Effects of Phosphorus and Organic fertilizers on the Yield and Proximate nutrient composition of lettuce (Lectuca sativa) in Southwestern Nigeria  

K. O. Sanni1 , B. S. Ewulo2
1. Department of Crop Production and Horticulture, Lagos State Polytechnic, Ikorodu, Lagos State, Nigeria
2. Department of Crop, Soil and Pest Management, Federal University of Technology, P. M. B. 704, Akure, Ondo State, Nigeria
Author    Correspondence author
International Journal of Horticulture, 2015, Vol. 5, No. 1   doi: 10.5376/ijh.2015.05.0001
Received: 10 Dec., 2014    Accepted: 17 Jan., 2015    Published: 10 Feb., 2015
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Sanni and Ewulo, 2015, Effects of Phosphorus and Organic fertilizers on the Yield and Proximate nutrient composition of lettuce (Lectuca sativa) in Southwestern nigeria, International Journal of Horticulture, Vol.5, No.1 1-7 (doi: 10.5376/ijh.2015.05.0001)


A field experiment was conducted to evaluate the effect of different types of organic manure, phosphorus fertilizers and their combinations on the yield attributes and proximate nutrient composition of lettuce grown in Akure and Ikorodu, Southwestern Nigeria. The study involved three types of organic manure (poultry, pig and cattle manures) and two types of phosphorus fertilizer (Ogun Rock Phosphate (RP) and Single Super Phosphate (SSP)) and a control with no fertilizer application resulting in sixteen (16) treatment combinations. The experimental design was Randomized Complete Block Design (RCBD) with three replicates. Yield was determined by weighing the fresh harvested lettuce and dry weight at 50 days after transplanting (DAT) and lettuce harvested were subjected to proximate composition analysis. The result of the study showed that lettuce grown on soil amended with organic manure combined with SSP and RP at 8t/ha-1 and 4t/ha-1 respectively exhibited significant higher yield. It also influenced the proximate composition positively at harvest compared to control and sole organic manure application. The result obtained clearly indicated that organic manure combined with phosphorus fertilizers prove to be a sound soil fertility management strategy in the study area. This should be adopted by would-be farmer of lettuce, as combinations of organic manures increase solubility of phosphorus and improve lettuce proximate nutrient composition.

Organic manure; Phosphorus fertilizers; Proximate composition; Soil fertility management; Treatments’ combinations

Continuous crop production in the tropics has rendered vast area of land unproductive as a result of physical degradation, loss of soil organic matter and decreased cation exchange capacity (CEC) and as well as increased Al and Mn toxicity. As these soils suffered multi-nutrient deficiencies, application of organic and inorganic fertilizers has become mandatory to increase crop growth and yields (Sanni and Adesina, 2012). Crop’s response to fertilizer is a function of the type of fertilizer applied (Ayodele and Olaniyan, 1982), soil status and the type of preceding crops.

