Effect of Organic Manures Application on Soil Physicochemical Properties of Coarse-textured Ultisol and Okra Productivity in Nsukka, Southeastern Nigeria

Sustainable nutrient management is necessary for maintaining good agronomic production as well as soil quality (Jagdish, 2015). This is even more relevant now that soil degradation is seen as one of the 21^st century global problem with its severe impact felt more in the tropics and sub-tropics (Lal, 2015). Inherent low soil nutrient has been a major constraint to crop productivity of most arable soils in Nigeria especially those of Nsukka ecological zone (Igwe, 2004). These soils are further degraded owing to continuous cultivation, deforestation and inappropriate farming practices (Oshunsanya, 2011). These unsustainable land use practices often result to low soil organic matter (SOM) content that makes the fragile soil become more prone to compaction and erosion (Alyelari and Oshunanya, 2008).
       
Okra (Abelmoschus esculentus L.) is one of the most important vegetables grown in Africa, Asia as well as in tropical America and perhaps Australia (NRC, 2006). It is grown mainly as fruit vegetables in the tropics and consumed in both green and dried state (Awurum and Okorie, 2011). Okra is utilized for several purposes such as soup thickener, which may also be served with rice and other food type, medically for clearing throat and curative medicine for ulcer and hemorrhage (Attigah et al., 2013). The production and productivity of okra have declined recently due to low soil fertility (Adeyemi et al., 2008), poor soil management and cultural practices by the farmers.
       
Low soil fertility has been regarded as a crucial problem facing agricultural development and food security in sub-Saharan Africa (Sanchez, 2002). In order to ensure high and sustainable okra yields, good soil fertility management is required to facilitate its production.
       
The use of organic manures and inorganic fertilizers increased SOM, improve soil structure and water holding capacity, improved nutrient cycling and maintain soil nutrient status (Saha et al., 2008). Olaniyi et al., (2010) also reiterated that the application of organic and inorganic fertilizers is necessary for enhancing the nutrients status of soil and increasing the crop yield.
       
However, high cost of fertilizer and its scarcity, poor technological expertise, increased soil acidity and nutrient imbalance have hindered its sustainable use (Ojeniyi, 2000). In addition, there are growing concerns about the negative implications of conventional agriculture involving use of chemical fertilizers and pesticides to human health and the environment. It has also been reported that organic manures improve the growth and yield parameters of crops when the required amount is judiciously used (Senjobi et al., 2010). These beneficial effects of organic manure on soil properties and yield of most crops may vary depending on organic manure and its nutrient composition, quality and quantity of organic manure applied. However, information on the recommended rate of organic manures application for okra production in the derived savanna region of southeastern Nigeria is scarce.  Therefore, the objective of this study was to compare the effect of different rates of organic manures and recommended rate of NPK 15-15-15 fertilizer on soil physico-chemical properties and okra yield in Nsukka, southeastern Nigeria.

Description of experimental site
 
The study was carried out at the Department of Soil Science Teaching and Research Farm, University of Nigeria, Nsukka. The location falls within the derived savannah agro-ecological zone of Nigeria (6^o51' N, 7^o25' E) at an average elevation of about 436 m above sea level. The rainy season starts from April to October while the dry season begins from November to March. There is usually a short break in the month of August. The average annual rainfall amount is about 1600 mm and 85% of this takes place during the rainy season. The average minimum and maximum temperatures are about 22^oC and 30^oC respectively. The relative humidity was rarely below 60% (Asadu, 2002). The soils of the area are mostly Ultisols which belongs to Nkpologu series (Nwadialo, 1989). The soil is very deep, dark-reddish brown at the top layer and reddish in the subsoil. It is coarse to medium sandy loam (clay content ranged from 5-25%), acid in reaction (pH range of 4-7) and low in nutrient status.
 
Experimental layout
 
The experiment was first carried out in the greenhouse and later evaluated in the field. The greenhouse experiment was laid out in a completely randomized design (CRD), while the field experiment was laid out in a randomized complete block design (RCBD). The treatments applied were poultry manure (PM) and pig slurry (PS) at 10, 20 and 40 t ha^-1 respectively, NPK 15:15:15 at 300 kg/ha and control with no amendment.
 
