Improving the growth and yield of okra by intercropping with varying populations of legumes

An experiment was conducted at the Vegetable Research Farm of the National
 Horticultural Research Institute, Ibadan, Oyo State, Nigeria, in the
 rain-forest agro-ecological zone in 2016 and 2017 to determine suitable
 cropping systems to increase the yield of okra. The seed of okra
 (Abelmoschus esculentus (L.) Moench), cv. LD-88, was planted at a spacing of
 60 ? 40 cm as an intercrop and monocrop to produce an average density of 4.2
 plants?m-2; the intercrops cowpea (Vigna unguiculata (L.) Walp.), var. Ife
 brown, and peanut (Arachis hypogaea L.), var. Kampala, were planted to
 provide average densities of 5.6 plants?m-2, 4.2 plants?m-2, 3.3 plants?m-2
 and 2.7 plants?m-2. Data were collected on plant height, number of leaves,
 leaf area, stem diameter and fruit yield of okra. Year affected plant
 height, number of leaves, leaf area, stem diameter and fruit yield of okra
 intercropped with legumes at different densities. Legume densities affected
 plant height, number of leaves, stem diameter, leaf area and fruit yield.
 The interaction of year ? legume densities affected plant height, number of
 leaves, stem diameter, leaf area and fruit yield of okra. Intercropping okra
 with peanut at the density of 2.7 plants?m-2 enhanced its growth and yield
 and appeared to be the best configuration for these crops.

The appropriate crop and sowing densities are important in intercropping. The success of this way of farming depends on interactions between component crops and environmental conditions (Lithourgidis et al., 2011). Intensification in space and time, competition between and among system components for light, water and nutrients are concerns related to intercropping (Tajudeen, 2010).
Okra is a widely cultivated vegetable crop and very important in the diet of Africans (Omotoso and Shittu, 2008;Adewole and Ilesanmi, 2011). Fresh edible okra pods provide the supplementary vitamins A, B-Complex, C, iron, and calcium (Akanbi et al., 2010;Jaibir et al., 2004;Chutichudet et al., 2007). The pod mucilage has its medicinal properties as an emollient, laxative and expectorant (Khan et al., 2000). Many problems have been known to arise from the sole cropping system such as a build-up of pests and diseases and depletion of soil nutrients which have been reported to reduce the growth and yield of crops like okra (Iyagba et al., 2012). Very little has been reported on compatibility and suitable spacing of legumes intercropped with vegetables like okra. There is the need to investigate the appropriate density of legumes intercropped with okra to improve its yield.

Materials and Methods
The experiment was conducted in 2016 and 2017 at the Vegetable Research Farm of the National Horticultural Research Institute (NIHORT), Ibadan, Nigeria, in the rain-forest agro-ecological zone at 7°33'N and 3°56'E at 168 m above sea level. Soil samples were collected randomly at a depth of 0-15 cm with a soil auger before herbicide application and taken to El-Alpha Mega Services laboratory, Ibadan, Nigeria for analysis of physical and chemical properties.
Soil pH was determined in distilled water. About 10 g of air-dried soil (< 2 mm fraction) were put into separate 50-ml beakers, and 10 ml of distilled water were added into each beaker to attain the 1:1 ratio and allowed to equilibrate for 30 minutes with occasional stirring. The electrode was calibrated with pH buffers 4.0 and 7.0 before insertion into the suspension, and the reading was taken with a digital pH meter (Corning Mosel 220 digital -the United Kingdom). The average of two readings taken to one decimal place was recorded as the pH of the soil in water (Bates, 1954).
Particle size distribution was determined according to Bouyoucos (1951), where 100 g of air-dried 2-mm sieved soils were weighed into a dispersion cup, 50 ml of 5% sodium hexametaphosphate (Calgon) solution and 200 ml of distilled water were added and stirred with a glass rod. After 30 minutes, the suspension was stirred for 15 minutes with a mechanical stirrer, poured into a 1000-ml glass cylinder and distilled water was added to make 940 ml. The cylinder was vigorously shaken in a back-and-forth manner, placed on a table and the hydrometer inserted. The first hydrometer reading was taken after 40 seconds, and the temperature was also recorded. After two hours, the second hydrometer and temperature reading was taken, and the percentages of sand, silt and clay were determined thereof. The textural class of the soils was determined by using the USDA soil textural triangle.
