EFFECTS OF DIFFERENT SOIL AMENDMENTS ON THE GROWTH AND YIELD OF OKRA IN A TROPICAL RAINFOREST OF SOUTHWESTERN NIGERIA

This study investigated the effects of different soil enhancers on the growth response of okra [Abelmoschus esculentus (L.) Moench] cultivated on a ‘contaminated’ field with sewage sludge from the two oxidation ponds of the Obafemi Awolowo University (OAU), Ile-Ife, Nigeria. This was with a view to assessing the growth performance and yield of the test crop under different soil amendments. Okra variety, NHAe 47-4 with NPK 12-12-17 (IO), compost organic fertilizer (OR), Glomus mosseae mycorrhiza (MY) and zero fertilizer applications as control (CT) was laid out in a completely randomised block design and each treatment plot (4 x 2 m) was replicated four times. Selected weather parameters were collected from a meteorological station in OAU campus during the period of the experiments. Growth parameters such as plant height, stem girth and number of leaves of okra increased with added soil amendments from four weeks after planting in the order: IO > OR > MY > CT. In 2010, the highest mean yield of 16.3 t ha obtained with 6.0 t ha of MY was not significantly higher than 15.4 t ha obtained with application of 0.2 t ha of IO, but significantly (p < 0.05) higher than 13.1 and 10.4 t ha obtained with applications of 6.0 and zero t ha of OR and CT respectively. Comparative okra yield, though relatively higher with mycorrhizal inoculation, but lower with no soil amendment was obtained in 2011. The study concluded that a direct linear relationship existed between solar radiation and okra productivity. Also, for a moderately ‘treated field’ with sewage sludge from domestic wastes, arbuscular mycorrhizal fungi can be integrated into soil fertility management to achieve low-cost sustainable agricultural systems for enhanced productivity of okra.


Introduction
Improved soil fertility through the application of fertilizers is an essential factor that enables the world to feed the billions of people that are added to its population (Brady and Weil, 1999). Increasing population and the consequent increased demand for food production and food quality in Nigeria require that proposed agronomic strategies for improvement should in general avoid high input costs. Despite the inherent low fertility status of the soils in Nigeria, carbon farming has not been well understood. Obi et al. (2005) reported that okra [Abelmoschus esculentus (L.) Moench] yield and yield components were lower without the application of NPK, and that with increase in rate of NPK application, yield of okra increased. The use of inorganic fertilizers alone has not been helpful under intensive agriculture because they aggravate soil degradation (Sharma and Mittra, 1991).
The soil degradation is brought about by loss of organic matter which consequently results in soil acidity, nutrient imbalance and low crop yields. The restoration and rehabilitation of degraded soils for high crop productivity using offfarm organic wastes such as sewage sludge, crop wastes, domestic and municipal wastes could help to restore the lost organic matter sustainably. The importance of organic manure and mineral fertilizer in tropical agriculture for increased world food production cannot be over-emphasized. The need to minimize the dependence on chemical fertilizers and to encourage the use of bio-fertilizers on a large scale in farming communities has been proposed as diagnostic condition for soil health and high crop productivity (Parr and Hornick, 1992).
Also, the levels of mycorrhizal colonization were greater under organic treatments than under the conventional practices (Kafkafi et al., 1988). Increase in soil organic matter has been found to enhance the available phosphorus in the soil through the organic anion, thus preventing P fixation and replacing the P bound to the soil (Nagarajak et al., 1970). Maintenance and improvement of soil quality are vital if agricultural productivity and environmental quality are to be sustained for future generations (Reeves, 1997). Arbuscular mycorrhizal fungi (AMF) can be integrated into soil management to achieve low-cost sustainable agricultural systems (Hooker and Black, 1995). AMF improve plant-water relations and thus increase the drought resistance of host plants (Nelsen, 1987), improve disease control (Khan, 2006) and increase mineral uptake and yield of crop (Adewole et al., 2010), hence, reduce the use of fertilizers. Mycorrhiza is undoubtedly of extraordinary importance in plant production, plant and soil ecology and plays a key role in sustainable agriculture (Bethlenfalvay et al., 1998).
Despite the importance of bio-fertilizers, such as compost organic fertilizer and Glomus mosseae for improved crop yield, there is a dearth of information on these soil enhancers when okra is cultivated in an Alfisol sewage sludge-treated soil of the tropical rain forest. This study was therefore aimed at investigating the influence of weather variation and different soil amendments on the growth and yield of okra in a sewage sludge-treated soil.