Lettuce (Lectuca sativa) is a rapid growing plant that needs a well-balanced nutritional program in order to be high yielding and of high quality. Lettuce marketable and nutritional qualities depend heavily on the agronomic strategy used. Vast of nitrogen (N) from fertilizers or a surplus of N can lead to an increase in nitrate content of plant tissues, synthesis of N-containing compounds and a decrease in beneficial phytochemicals (Sorensen et al., 1999). Lettuce treated with mineral N-fertilizer had higher nitrate content than the organically fertilized lettuce (Sanchez et al., 1988). Synthetic fertilizers are believed to contribute substantially to human, animal food intoxication and environmental instability/degradation (Masarirambi et al., 2012). Organic fertilizers can therefore be used to reduce the amount of toxic compounds (such as nitrates) produced by conventional fertilizers in vegetables like lettuce; hence, improving the quality of leafy vegetables produced as well as human health. There is increased demand of organically produced vegetables in view of its health and nutritional benefits. Since total eradication of mineral fertilizers is not possible, there is need to integrate them with organic manures for sustainable crop production. There is little attention on the combination of organic and inorganic fertilizer but the little information on crop’s response to joint application of chemical and organic manure centered on the agronomy of production (Adeniyi, 2001; Akanbi et al., 2010). Therefore the objective of this study is to assess the effects of different sources and variable rates of organic manure and phosphate fertilizer on yield and proximate nutrient composition of lettuce (Lactuca sativa) in South Western Nigeria.
1 Results and Discussion
1.1 Pre cropping soil analysis
The data on the initial chemical properties of the soils used for the study are presented in Table 1. Particle size fraction showed that the soils were sandy clay loam (Akure) and loamy sand (Ikorodu) in texture and low in fertility as reflected by the low content of organic carbon, and total nitrogen. The available phosphorus (P) and exchangeable bases (Ca, Mg and K) were seemingly low based on the ratings of FMANR, (1996) for this agro-ecological zone. The low fertility status of the soils is a true reflection of most humid environment that are strongly weathered with low activity of clay mineralogy and high acidity due to intense precipitation with its associated erosion and leaching in the environment. The pH value was within the pH range 5.50 – 6.50 and is good for good performance of vegetables (Pureseglove, 1991), most nutrient elements were below the critical range (Adeoye and Agboola, 1985). These revealed that there is need for soil amendment in form of fertilizer application to improve the growth and yield of lettuce. Cooke (1972) reported that the major nutrients required by the crop are Nitrogen (N), Phosphorus (P) and Potassium (K). Inadequate supply of any of these nutrients during crop growth would have negative impact on the reproductive capability, growth and yield of the plant (Vine, 1953; Solubo, 1972) and supplementary amount of nutrients can be added to soil inform of organic or inorganic fertilizer to correct inadequate supply of nutrients to the crop (Dirk and Hogarth, 1984).

Table 1 Pre cropping soil analysis

1.2 Effects of soil amendment on lettuce yield attributes

Sole application of organic manure and combinations with phosphorus exerted significant (P<0.05) influence on yield attributes of lettuce in both locations. In Ikorodu, 4t PM + RP plot had the lowest fresh weight of harvested lettuce (0.29kg/plant) and 12t PM recorded the lowest fresh weight in Akure (0.04kg/plant). Highest fresh weight of 0.83kg/plant was recorded in plot treated with 8t CM + SSP combination followed by 0.79kg/plant recorded from plot treated with 4t CM + SSP combination for Ikorodu and 0.67kg/plant was recorded in plots treated with 4t PGM + RP and 12t CM followed by 0.13kg/plant recorded from 8t PM + RP and 8t PGM + SSP combinations and 0.11kg/plant from control and 8 t PGM + RP combination plots respectively (Figure 1).

Figure 1 Fresh weight of harvested lettuce

There was a significant (P<0.05) difference on dry weight among treatments. Dry matter content was higher in plants from
8t CM+RP 43.67g/plant and followed in decreasing order: 8 t PGM+RP (40.00g/plant), 8 t PGM + RP (36.33g/plant), 4t PGM + SSP (35.00g/plant), 12t PGM (34.33) and lastly 4t PM+RP (17.33g/plant) was observed in Ikorodu. While from Akure dry matter content was higher in plant obtained from 4t PM + SSP (14.33g/plant) followed in decreasing order: 8 t PGM+RP (13.33g/plant), 4t PM+RP and 12t CM (13.00g/plant), 12t PGM (12.67g/plant)and lowest dry matter content was recorded from 4t PGM + SSP (6.67g/plant) and 12t PM (6.0g/plant) (Figure 2).

Figure 2 Dry weight of harvested lettuce

The results of the present study show that SSP amended with CM was superior to the control or other organic manure combination. This shows that the effectiveness of phosphorus fertilizer on crop production was remarkably improved through the solubilizing effect of cow manure. The result from the study supports the findings of Akande et al (2006) who reported that complementary use of phosphorus fertilizer with 3 t ha-1 cow dung manure gave the optimum yield in maize and okra.