The Walkley-Black method (1934) as modified by Heanes (1984) was employed, 0.5 g of 0.5 mm sieved soil was weighed into a 50-ml glass beaker, and 5 ml of 1M potassium dichromate were added and swirled to mix thoroughly. Thereafter, 10 ml of concentrated sulphuric acid were added into the suspension, and the mixture heated for exactly 30 minutes on a hot plate at 150 °C. After the mixture had cooled down, it was diluted to 50 ml with distilled water and allowed to stand overnight. This is to allow for a clear supernatant solution. Standard carbon solutions were prepared from oven-dried sucrose, mixed with the same volume of potassium dichromate and concentrated sulphuric acid and digested as the soils. The standards and samples were read on a spectrophotometer (Labomed 20D Spectrophotometer -the United States of America) at a wavelength of 600 nm using a 1-cm cell. The amount of C in the samples was determined from a standard curve.
Nitrogen was determined by the macro-Kjeldahl method. About 0.5 g of 0.5mm sieved soil was weighed into a digestion flask together with 0.5 g of the salt/catalyst mixture of sodium sulphate and copper sulphate (ratio 10:1) in 5 ml of concentrated sulphuric acid and digested for about 3 hours (Amin and Flowers, 2004). The digested solution was made up to 50 ml with distilled water and shaken in a back-and-forth manner. Thereafter, an aliquot of the digest was taken and the N content determined by the colorimetric Technicon (Technicon, 1973) auto analyser method with a spectrophotometer (Labomed 20D Spectrophotometer -the USA) at 630 nm.
Available phosphorus was determined by extractants as enumerated earlier in Mehlich-3 (Mehlich, 1984). One g of soil was extracted with 10 ml of the extractants (ratio 1:10) on a reciprocating shaker for five minutes. Extracts were filtered through Whatman No. 42 filter paper. A 5-ml aliquot of the extracts was taken into a 25-ml volumetric flask, 5 ml of ascorbic acid (Watanabe and Olsen, 1965) were added, shaken and made to mark with distilled water. Phosphorus content was determined with the aid of a spectrophotometer (Labomed 20D Spectrophotometer -the USA) on a wavelength of 882 nm (Bray and Kurtz, 1945).
Exchangeable potassium was determined in Mehlich-3 as for available P. Ca, Mg, K and Na were determined by the Atomic Absorption Spectrophotometer (AAS) (Buck Scientific AAS Model 210 VGP -the United States of America). Micronutrients (Zn, Cu, Mn and Fe) were also determined in Mehlich-3 extractant by the Atomic Absorption Spectrophotometer (AAS) (Buck Scientific AAS Model 210 VGP -the United States of America). Exchangeable acidity (Al 3+ + H + ) was determined using the 1 N KCl extraction method, and titrated with 0.01 N NaOH (Black, 1965). About 2 g of 2-mm sieved soil were weighed into a beaker while 20 ml of 1 N KCl were added and stirred with a reciprocating shaker for 5 minutes. It was filtered with Whatman No. 42 filter paper, to get the filtrate. Two drops of phenolphthalein were added, and the solution was titrated with 0.01 N NaOH until the pink colour was observed. Effective cation exchange capacity (ECEC) was calculated by summation of exchangeable bases and exchangeable acidity.