Study area, experimental design and agronomic details
The study was conducted on an Alfisol farmland (07°30.362ˊN and 004°30.747ˊE) around the oxidation ponds of the Obafemi Awolowo University (OAU), Ile-Ife, Nigeria during the wet season of 2010. The study area falls within the tropical rain forest of south-western Nigeria. Viable seeds of okra variety, NHAe 47-4 were purchased from National Horticultural Research Institute, Ibadan, Nigeria. The 'minimally contaminated' experimental site with treated sewage sludge was manually cleared with hand hoe and cutlass two times at two weeks interval. The sewage sludge was a product of domestic/kitchen wastes, majorly from the students' hostels and the staff quarters of the OAU channelled into the two oxidation ponds. These wastes were allowed to undergo oxidation inside the ponds before dislodgement to the open land near the ponds by the university authority.
Using random sampling technique, three surface (0-15 cm depth) soil samples were collected with soil auger from the cleared farmland. The pre-planting soil samples were air-dried for 7 days, sieved through a 2-mm mesh and analysed for soil physical and chemical properties using standard methods. The mean values of selected soil properties before planting in 2010 and 2011 are presented in Table 1. The experiment consisted of four, 19 x 2 m blocks; each block was in turn divided into four plots of 4 x 2 m with an alley of 0.5 m between blocks and 0.5 m within plots; laid out in a completely randomised block design with four treatments and each treatment was replicated four times. The treatments consisted of 6.0 t ha -1 organic fertilizer (OR); 0.2 t ha -1 NPK 12-12-17 (IO); 6.0 t ha -1 of Glomus mosseae mycorrhiza (MY) and zero soil amendment as control (CT). The NPK and organic fertilizers were purchased at a local market. The mycorrhiza used was an inoculum of soil containing spores, hyphae and maize roots infected with Glomus mosseae.
Sowing was done with four seeds per hole using 0.6 x 0.3 m planting distance. The soil amendments were applied at sowing. The okra seedlings were thinned to two stands per hole at two weeks after planting (WAP) to give plant population of 111,111 plants per ha. Three manual weeding treatments of the plots were carried out at 2, 5 and 7 WAP using hand hoe. Collection of data on growth parameters such as stem girth, number of leaves, and plant height of okra commenced at 2 WAP and thereafter, continued weekly. Harvesting of okra pods began 45 days after planting and continued until 15 WAP, when the experiment was terminated.
At each time of harvest, the pods were counted per plot to get the number of pod and weighed to get the fresh pod weight. A repeat experiment was conducted on the same plot of land during the wet season of 2011. The soil pH was electrometrically determined (McLean, 1982). Nitrogen was determined by Kjeldahl method (Bremner and Mulvaney, 1982), available phosphorus was determined using Bray P1 method (Olsen and Sommers, 1982). Exchangeable cations (K + , Na + , Ca 2+ and Mg 2+ ) were extracted with 1 M ammonium acetate and the concentrations of Ca 2+ and Mg 2+ in the soil extracts were read using Perkin-Elmer Model 403 (Shelton, Connecticut, USA) Atomic absorption spectrophotometer (AAS) while K + and Na + were read on Gallenkamp flame photometer (Thomas, 1982). The concentrations of Cu, Pb and Cd in the soil extracts were read on AAS.
Chemical composition of compost organic fertilizer used Nitrogen was determined by Kjeldahl method (Bremner and Mulvaney, 1982). Available phosphorus was determined using Bray P1 method (Olsen and Sommers, 1982). Calcium ions, Mg 2+ and K + were extracted using 1 M ammonium acetate buffered at pH 7.0 as extractant (Thomas, 1982) and the concentrations of Ca 2+ and Mg 2+ in the compost extracts were read using AAS, while K + was read on flame photometer. The chemical composition of compost organic fertilizer was: N 6.4%, P 6.4%, K 4.6%, Ca 12.4% and Mg 5.9%.

