Agrowastes Potentials in Releasing Soil form of Nitrogen

A laboratory incubation study to determine the decomposition rate of Sunflower (SF), Gliricidia sepium (GL), Kola Pod Husk (KPH), and Cocoa Pod Husk (CPH) on soil forms of nitrogen was carried out at the Department of Agricultural Science, Adeyemi College of Education, Ondo South-Western Nigeria. The soil sample was collected from the farmers’ field in Ondo. The soil was air-dried and sieved through a 2mm diameter laboratory sieve. 200g of the soil samples were weighed into cups and 1g of each CPH, SF, GL and KPH was added. The treatments were replicated five times to give 45 cups of soil samples and arranged on a Completely Randomised Design. The cups were covered with absorbents. The treatments were analysed 30, 60 and 90 days to determine total N, NO3-N and NH4-N using destructive analytical method. Compared with control, all the treatments significantly increased total N and NO3-N while SF and GL significantly increased (p <0.05) NH4-N at 30 days of incubation. All the treatments significantly increased (p <0.05) total N, NH4-N and NO3-N at 60 and 90 days. Cocoa Pod Husk had the highest increase in total N while KPH recorded the highest NH4-N and NO3-N. Cocoa Pod Husk (CPH), KPH, GL and SF could be used to increase soil forms of N for optimum crop production in Ondo, South-Western Nigeria.


INTRODUCTION
Nitrogen can be added to the soil through the application of inorganic nitrogenous fertilisers and organic fertilisers, such as animal and plant residues. Some agrowastes have been reported to have the tendency of releasing higher ammonium nitrogen and nitrogen nitrate used for crop production (Ayeni et al. 2015). Mineral fertilisers are very easy to use, but they have a detrimental effect of the soil at long use. Also, farmers must be cautious of the amount of nitrogen fertilisers they add to the soils because of their negative effects on the environment in generality and aquatic ecosystems. Excess nitrogen has a negative effect on plant growth and yield as well as drinkable water and water bodies. Excess N in water causes eutrophication and causes environmental pollution during burning by releasing poisonous nitrogen monoxide.
Farmers in southwestern Nigeria commonly use sunflower and gliricidia leaves as mulching materials while cocoa pod husk and kola pod husks are popularly used as organic fertilisers. Gliricidia plants are good as staking materials and are also used in silviculture. There is need to determine the rate at which nitrogen is released when cocoa pod husk, kola pod husk, sunflower and gliricidia are applied into the soil and the amount the organic wastes release to the soil to guide against their negative impacts on the soil and man. Research works have shown that the selected agrowastes are sources of plant nutrients (Ayeni et al., 2008;Titiloye et al., 1985;Wallace, 1994 andOdedina et al., 2007). The objective of this study was to determine the rate of nitrogen mineralisation by cocoa pod husk, kola pod husk, sunflower and gliricidia in Ondo Southwestern Nigeria.

MATERIALS AND METHODS
The laboratory incubation study to determine the rate of N mineralisation from cocoa pod husk, gliricidia and sunflower were conducted in the laboratory of Agricultural Science Department of Adeyemi College of Education, Ondo, in 2016. The soil sample was collected from the Adeyemi College of Education Teaching and Research Farm. The soil samples were then air-dried and sieved through a 2mm diameter mesh. The experimental design was Completely Randomised Designed and replicated nine times.200g of air-dried soil sample was weighed into forty-five different plastic cups and labelled according to the treatment applied. The treatments applied were 0 and 1g to represent 0 and 10t/ha respectively were weighed into the plastic cups. An equal volume of water was added to the content of each container. The samples were then incubated for three months. Three months was assumed to be the maturity period for most arable crops in South-Western Nigeria. A descriptive-analytical method was used to determine the rate of mineralisation of the selected agrowastes at 30, 60 and 90 days of incubation.

Soil Nutrient Determination and Data Analysis
Soil pH was determined using a glass electrode pH meter in 1.2 soil-water ratios 10g of the soil sample was weighed into 100ml beaker (in duplicate) and 20ml of distilled water was added. Each beaker was stirred several times for about 30minutes. The pH of the sample was measured by immersing the glass electrode into a clear solution on top of the suspension.
Total nitrogen (N) was determined by Micro-Kjedahl method NH4-N and NO3-H by steam distillation technique of Bremner and Keeney (1966). Available Phosphorus (P) was extracted by Bray-1-method and determined colourimetrically. Exchange bases (K, Ca, Mg and Na) were extracted with neutral ammonium acetate. Calcium (Ca), magnesium (Mg), potassium (K) and sodium (Na) were determined from the filtrate by atomic absorption spectrophotometer (AAS). Organic carbon was determined by Walkey Black wet oxidation method.
Data collected were subjected to analysis of variance (ANOVA) and separated by the least significant difference. Table 1 shows the soil properties before the experiment was conducted. According to Ayeni (2012), the soil was neutral to slightly alkaline, low in organic matter with adequate nitrogen level. The soil was high in available K.  Table 2 shows that the chemical analysis of the content of plant nutrients in sunflower, gliricidia, kola pod husk and cocoa pod husk. This is in agreement with the work of Partey et al. (2011), Kumar and Mishra (2013), Ayeni et al., (2015), Makinde et al. (2013) and Adeosun et al. (2013) that sunflower, gliricidia, kola pod husk and cocoa pod husk contain N, P, K, Ca and Mg that could be used to increase soil nutrients. Partey et al. (2011) worked on gliricidia and sunflower and observed that they contain low C/N ratios that enhance early nutrients such as N mineralisation. Adu-Dapaah et al., (1994), Adeoye et al., (2001) and Odedina et al. (2011) reported that cocoa pod husks are being used as fertilisers and soil amendments to suppress nematode population in cocoa soil because of its high K content. Obatolu (1995) reported the use of cocoa pod husk as fertiliser for coffee and maize production.   The rate of nitrogen mineralisation of Sunflower, Gliricidia sepium, Kola Pod Husk and Cocoa Pod Husk after 60 days of incubation is shown in Table 4. Compared with control, all the treatments significantly increased (p <0.05) total N, NH4-N and NO3-N at 60 days of incubation. Kola Pod Husk had the highest GL. The soil samples treated with KPH recorded the highest NH4-N, followed by CPH. Application of 10t/ha of KPH had the highest significant NO3-N release rate followed by CPH.