Exchange acidity = H + + Al + . Base saturation (BS) was calculated from the formula: (1) In both years, the experiment was conducted between July and September. Rainfall, temperature and relative humidity varied between years ( Table 2). The field was disc plowed twice, harrowed and treated with the systemic herbicide Force-up ® , a.i. glyphosate, at 250 mL to 18 L of water, using a knapsack sprayer before planting. No fertilizer was applied. The experiment comprised 17 treatments arranged in a randomized complete block design replicated 3 times. The seeds of okra, var. LD-88, obtained from NIHORT were planted at a spacing of 60 × 40 cm in intercrops and monocrops corresponding to a plant population of 4 plants•m -2 . Cowpea, var. Ife brown, and peanut, var. Kampala, were planted at densities of 5.6 plants•m -2 , 4.2 plants•m -2 , 3.3 plants•m -2 and 2.7 plants•m -2 . The plot size was 2.4 × 2 m (4.8 m 2 ). Weeding was carried out manually at 6 and 8 weeks after sowing. The insecticide DD-force ® , Dichlorvos 1000EC, was applied to cowpea at 1.9 mL to 0.75 L of water using a hand sprayer.
Five okra plants were randomly selected per plot and tagged for data collection. Data on plant height, number of leaves, leaf area, stem diameter and fruit yield of okra were collected. Data collected were subjected to analysis of variance using SAS (ver. 9, SAS Institute, Cary, NC) software at the 5% level of probability.

Results and Discussion
The chemical and physical properties of soil in the experimental location as presented in Table 1 showed that organic carbon, total nitrogen, the macro-and micronutrients of the site were below the minimum requirement for plant growth and yield.
Results of the analysis showed that the year had a significant (P < 0.01) effect on the growth and yield of okra intercrop with different densities of legume (Table  3). Okra was significantly taller with significantly higher leaf area and stem diameter in 2016. Also, the significantly higher fruit yield of 7.3 t. ha -1 was obtained in 2016, while 4.9 t.ha -1 was obtained in 2017 (Table 4).  Improving the growth and yield of okra by intercropping with varying populations of legumes 219 Intercropping okra with legumes at different densities significantly affected the growth and yield of okra. Okra/no legume was the tallest (42.7 cm) though not significantly different from okra intercropped with cowpea at the density of 4.2 plants•m -2 that was 39.8 cm (Table 4). Also, okra intercropped with peanut at the density of 2.8 plants•m -2 produced a significantly higher number of leaves (16.1), but not significantly different with okra/no legume (15.3). IIntercropping okra with cowpea at 5.6 plants•m -2 and 4.2 plants•m -2 significantly reduced the number of okra leaves (Table 4). However, leaf area of okra intercropped with peanut at 2.8 plants•m -2 814.6 cm was significantly higher. The stem diameter of okra was also significantly (P < 0.01) affected by intercropping with legumes at different densities (Table 3) as okra/no legume (2.4 cm) and okra intercropped with peanut at 3.3 plants•m -2 had the highest stem diameter of 3.5 cm ( Table 4). The fruit yield of okra was also significantly (P < 0.01) influenced by different densities of legumes (Table 3) as okra/no legume and okra intercropped with peanut at 2.8 plants•m -2 had the significantly higher fruit yields of 7.8 and 7.4 t/ha, respectively ( Table 4).