Selected weather parameters
The selected monthly mean weather data (surface soil temperature, rainfall, relative humidity and solar radiation) collected from the Space Applications and Environmental Science Laboratory, OAU, Ile-Ife, Nigeria during the cropping periods in 2010 and 2011 are presented in Table 2. Statistical Analysis The data obtained were subjected to descriptive and one-way analyses of variance to test for their treatment effect. Test of significance for differences in means was statistically compared using Duncan's multiple range tests at 5% level of probability.

Results and Discussion
Physical and chemical characteristics of soil The mean soil pH (1 : 2 Soil -1 M CaCl 2 ) was 5.00 indicating acidic soil conditions. Some of the other mean values obtained were: organic carbon 9.92 g kg -1 ; total nitrogen 3.85 g kg -1 ; and CEC 11.59 cmol kg -1 which were considered moderate for okra cultivation in Nigeria (Olaniyi et al., 2010). At 4 WAP, the plant height had significantly (p < 0.05) lowest values in plots with OR and CT fertilizer applications than in IO and MY treatments. However, from 6 WAP, only plants in the CT had significantly (p < 0.05) lowest height. The N and P in the inorganic fertilizer were easily absorbed by the okra plants which would have enhanced the rapid plant growth. Also, the ability of added arbuscular mycorrhiza prevented the native P from being fixed, but enhanced P availability could have been responsible for okra rapid growth (Sharma and Mittra, 1991). The absorption of mineralized OR became manifested from 6 WAP. The order of increase in okra plant height was IO > OR > MY > CT until 8 WAP. However, from 10 WAP, plots with OR were leading, probably because the organic fertilizer used had just fully mineralised for the okra plants to use.

Stem girth of okra
A similar trend recorded in the height of okra plants was obtained in stem girth from 2 to 12 WAP (Figure 2). At 4 WAP, the stem girth was significantly (p < 0.05) highest in plots with IO fertilizer application than in other treatments, but not significantly (p < 0.05) different from 6 WAP, except for the CT treatment. Except for IO, gradual decrease was observed at 12 WAP due to shedding of leaves in the other treatments. Plants in the control plots did not perform too well in terms of growth as they had to rely only on the native soil nutrients.

Yield of okra
In 2010, the highest okra mean yield of 16.3 t ha -1 obtained with mycorrhizal inoculation was not significantly higher than 15.4 t ha -1 obtained with application of inorganic fertilizer, but significantly (p < 0.05) higher than 13.1 t ha -1 obtained with application of organic fertilizer and 10.4 t ha -1 obtained when no soil amendment was applied (Table 3). Ryan and Angus (2003) earlier obtained improved biomass, plant growth and yield of okra with AMF applications. Comparative okra yield, though relatively higher with mycorrhizal inoculation, but lower with no soil amendment was obtained in 2011. The residual effects from the previous soil amendments with the new additions could account for these differences. The control plots depended only on the native soil fertility; hence the lower okra yield obtained during the second cultivation was not out of place. Glomus mosseae (an arbuscular mycorrhiza) proved most effective for good growth performance and yield of okra. Previous studies of Shaheen et al. (2007), and El-Shaikh and Mohammed (2009) showed an increase in agronomic growth performance and yield of okra when bio-inoculants were applied to soil. The roots of plant that developed well and had enhanced access to soil nutrients when soils are treated with arbuscular mycorrhiza (Adewole et al., 2010) could be attributable to increased okra yield.
Typical tropical weather conditions are highly variable (Nathaniel, 2011). The weather parameters measured in 2010 and 2011 were greatly different, thus confirming the high variability of weather conditions of the study area, a typical tropical environment. The solar radiation is a function of radiation use efficiency and this actively participates in the process of photosynthesis and it has a linear relationship to crop productivity (Kumar et al., 2008). In 2011, with the exception of April, the monthly mean solar radiation measured during other months was higher than their corresponding months in 2010. This may have accounted for higher okra yields in 2011 for all the treatments, except for the control plots whose low native soil nutrients might have been the major contributing factor to low yield of okra.

Conclusion
The mean yield of okra (t ha -1 ) was in the order: MY > IO > OR > CT, suggesting that treated sewage sludge, in combination with Glomus mosseae can be integrated into the soil fertility management. We therefore conclude that for a moderately 'contaminated field' with treated sewage sludge from domestic wastes, arbuscular mycorrhizal fungi can be integrated into soil fertility management of the humid tropics to achieve low-cost sustainable agricultural systems for enhanced productivity of okra. Also, higher solar radiation enhanced the yield of okra.