An experiment was conducted to determine the effect of different levels of nitrogen on the yield and yield components of colocasia at District Malakand village Garhi Usmani Khel during 2013. The study was consisted of five level of nitrogen 0, 60, 90, 120 and 150 kg・ha⁻¹. The experiment was conducted in randomized complete block design with three replications having plot size 2.74 m × 2.43 m. Local colocasia variety was sown on a well prepared clay soil. The spacing between plant to plant 12 cm and row to row was 30 cm. In each row 12 colocasia tubers were planted. The entire quantity of nitrogen in the form of urea was applied to the respective plots at seed bed preparation. The crop was sown in February and then harvested in October. Data on number of colocasia plot⁻¹, 1000-tubers weight, length, diameter and total yield of colocasia was recorded. Manual weeding was performed three times in the cropping season. After 3 months of the emergence of colocasia ridges were performed for better production of colocasia tubers.

During month of May fungal disease called blight of colocasia appeared and was controlled by a fungicide Metalixal + Mancozeb 72% wp@ 3 gm per liter three times at the interval of 10 days the whole month. In month of June crop was again attacked by insects (worms) and Chlorpyrifos 40 ec + Acetamiprid 20% sp insecticide was used at 3 gm per 10 liter. A high rainfall in the month of august caused a severe brownish spots on leaves that was a fungal attack. Metalixal + Mancozeb 72% wp@ 3 gm per liter and Acetamiprid 20% sp of 3 gm per 10 liter was used 3 time at 10 days interval the whole month. The colocasia was harvested at the month of October. All other agronomic practices such as irrigation and plant protection measures etc were kept normal and uniform for all the experimental units. Data on tuber yield (kg・ha⁻¹), number of tuber per plant, weight of tubers and size of tuber were recorded.

The data was analyzed, thus Analysis of variance (ANOVA) procedure for randomized complete design by a statistic 8.1 software. The variations between the treatments were compared by LSD test of significance [11] .

The Hydrometer was used to investigate texture of soil as presented by [12] . Take 50 g soil and then added 10 ml of Na₂CO₃ and water into a dispersion cup. Shake for 5 minutes through dispersing machine to break up all the aggregates. Reading was noted in hydrometer after 40 second and after 2 hours along with the temperature. First reading mean after 40 second represent silt and clay while 2 hour reading shows clay in the suspension.

Ten grams of soil was added to 50 ml of distilled water and then shake for 20 - 30 minutes to make a 1:5 suspension. The pH was determined in the suspension using pH meter (Inolab WTW Series pH 720) after calibrating the instrument with standard buffers of 4.0 and 10.0.

The electrical conductivity was calculated in soil suspensions of 1:5 by using EC meter after calibration with KCL standard solution as presented by [13] .

Organic matter was calculated by [14] . 1 gm of dried soil was taken in a conical

flask along with 10 ml of 0.5 N K₂Cr₂O₇ and 20 ml of conc. H₂SO₄. It was then allowed to stand for 30 min to complete the reaction. After sometime add 200 ml of distilled water and the suspension was filtered. Then 2 - 3 drops of Ortho phenolphthalein indicator was added to the filtrate. It was then titrated against 0.5 N Fe₂SO₄・7H₂O until the color changed to dark brown, representing the end point. The percent organic matter was calculated using the following formula.

%OM= [ ( mLofK 2 Cr 2 O 7 × N ) − ( mLofFeSO 4 ⋅ 7H 2 O × N ) ] Weightofsoil × 0 .69

P concentration in soil sample was determined by extracting soil solution with AB-DTPA as presented by [15] . 10 g of soil and 20 ml of AB-DTPA solution was added to a conical flask and placed on shaking machine for 15 minutes. Then the samples were filtered with Watman No. 42. One ml sample along with 5 ml of ascorbic acid was taken and made the volume up to 25 ml. Then with the help of spectrophotometer (Lambda-35), P was determined at 880 nm after proper color development.

Kjeldhal method of [16] is used to evaluate total N in soil and plant samples. Finely 0.2 g ground sample of dry materials were digested with 3 ml of concentrated H₂SO₄ in the presence of 1.1 g digestion mixture containing CuSO₄, K₂SO₄ and Se on a heating mantle for 1 hour. The digest was transferred into the distillation flask and distilled in the presence of 10 ml of 10 M NaOH solution. The distillate was then taken in 5 ml boric acid mixed indicator solution and then titrated against 0.01 M HCl. Calculation for total nitrogen in soil are as follow:

TotalN ( % ) = ( Sample-Blank ) × 0 .005 × 0 .014 × 100 × 100 / Wt .ofsoil × 20nt

