporal shoot architectural differences. During the early development stage of the potato, the lower–lying canopy of Mucuna could have enabled the potato to obtain enough solar radiation for photosynthesis, resulting in higher tuber yield. Additionally, increased ground cover due to relatively higher Mucuna biomass at later stages of potato development could have further benefited potatoes by reducing water loss through evaporation and lowering the temperature within the canopy [24], [25]. These results concur with earlier findings of Wang et al. [26] and Jin et al. [27] who have stated that lower temperatures promote translocation of photo-assimilates to the developing tubers leading to higher tuber yield. In addition, Addo-Quaye et al. [28] found that the spatial arrangement of a single row of maize alternating with a single row of soybean gave a higher maize grain yield compared to maize planted with double rows of soybean. The reduction of yield under 1:2 and 2:1 intercropping patterns could be due to interspecific competition between the intercropped components for light, water, air, and nutrients, and also the aggressive effect of Mucuna pruriens on potatoes. It also might be attributed to the highest plant population. This corroborates with the works of Kidane et al. [29], who showed a significant increase in the productivity of sole-cropped potatoes in a maize–potato intercropping system, compared to intercropped treatments.

Potato equivalent yield was higher in intercropping patterns than in sole crops and the 1:1 intercropping pattern had the highest PEY (Table 2). This could mainly be attributed to the additional Mucuna grain yield and its high market price (2,500 CFA kg⁻¹). Similar results were found by Gitari et al. [14] and Rahman et al. [30] when intercropped potato with legume and brinjal, respectively. Furthermore, this high PEY in the intercropping patterns could be attributed to growing spaces being varied; temporal growth variance between two varying crops; and a combined increase in making better use of light, soil moisture content, and nutrients.

4.3 Biological efficiency on potato–Mucuna intercropping pattern

LER values recorded under different intercropping patterns were greater than 1 (Table 3), indicating a yield advantage of intercropping potato with Mucuna over monocropping. Also, the integration of Mucuna into the potato cropping system promotes growth and yield of the companion crop. However, the 1:1 intercropping pattern gave the highest LER followed by 2:1 and 1:2, which shows an advantage of 117, 90, and 38% respectively over pure potato stand concerning the use of environmental resources for plant growth and yield as indicated earlier [31,32]. In all intercropping patterns, the ATER values were lesser than LER values indicating the over-estimation of resource utilization may be due to the wide variations in the maturity periods of the crops of which Mucuna stayed longer on the land and had enough time to compensate for the potato competition. Bitew et al. [33] found parallel results with lupine–wheat and lupine–barley cropping systems.

ATER varied significantly among intercropping patterns (Table 3). It was higher than the unity in 1:1 and 2:1 intercropping pattern with the highest on 1:1 which show that 1:1 intercropping pattern provides the best yield advantage of cultivating potato and Mucuna compared to the monocropping by taking into consideration the time taken by the components crops under the intercropping systems in the field. However, the 1:2 intercropping pattern gave an ATER value less than unity; this indicates the inefficient biological efficiency of this cropping combination. This might properly be due to the long cropping period (180 days) of Mucuna pruriens and somehow the interspecific competition for available resources such as soil nutrients, sunlight, and water. This result is in accordance with those of Doubi et al. [34].

4.4 Competitive intensity on potato–Mucuna intercropping pattern

Intercropping patterns had a significant effect on potato aggressivity (Ap) and CR (Table 4). In the 1:2 intercropping pattern, the potato was a dominant crop in terms of both aggressivity and CR. This could be mainly explained by the intraspecific competition instead of interspecific competition due to the planting distances. The negative values for potato aggressivity and CR in 1:1 and 2:1 revealed that the potato was less competitive than Mucuna. This was an indication of yield loss, probably due to the competitive effect of Mucuna on potatoes when grown in an association. The competitive nature of Mucuna may be attributed to the invasive nature of Mucuna above-ground canopy which could have affected light interception by potatoes. A similar result was reported by Ibrahiem [35] on intercropping of roselle and cowpea. CR is only used as a measure of intercrop competition (inter-specific competition) [36].