Greenhouse study
 
The soil used for the greenhouse study was collected from the Teaching and Research Farm of the Department of Soil Science, University of Nigeria, Nsukka. Air-dried sieved (2 mm) soil (3.5 kg) was placed in each perforated pots following CRD with eight (8) treatments replicated five times. Poultry manure and pig slurry were added at 10, 20 and 40 t ha^-1. All the pots were watered to field capacity for two weeks (3 times per week). At the end of two weeks, the okra (Spineless species) seeds were planted at the rate of two seeds per pot. The germinated seedlings were thinned down to one per pot two weeks after planting (WAP). The recommended rate of NPK 15-15-15 fertilizer (300 kg ha^-1) was applied two weeks after planting (WAP) to five pots while the remaining five pots with no amendment served as the control. The okra plants were harvested at eight WAP.
 
Field study
 
The field evaluation was carried out from 10^th April, 2018 to 15^th July, 2018 at the Teaching and Research Farm of the Department of Soil Science, University of Nigeria, Nsukka. Each of the plots (3 m × 1.5 m) were properly demarcated using earthen bunds with a spacing of 0.5 m between each plot and a spacing of 1 m between each blocks. There were 40 experimental plots under randomized complete block design (RCBD) with five replications. The plots were manually tilled to 20 cm depth using hand hoe. The poultry manure and pig slurry at 10, 20 and 40 t ha^-1 were thoroughly mixed in plots as per treatments before planting. The okra (Spinelees species) seeds were planted at the rate of two seeds per hole using a plant spacing of 60 cm × 50 cm and thinned to one, two weeks after planting. The recommended NPK 15-15-15 fertilizer (300 kg/ha) was applied two weeks after planting to five plots while the remaining five plots without any amendment served as control. Weeding was done by hand and hand hoe picking. Subsequent weed controls were done by a combination of hand picking and use of hand hoe method at two weeks interval.
 
Data collection and soil sampling
 
The agronomic parameters such as plant height and number of leaves were measured at 2, 4, 6 and 8 weeks after planting (WAP) in greenhouse and field studies. Fresh and dry matter yields were also measured. Both disturbed and undisturbed soil samples were collected at 0-15 cm depth with the help of core samplers and soil auger. The soil samples were collected in black polyethylene bags. The disturbed soil samples were air dried, sieved with 2 mm sieve while the core samples were trimmed with spatula and saturated for at least 48 hours before analysis. The soil analysis were done using standard laboratory procedures.
 
Laboratory analysis
 
Particle size distribution of the soil was determined using the Bouyoucous hydrometer method as described by Gee and Bauder (1986). Bulk density was determined by core method as described by Blake and Hartge (1986).
       
Saturated hydraulic conductivity (Ksat) was determined by the constant head permeameter method (Klute and Dirksen, 1986). Darcy’s equation for analysis of constant head method, as described by Youngs (2001) was used for the computation of Ksat as follows:
  
Where
Q is steady state volume of outflow from the entire soil column (cm³), L is the length of soil column (cm), A is the interior cross-sectional area of the soil column (cm²), T is the time of flow (sec), ΔH is the change in hydraulic head or the head pressure difference causing the flow (cm).
       
Total porosity (P) was computed from bulk density (Bd), using the equation below (assumed particle density p_s = 2.65 Mg m^-3):
                                                           
The size distribution of water stable aggregates (WSA) was determined by the wet sieving method (Kemper and Rosenau, 1986). In this method, 25 g of the air-dried soil sample retained on 2 mm sieve was weighed out and put on the topmost of the nest of sieves of 2, 1, 0.5 and 0.25 mm. The content was soaked initially for 5 minutes to prevent slaking and thereafter oscillated vertically in water for 5 min at the rate of 30 oscillations per minute and at an amplitude of 4 cm. Thereafter, soil aggregates left on each sieve were oven dried at 105^oC for 24 hours and weighed. The aggregate stability (AS) as percent of water-stable aggregates (WSA) > 0.5 mm on each sieve were determined thus:
                                           
Where
Mwsa = Mass of water stable aggregates > 0.5 mm plus sand (g).
Ms = Mass of the sand fraction alone (g).
Mt = Total mass of the sieved soil (g).
       
The MWD of WSA was calculated based on the equation proposed by Chaney and Swift (1984) as:
 
Where
x_i = The mean diameter of any particular size range of aggregates separated by sieving and w_i = The weight of aggregates in that size range as a fraction of the total initial dry weight (25 g) of soil used.
       