Application of organic based fertilizer and inorganic fertilizer enhanced plant yield when compared to untreated controls. The nutrient use efficiency of crops is better with a mix of manure and inorganic fertilizer (Murwira and Kirchmann, 1993). The organic manure mixed with phosphorus based fertilizer supported lettuce growth better than sole organic manure.
Considerable variations in leaf dry matter of lettuce were observed amongst the treatments. Magkos et al., 2003 observed that vegetables cultivated on soils with higher amount of organic fertilizers had high dry matter as compared to those produced conventionally. But findings from this study negate such assertion, because all lettuce grown on sole organic manure recorded low dry matter.
1.3 Effects of soil amendment on Lettuce Leaf Proximate composition
In Akure study, the lowest moisture, crude protein, ash, crude fibre, carbohydrate and fat contents, were recorded from lettuce treated with 4t CM + RP, 12t PM, control and 12t CM, 4t CM + RP, 12t PM and 4t CM + SSP respectively, while lettuce treated with control and 8t CM + RP had highest moisture content; 8t CM + RP and 4t PM + SSP had highest crude protein content and 8t CM + RP had highest ash content which is closely followed by lettuce in soil amended with 4t PM + RP and 8t PGM + RP. Crude fibre content was highest in lettuce that received 8t PM + SSP and followed by 12t CM and 4t PM + SSP respectively. 8t CM + RP and 4t PM + SSP plots produced lettuce with high carbohydrate content while 4t CM + RP produced that with highest fat content (Table 2).

Table 2 Proximate content of lettuce grown in Akure

For study conducted at Ikorodu, moisture content was high in lettuce produced from 12t PGM and 4t PM + SSP and low in 8t PM + RP amended plots. Plots that received 8t CM + RP produced lettuce with highest crude protein content and lowest in control plot. Ash content of 26.82% was observed from 12t CM treated plots with control plot producing the lowest ash content. Lowest crude fibre content was observed from 4t PM + RP and highest content from 8t PGM + RP. Lettuce harvested from 8t PGM + RP plots produce the lowest carbohydrate content, while those from 8t CM + RP and 8t PGM + SSP had the highest carbohydrate content. Fat content was lowest in plots fertilized with 4t CM + SSP and highest in control plot (Table 3).

Table 3 Proximate content of lettuce grown in Ikorodu

The positive response and increased yields attributes and proximate composition of lettuce with applied fertilizer treatments might be due to the low initial nutrient status in the used soils, which revealed the vital role played by the soil nutrient status in the crop production. The result obtained clearly indicated that organic manure combined with phosphorus fertilizers prove to be a sound soil fertility management strategy in the study area. This should be adopted by would-be farmer of lettuce as combination of organic manure increase solubility of phosphorus. This complementary application reduces the dependence of the farmer on inorganic fertilizer use. It also reduces the exposure of the soil to the consequences of inorganic fertilizer application.

2 Materials and Methods
2.1 Experimental Site
The experiment was conducted at two locations in the rain forest of Nigeria during year 2011/2012 cropping season. The first experiment was conducted at the Teaching and Research Farms of Lagos State Polytechnic, Ikorodu. The geographical location of Ikorodu is between latitude 5° 10' N and longitude 3° 16' and 3° 18' E. The soil type has been classified as Fleventic Eutroctrept (Soil Survey staff), Gleyic Cambisol (FAO) and Idesan series (Fasina, 1996, personal communication); while the second trial was carried out at the Horticultural Unit, Ministry of Agriculture, Ondo road, Akure; Latitude 7° 32' N and Longitude 5° 53' E area of rain forest agro-ecological zone of Southwestern Nigeria. The soil was classified as Oxic tropudalf (Adepetu et al., 1979), or Luvisol (FAO). The climate of Akure is essentially humid with rainfall varying from 2,000 – 5,500mm per annum. Two distinct seasons exists namely – rainy and dry seasons. The rainy season started in February and ends in October. It has a bimodal distribution pattern with distinct peaks in July and September. Dry spell occurs between late July and middle of August. Annual air temperature varies between 27°C - 35°C.
2.2 Pre Soil Analysis
Prior to the commencement and at the end of the experiment core soil samples were collected at random from the sites at a depth of 0-15cm. The soil was air dried, crushed and sieved with a 2mm sieve mesh. It was then analyzed in a laboratory as follows: Soil pH was determined elecrometrically in 1:2 suspension of soil 0.1M KCL (Thomas, 1996). Particle size distribution was determined by the hydrometer method (Gee and Or, 2002) and total porosity was calculated from the bulk density as the fraction of total volume not occupied. Organic carbon content was determined using Walkey/Black method procedure (Nelson and Sommers, 1996) and Organic matter estimated by multiplying with a factor of 1.724. Total nitrogen content was determined by the Kjeldahai digestion procedure (Bramner 1996). Exchangeable bases (Na, K, Ca and Mg) were determined after leaching with 1 N ammonium acetate (NH4OAC) and were read from the atomic absorption spectrophotometer while K and Na were read from the flame photometer (Jackson 1968). Available phosphorus was determined with the Bray 1 method (Bray and Kurtz 1965) and exchangeable acidity by KCL extraction method (Mclean, 1965).
2.3 Experimental Design
The experimental design was Randomised Complete Block Design (RCBD). The treatment consisted of flat rate of phosphorus fertilizer application (Rock phosphate (RP) and Single Super Phosphate (SSP) (150kg ha-1) combined with manure: poultry (pm), pig (pgm) and cattle (cam) manure each at different levels (0 t/ha-1, 5t/ha-1 and 10t/ha-1) were incorporated into the soil 2 weeks before planting and at planting respectively. A control was established. The combination resulted in 16 treatment combinations viz:

2.4 Crop Establishment and Management
The experimental plots were manually cleared and raised seedbeds prepared. Organic manures (PM, PGM and CM) at different levels were incorporated into the soil at 14 days before transplanting, while phosphate fertilizer (Ogun Rock Phosphate (ORP) and Single Superphosphate (SSP) was applied at a fixed rate of 150kg/ha. Great Lake lettuce cultivar was obtained from Jos, Plateau State, Nigeria and raised in the nursery. Lettuce seedlings were transplanted four (4) weeks after sowing at 25cm x 25cm between rows and plants; thus giving a plant population of 160,000 stands per hectare. Weeding was carried out as at when due using hand hoe, while foliage insect pest control was achieved by spraying Cypermethrin at 5 liters/ha mixed in 80 litres of water at 2 weeks interval up till 14 days before harvesting.
2.5 Data Collection
Five (5) plants were randomly selected and tagged from each plot for data collection at 40, 60 and 80 days after transplanting (DAT). The data for the following parameters were collected: plant height, number of leaves, leave area, marketable yield, shoot dry matter, and the nutrient composition of fresh lettuce (leaf tissue analysis). Plant height was determined with a meter rule at the distance from the soil level to the terminal bud; number of leaves was determined by visual counting of the leaves. Leave area was measured by tracing the margin of the leaf on a graph paper and the total leaf area/plant was obtained by counting the leaf number of 1cm squares (Edje and Osiru, 1988). Shoot dry matter was taken at 80 DAT, by cutting, oven drying and weighing the entire above ground vegetation. Marketable yield was determined by weighing harvested lettuce at 50 DAT with digital weighing balance and the average of their respective weights were taken for each treatment. This was extrapolated to evaluate total yield per hectare.
2.6 Mineral Content Analysis of Leaf Tissue
A microwave was used for digestion of lettuce plant tissue. A sample of 0.5g of each treatment was weighed and put in the microwave bomb (Microsoft® Encarta® 2003). For Calcium (Ca) and Zinc (Zn) analysis, 10ml 6M HCL and 10ml 6M HNO3 were used for digestion. For iron (Fe) analysis, 10ml 6M H2SO4 and 10ml 6M HNO3 were used for digestion. These acids were added into the bombs containing the samples and digested in the microwave for 2 minutes. The samples were cooled to room temperature in a water bath and filtered into 50ml volumetric flask. Dilution to the mark was done using deionized water (Masarirambi et al., 2010, Masarirambi et al., 2012).
 Standard solutions of 0.5 ppm, 1.0 ppm and 2.0 ppm were prepared from a 1000 ppm stock solution of Ca and Fe using the formula: C1V1 = C2V2. With a known concentration (C2) and volume (V2) required, the volume of 1000ppm stock solution was calculated and necessary dilution done to make V2. Standards were run in the atomic absorption spectrophotometer (AAS) to determine their absorbance (Masarirambi et al., 2010, Masarirambi et al., 2012).
2.7 Statistical analysis
The data collected were analysed using MSTAT-C (Niseen, 1989) statistical package. Analysis of Variance (ANOVA) was undertaken on data collected so as to determine if there were any significant differences amongst treatments. Mean separation where significant differences were detected was done by Duncan’s Multiple Range Test (DMRT) (Gomez and Gomez, 1984)
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