In terms of the advantage of intercropping systems, the relative crowding coefficient (K) indicates a similar trend with LER. K did not vary significantly between the intercropping patterns but had values greater than unity (Table 4). These results indicate an advantage of intercropping for exploiting the resources. Doubi et al. [34] reported a similar result with K-values greater than 1 gourd and cassava intercropping systems.

4.5 Effects potato–Mucuna intercropping pattern on the incidence of late blight

The results of this study showed that generally potatoes intercropped with Mucuna had a lower disease incidence (Table 5) compared to potato sole crop (P < 0.05). Gaba et al. [37] stated that multiple cropping systems as well as intercropping can regulates pests in the broadest sense by preventing their growth, reproduction, or dispersal. Pest is less efficient in locating and colonizing its host due to the resource dilution of a host plant in the plant mixture [37]. The different intercropping patterns showed that increasing Mucuna rows among potato rows lead to a decreasing late blight incidence (Table 5). Kassa and Sommartya [38] obtained similar results and reported that, among the proportions, 75% garlic with 25% potato (3:1) intercropped plots showed significantly (P < 0.05) low disease development and high tuber yield.

4.6 Soil physicochemical properties as influenced by potato–Mucuna intercropping patterns

Cropping systems did not have significant effects on soil physical properties (Table 6). The high biomass production, which protected the soil particles from raindrop impact and being washed away by run-off, had led to improve their effects compared to the initial physical characteristics. Furthermore, Nyawade et al. [39] and Gitari et al. [14] found that, under intercropping systems, there is high root density, which binds soil particles, hence reducing the susceptibility of the soil to erosion.

In this study, the increasing pH, OC, and N values in potato–Mucuna intercropping systems (Table 6) support the importance of legume residues in improving soil fertility.

The high N content in potato–Mucuna treatments could be attributed to the high-quality biomass that contains high levels of N. This is in accordance with other findings reported by Muthomi et al. [40] that when legume residues are ploughed into the soil as green manure, they decompose rapidly to release N that is bound in them to the soil. This can explain the increasing pH, N, and OC values that were observed under intercropping systems, compared to initial values (Table 6). The carbon to nitrogen ratio (C:N) is an index of nutrient mineralization and immobilization whereby a low C:N ratio indicates a higher rate of mineralization and a higher C:N ratio indicates greater rates of immobilization [41]. The lowest C:N ratio was obtained from the 1:1 intercropping pattern. The 2:1 intercropping pattern recorded the highest CEC than sole treatments and other intercropping patterns. This might be attributed to the high accumulation of organic matter in this arrangement. In addition, the surface layer of organic matter which has negatively charged surfaces may have acted as an adsorption site for positively charged cations (Ca²⁺, Mg²⁺, K⁺, Na⁺).

4.7 Relationship between plant traits and soil physical and chemical properties

Whilst potato growth variables, yield, and PEY were not significantly (P > 0.05) correlated with proportions of sand, a positive correlation was nevertheless observed. This could be due to the fact that the reduction of proportions of sand after harvest, reduced water losses through evapotranspiration which facilitated nutrient uptake and enhanced crop growth and yield. A negative correlation between potato growth and yield with an increase in silt and clay could be due to the higher moisture-holding capacity of clay and silt which could have negatively affected crop emergence [42]. Furthermore, potato growth and yield had a positive correlation but non-significant (P > 0.05) with an improvement of chemical properties which indicates the efficient use of nutrients by potatoes when intercropping with Mucuna pruriens. Moreover a significant (P < 0.05) and negative correlation was observed between Mucuna pruriens yield and C/N, which is due to the high nitrogen production of Mucuna pruriens through symbiosis with rhizobia soil bacteria and higher Mucuna pruriens biomass production. This result is also observed with the positive correlation with Mucuna pruriens yield and N (%).