Soil pH was determined using digital pH meter in a soil solution ration of 1:2.5. Organic carbon was determined using the modified Walkley and Black method as described by Nelson and Sommer (1996). The soil organic matter was obtained by multiplying with a correction factor of 1.724. Total nitrogen was determined using the Kjeldhal method as described by Bremner (1996). Available phosphorus was determined using Bray II method as described by Bray and Kurtz (1945). Exchangeable calcium, magnesium, sodium and potassium were extracted with NH4OAc. Calcium and magnesium were determined using Ethylene Diamine Tetra-acetic acid (EDTA) titration method while potassium and sodium were determined using flame photometer. Cation exchange capacity was determined titrimetrically using 0.01N NaOH. Exchangeable acidity was determined titrimetrically using 0.05 N NaOH.
 
Data analysis
 
The soil and agronomic data were analyzed for variance (ANOVA) in RCBD and in CRD for the field and the greenhouse study as outlined by Steel and Torrie (1980) using GENSTAT 4.0. Separation of treatment means was done using the F-LSD at 5% probability level as described by Obi (2002).

Effect of poultry manure, pig slurry and NPK on soil physical properties
 
The result of greenhouse and field studies (Tables 1 and 2) indicated that the rates of poultry manure and pig slurry significantly (p<0.05) improved soil physical properties than the control and the recommended rate of NPK 15-15-15 fertilizer. The results are in agreement with the findings of different researchers reporting that organic manure improved the stability of soil aggregates (Ardeshir et al., 2010), Ksat (Wanas, 2002a), increased total porosity and reduced bulk density (Agbede et al., 2017; Arruda et al., 2010; Mahmood et al., 2017) and improve mean weight diameter (Mbah and Onweremadu, 2009).
 

 

 
Effect of poultry manure, pig slurry and NPK on some soil chemical properties
 
The result of greenhouse study showed that different levels of poultry manure and pig slurry applications significantly (p<0.05) improved the soil chemical properties except CEC than the control plot with no amendment (Table 3).
 

       
Similarly, the results of the field study (Table 4) showed that different levels of poultry manure and pig slurry applications significantly (p<0.05) improved the soil chemical properties; soil pH, SOM, TN, Av.P and CEC than the control plot with no amendment. The results are in conformity with the findings Onwu et al., (2014) and Mbah and Onweremadu (2009). Han et al., (2016) and Liu et al., (2010) noted that organic manure increased soil pH while NPK fertilizer decreased soil pH. The cumulative effect of poultry manure and pig slurry applications indicate that organic manure adds organic matter and nutrients to soil and in turn improves physical and chemical properties of the soil for higher productivity.
 

 
Effect of poultry manure, pig slurry and NPK on the yield component of Okra
 
The results of greenhouse and field studies showed that the biomass (fresh and dry matter), fruit yield, number of leaves and plant height were significantly (p<0.05) improved due to increasing levels of poultry manure and pig slurry (Tables 5 and 6). The results are in agreement with the findings of Premsekhar and Rajashree (2009). Adesina et al., (2014) and Nweke et al., (2013) had also reported that plant height and number of leaves increased with increase in the rate of organic manure applied. The significant influence on growth and yield characteristics of okra may be due to the improvement in soil physical properties and enhanced uptake of nutrients by the applications of poultry manure and pig slurry. However, poultry manure showed its superiority over pig slurry. Poultry manure has been reported to increase the fruit yield and plant height of okra compared to other sources of organic manure (Fagwalawa and Yahaya, 2016).
 

 

 
It has been reported that poultry manure contains high amount of nutrients especially nitrogen and phosphorus that are easily taken up by plants for fast growth (Ewulo, 2005; Awodun, 2007). 

The application of organic manures improve significantly (p<0.05) soil pH, available phosphorous, total nitrogen, organic matter, cation exchange capacity, aggregate stability, bulk density, porosity and saturated hydraulic conductivity. The improvement in soil physical and chemical properties through the applications of poultry manure and pig slurry led to significant (p<0.05) improvement in the uptake of nutrients and, growth and in turn, growth and yield of okra. However, poultry manure have greater influence on soil properties and okra productivity than pig slurry. It may be concluded that the addition of 20 t ha^-1 of poultry manure and 40 t ha^-1 of pig slurry is best for sustainable soil management and optimum productivity of okra in the coarse textured Ultisols in Nsukka, southeastern Nigeria.