Okra, (Abelmoschus esculentum L.), Moench, var.: Corazon F1) commonly called Lady’s Finger, belongs to the family Malvaceae. The geographical origin of okra is disputed, with supporters of West African, Ethiopian, and South Asian origins. It is grown throughout the tropical and sub-tropical and in the warmer parts of the temperate regions. It is cultivated for its edible green seed pods. The stem is useful as fiber. The leaves are considered as good cattle feed and sometimes consumed. The seeds are roasted and used as substitute for coffee [1].

Okra is filled with anti-oxidants a characteristic that makes it an excellent anti-fatigue food. It contains adequate fiber to boost digestion, lower cholesterol level which contributes to heart health. Fibre also cuts the risk of cardiovascular disease and stroke. It contains a substance that can increase sugar absorption by the muscles. When this happens effectively, there are low blood sugar levels. It also contains pectin, a protein that could fight human breast cancer cells [2].

Nitrogen (N) plays a major role in plant metabolism system. Nitrogen is involved in all vital processes in plants in association with proteins and hormones. In obtaining high yields and quality crop products, therefore, nitrogen application is indispensable [3]. 

Nitrogen enhances photosynthetic processes, leaf area production, leaf area duration as well as net assimilation rate [4]. Nitogen is crucial to the growth and development of vegetables. Especially with fruit vegetables, a direct and positive correlation between aboveground part and yield is noticed with balanced N supply [5]. Nitrogen deficiency in okra could result to decrease in leaf size, stem diameter and height of plant, intense falling of floral buds and limiting fruiting [6]. Also, N stimulates the absorption of other nutrients. Nitrogen directly influences source-sink relationships, because it changes the distribution of assimilates between reproductive and vegetative parts [7]. Nitrogen is easily leached and lost especially in sandy soils and under rain-fed agriculture or in uncontrolled irrigation systems, therefore prompting use of slow release N fertilizers or split application of other inorganic N fertilizers [5]. 

Highest fruit yields of Okra crop under N fertilization had been recorded for 120 to 200 kgha^-1N [8,9,10].

Research has shown that Organic fertilizers (as carbon neutral alternative) could be used in place of liquid fossil fertilizers as an environmental safeguard. Organic matter improves soil structure, increases the water holding capacity and promotes biological transformations such as N-mineralization [11]. Benefits of the combined application of organic manure and N fertilizers in minimizing leaching of N have been shown [12]. Notwithstanding, confounding results on how untreated organic manures and N fertilizer combinations lead to increased leaching of NO₃^– N have also been shown [13,14].

In Sierra Leone okra is cultivated using different soil amendments such as, green manure, compost, public garbage matter, household refuse, farmyard manure or inorganic fertilizers. Sometimes a combination of organic and inorganic fertilizers is used. These treatments must have different effects on the growth and yield characteristics of okra critical for production economics and marketability. Keeping this point in view, the present research entitled ”Growth and Yield of Okra as Influenced by Nitrogen, Organic and Integrated fertilizers in Loamy Sand Soil’’ was undertaken with the following objectives:

• To assess the effect of selected Nitrogen, Organic and Integrated fertilizers on growth characteristics and yield of okra

• To assess the effect of selected Nitrogen, Organic and Integrated fertilizers on the yield of okra

Experimental Site 

A field experiment was conducted at the Ernest Bai Koroma University of Science and Technology, Makeni University College (Lat. 8.53N, Lon. 12.02W and 100m above sea level), Northeastern Sierra Leone, in the rainy season of 2018. The climate is humid-tropical with distinct dry and wet seasons and mean annual temperature of 30⁰C. Annual rainfall ranges from 2500 – 3000mm [15]. The rainy season lasts from May to November and the dry season, December to April. The experiment started 1^st June, 2019 with land clearing and ended 12^th September, 2018 with last harvesting of fruits.

The site was fallowed for three years following continuous cropping of maize before the start of this trial. The dominant vegetation was Guinea grass. The experimental site was cleared of vegetation and ploughed before manures were incorporated. 

Experimental Design, Plots, Planting and Fertilizer Application

Experimental Design: The experiment was laid out using the randomized complete block design with three replications. Five fertilizer treatments were used: ‘no fertilizer’ applied, 10tha^-1 Public garbage matter (PGM), 10tha^-1 Guinea grass green manure, 200kgha^-1 NPK 19:19:19, and 200kgha^-1 NPK 19:19:19 + 10tha^-1 Guinea grass green manure (GGGM). The randomization of treatments was done with aid of random number tables [16]. 

Plots

Total number of plots was 15 and plot size was 2m x 2.5m. Plot-to-plot distance was 50cm and replication border was 1m. Length and width of field were 10m and 9.5m, respectively.

Planting and Fertilizer Application

The public garbage matter was obtained from the Makump dump site near the City of Makeni two weeks before application and was immediately freed of all inorganic solids manually and kept as a heap in open air. The Guinea grass green manure was cut fresh at the experimental field, weighed fresh, and applied at day of harvest, incorporated to soil. The experimental site was laid out and the manures were equally distributed among plots, each receiving 50kg of the designated organic substance. These were incorporated in the furrows dug on 8^th June, 2019 on which rows were established and seeds were planted on 18^th June, 2019. The nitrogen fertilizer was applied split – at two weeks after planting and at the onset of flowering. The okra seeds were purchased from Seed Tech Co., Freetown, Sierra Leone. Three seeds were planted per hole, which was 2cm deep at a spacing of 25cm x 50cm, 25cm away from each border and later thinned to one seedling per stand after germination. There were four rows of okra per plot and eight  holes per row giving a total of thirty-two plants per plot. 