Interaction of year × intercropping as shown in Table 5 revealed that in 2017, okra/no legume was significantly taller (54.3 cm) while the shortest plant was observed with okra intercropped with peanut at the densities of 4.2 plants•m -2 and 2.8 plants•m -2 in 2016 (23.6 and 22.2 cm). Also, okra intercropped with peanut at 2.8 plants•m -2 had a higher number of leaves (16.1) in 2016 while in 2017 okra intercropped with cowpea at 5.6 plants•m -2 had the lowest number of leaves (5.9). However, the highest and lowest numbers of leaves were observed in okra/no legume in 2017 (17.9) and in okra intercropped with cowpea at 5.6 plants•m -2 (5.9). Okra intercropped with peanut at the density of 2.8 plants•m -2 in 2016 had significantly larger leaf area (1148.2 cm 2 ) while okra intercropped with cowpea at 5.6 plants•m -2 in 2017 had the lowest leaf area (231.0 cm). The highest stem diameter (3.7 cm) was observed in okra intercropped with peanut at 3.3 plants•m -2 which was significantly higher compared to okra intercropped with cowpea at all the densities and sole okra in 2016. No significant difference was observed in fruit yield of okra/no legume and okra intercrop with peanut and cowpea at 2.8 plants•m -2 , and cowpea at 5.6 plants•m -2 in 2016. Okra intercropped with cowpea at 5.6 plants•m -2 and 4.2 plants•m -2 in 2017 had lower fruit yield though not significantly different from okra intercropped with peanut at 5.6 plants•m -2 and 4.2 plants•m -2 and also okra intercropped with cowpea at 2.8 plants•m -2 (Table 5). Obasi (1989) and Orkwor et al. (1991) have observed that the most important feature of plants that determines their competitive ability for light is height. They have concluded that a successful competitor for light is the component that has its foliage at a higher canopy layer. Palaniappan (1985) and Olasantan and Lucas (1992) have also noted that canopy height is one of the important features which determines the competition ability of plants for light. Palaniappan (1985) has observed that when one component is taller than the other in an intercropping situation, the taller component intercepts the major share of the light such that growth rates of the two components will be proportional to the quantity of the photosynthetically active radiation they intercept. From this study, okra sown as a sole crop was observed to be significantly taller than okra intercropped with either groundnut or cowpea. This could probably be due to the fact that there was an early onset of inter-specific competition between okra and component crops and these component crops had a smothering effect on okra that made the growth and development of okra be hindered compared to okra sown as a sole crop that did not experience any inter-specific competition. This result was contrary to the report of Njoku et al. (2007), who reported that intercropped okra was taller than the sole crop. Muoneke et al. (1997) also reported that the taller okra plants obtained when intercropped with maize was in a bid to display their leaves for solar radiation. This result implies that okra intercropped with either groundnut or cowpea had less ability to compete for light, unlike okra sown alone. Okra intercropped with groundnut at the high density (4.2 plants•m -2 ) was observed to be significantly taller, whereas okra intercropped with either groundnut or cowpea at the low density (3.3 plants•m -2 ) was observed to be the shortest. This implies that intercropping okra at close spacing initiated a competition to the extent they grow taller than those intercropped at wide spacing. This result corroborates the report of Ibeawuchi et al. (2005), who also reported that okra plant height decreased as row spacing increased.
Okra sown as a sole crop and okra intercropped with groundnut at the low density (2.8 plants•m -2 ) had the wider stem diameter. This could be due to the fact that the level of competition in sole okra and okra intercropped at wider spacing was low that made these plants take more nutrients from the soil and had more space for growth. Okra intercropped at wider spacing was the shortest and probably this could have enhanced the wide diameter. This result is also in accordance with the report of Ibeawuchi et al. (2005), who also observed that wide row spacing with lesser plant population led to an increase in the girth of okra stems. Okra intercropped at the high density (4.2 plants•m -2 ) had the least stem diameter. Okra plant was able to compete favorably with groundnut at close spacing but not with cowpea. This showed that cowpea initiated more competition than groundnut.
Sole okra had higher leaf area than okra intercropped with either groundnut or cowpea. In the intercrop, leaf area increased with decreasing plant density. This could be a result of less competition for nutrient, light and space and could also be a result of the aggressive growth habit of cowpea and groundnut. This result was corroborated by the report of Odedina et al. (2014) who stated that the aggressive growth habit of the cowpea variety used in their study could be responsible for the reduction of leaf area and LAI in okra + IT84S 2246-6 intercrop. In 2016, about 807.3 mm of rainfall fell during the crop cycle from June to September, however, in 2017 of the same cropping cycle there was more rainfall (1,674.3 mm). The better yield and performance of okra in 2017 may be attributed to the better rainfall during the entire crop cycle.

Conclusion
This study shows that okra fruit yield increased at wider spacing. Therefore, intercropping okra with peanut at the density of 2.8 plants•m -2 has proven to be suitable and appropriate and could therefore be recommended.