Soil Sampling and Analysis

Before planting, soil samples (0-20 cm deep), were collected in three locations in each replication with a soil-sampling augur. They were mixed to form one composite soil sample which was taken to the Njala University Soil Science laboratory and was analyzed as follows:

Data Collection and Harvesting

Data were collected from six plants that were randomly selected and tagged, in the two middle rows of each plot excluding the end plants, on plant height, stem girth, number of leaves, leaf length, number of fruits and fresh fruits weight. Leaf length measurements were made on the 4^th leaf of each tagged plant from the ground. Data on plant height, stem girth, number of leaves, and leaf length were collected on the 30^th, 45^th, and 60^th days after planting (DAP) and their averages were calculated. Harvesting, counting and weighing of fruits were done, every fourth, eighth, twelfth, and sixteenth day after fruit maturity, and their averages calculated.

Plant height was measured from ground level to the tip of the plant with help of a meter rule.

Stem girth was the measurement of the circumference of the stem 6cm above ground level, using a tape rule.

Number of leaves per plant was obtained by counting the number of leaves on every tagged plant in each plot each time data was collected.

To obtain the Leaf length the fourth tagged leaf of each of the six tagged plants of a plot was measured from the node to the tip of the middle leaf lobe using a tape rule.

Number of fruits per plot was determined by harvesting and counting of the fruits borne on the tagged plants.

Weight of fresh fruits per plot was obtained by the immediate weighing of harvested fruits from the tagged plants.

Plot Yield (t/ha) = (plot weight (g)/1000, 000) × plant population per hectare/No. of plants harvested per plot

Statistical Analysis

The General Linear Models Procedure of the Statistics Analysis for Microsoft Windows Release 15:37, Monday, June 15, 1998 (SAS Institute) was used to detect differences between treatments for all variables. The treatment means were subjected to Duncan’s Multiple Range Test to specify the particular pairs of treatments that differ significantly.

Some growth and yield attributes differ significantly by different treatments. There were significant differences between treatments 200kgha^-1 NPK 19:19:19:  + 10tha^-1

Table 1: Pre-cropping Physico-Chemical Properties of the Soil of the Experimental Plot

GGGM, 10tha^-1 PGM and  ‘no fertilizer’ applied in terms of plant height stem girth, fresh fruit weight per plot and fresh fruit yield per plot. There were no significant differences between treatments 200kgha^-1 NPK 19:19:19 + 10tha^-1 GGGM, 10tha^-1 GGGM and NPK 19:19:19 in terms of the same growth and yield components. However, among the treatments highest plant height (39.557 cm), largest stem diameter (4.300 cm²), largest number of leaves (8.800) and longest leaf (31.100cm) were recorded in 200kgha^-1 NPK 19:19:19 + 10tha^-1 GGGM as the growth traits, followed by 10tha^-1 GGGM, 200kgha^-1 NPK 19:19:19, 10tha^-1 PGM and ‘no fertilizer’ applied (see Table 2). The heaviest fresh fruits per plot and largest fruit yield per plot (206.110 g) and (2.748 t/ha) respectively, as yield traits were observed in 200kgha^-1 NPK 19:19:19 + 10tha^-1 GGGM treatment, followed by 200kgha^-1 NPK 19:19:19, 10tha^-1 GGGM, ‘no fertilizer’ applied and then 10tha^-1 PGM. 

Treatments produced no significantly different effect on the number of leaves per plant and number of fresh fruits per plot. This corroborates with the findings of Muchow [17] and Vos and Biemond [18] in their studies of Nitrogen fertilizer effects on growth of maize and sorghum, and potatoes (Solanum tuberosum L.), respectively, that nitrogen effects on leaf numbers were numerically small in maize and sorghum, and potato.  Direkvandi, et al., [19] reported that fruit number on tomato plants were not significant between nitrogen fertilizer treatments N₂, N₅, and N_6, explaining that bio-fertilizers did not have an effect on fruits number in plant. The similarity to this experiment may not be far from the consideration that many more of the fertilizers contained organic manure (GGGM or PGM or organic matter in the loamy soil) rendering them bio-fertilizer characteristics. The integrated nutrients management treatment of 200kgha^-1 NPK 19:19:19 + 10tha^-1 GGGM exhibited superior values regarding growth and yield characteristics. 

This result is supported by Mal, et al., [20] whose investigation revealed that maximum vegetative growth and fruit yield in okra were observed in integrated application of fertilizers as compared to the use of chemical fertilizers and organic manures. It is also supported from several experiments that no single source of nutrients, be it organic manure, chemical fertilizer or biofertilizers can meet the nutrient needs for modern intensive farming.

Table 2: Mean Growth and Yield Characteristics of Okra Plants Influenced by Nitrogen and Organic Fertilizer Mix

Integrated nutrient management is a holistic approach that considers all the available farm resources that can be used as plant nutrients. Nutrients added through combined inorganic and organic sources are better utilized than inorganic alone, besides reducing cost of production and maintaining the soil health [21]. Moreover, long term sustainability of productivity could be achieved only through the interaction of inorganic and organic sources of nutrients [22.23].

Therefore, based on the findings, the integrated use of the green manure and the inorganic fertilizer can be adopted by farmers in the study area to maximize the yield of okra.

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