Data regarding yield of colocasia are given in Table 2. Analysis of the data showed that nitrogen levels significantly (P ≤ 0.05) affected yield of colocasia. The highest yield of colocasia 14.15 t・ha⁻¹ was recorded in the plot treated with 60 kg N ha⁻¹, followed by 90 kg N ha⁻¹ with a yield of 13.59 t・ha⁻¹ of colocasia. The lowest yield with value of 9.63 t・ha⁻¹ was observed in control. Calculating the increase in yield, the application of 60 kg N ha⁻¹ increased the yield by 46.90% over the control (Table 2, Figure 1). However further application of N did not further increase this trend, rather it showed the decreasing trend. Both the increase over control and relative yield (Figure 1) showed the highest values at 60 kg N ha⁻¹ revealed that it could be the suitable and more economical level of N for colocasia under the prevailing soil and climatic conditions for the given variety. However, this level of 60 kg N ha⁻¹ seems much lower than the expected

and reported values of [17] who used and recommended much higher value of N for economical yield of colocasia and sweet potato. Optimum levels of nitrogen fertilization for tuber yield in colocasia were found to be 79.62 kg N [18] . The result is in contrast with [19] who reported that maximum yield attributes and yield were found with 120 kg N ha⁻¹. Similarly, [20] reported that the highest stolon yield (24.5 t・ha⁻¹) was recorded with 125 kg N ha⁻¹ and 125 kg K ha⁻¹, which is in agreement with the findings of our study. [21] also observed the highest corm yield with N₁₂₀K₁₂₀ kg・ha⁻¹. No doubt the N requirements of the crop varies from soil to soil, crop to crop and variety and also the lower N requirement in our condition could be associated to lower yield potential of the crop variety. The local variety (lower yielding) was used in the said study as no higher yielding variety was available at time of sowing in the market.

Thousand colocasia weight (kg) showed significant variation (P ≤ 0.05) with N applied. The maximum 1000-colocasia weight of 31.18 kg was recorded in the plot receiving 90 kg N ha⁻¹, followed by 120 kg N ha⁻¹ with a weight of 27.41 kg. The minimum weight of 22.92 kg was observed in control. Calculating the increase in yield, the application of 90 kg N ha⁻¹ increased the 1000 colocasia weight by 36% over control. (Table 3, Figure 2). Stolon length was increased with the application of more nitrogen [22] which in turn increases the 1000 stolon mass. Meanwhile, [23] reported the maximum number of leaves plant⁻¹ with 80 kg N ha⁻¹ and 120 kg K ha⁻¹ which is the reason for the increase in the 1000 stolon weight. However further application of N did not further increase this trend, rather it showed the decreasing trend. Both the increase over control and relative yield (Figure 2) showed the highest values at 90 kg N ha⁻¹ revealed that it could be the suitable and more economical level of N for colocasia under the prevailing soil and climatic conditions for the given variety.

The data showed that number of colocasia tubers plant⁻¹ was significantly (P ≤ 0.05) affected by different levels of nitrogen (Table 4). The maximum number 16.63 was observed in treatments receiving 90 kg N ha⁻¹ followed by 60 kg N ha⁻¹

with a number of 16.0 colocasia plant⁻¹. The minimum number of 10.99 colocasia tuber plant⁻¹ was recorded in control. Calculating the increase in number of colocasia plant⁻¹, the application of 90 kg N ha⁻¹ increased the number of colocasia plant⁻¹ by 51.33% over the control (Table 4, Figure 3). Increase in the colocasia per plant is due to N increased which is associated with vegetative growth of plants [24] . It is also linked with vigorous vegetative growth, photosynthesis, and dark green color 11 pigmentations [25] . Availability of usable N in most agricultural conditions is a major limiting factor of plant growth and in most tropical soils yields of taro may be improved when inorganic N fertilizers are used. Studies conducted by [26] revealed that taro requires relatively high N especially in its early stages of growth. However further application of N did not increase this trend, rather it showed the decreasing trend. Both the % increase over control and relative number of colocasia tuber plant⁻¹ (Figure 3) were maximum at 90 kg N ha⁻¹ revealed that it could be the suitable and more economical level of N for colocasia under the prevailing soil and climatic conditions for the given variety.

Both maximum length and maximum diameter of the colocasia increased with increase in N levels (Table 5). The highest length of 5.84 cm was obtained in plots treated with 150 kg N ha⁻¹, followed by 120 kg N ha⁻¹ with a length of 5.78 cm. The lowest length noted was 4.97 cm in a control. Calculating the increase in length, the application of 150 kg N ha⁻¹ increased the length by 17.62% over the 120 kg N ha⁻¹ (Table 5, Figure 4). Unlike other parameter, the length of tubers of colocasia increased with increase up to the higher level of N application as concluded by [20] , he concluded that maximum stolon length was recorded with application of N at the rate of 125 kg・ha⁻¹ (Figure 4). Increase in the stolon length might be due to increase in the photosynthesis as [27] observed the maximum number of leaves plant⁻¹ of Colocasia esculenta, with 100 kg N ha⁻¹. While, [23] reported the maximum number of leaves plant⁻¹ with 80 kg N ha⁻¹.

Similarly, the maximum mean diameter significantly (P ≤ 0.05) affected by different levels of nitrogen and the maximum mean diameter of 3.14 cm was recorded in plots treated with 90 kg N ha⁻¹, followed by 60 kg N ha⁻¹ with a diameter value of 3.10 cm. The minimum diameter of colocasia tuber was recorded in a control that was 2.66 cm. Comparing the increase in diameter over control, it was observed that application of 90 kg N ha⁻¹ increased the colocasia diameter

by 18.15% and it was maximum among all treatments (Table 5, Figure 4). Some researchers have shown that NPK fertilization enhanced growth and corm yield of taro [28] and [29] . It is known that taro consumes substantial amounts of potassium [30] . N increases the number and length of the internodes and thus results in progressive increase in plant height. Different researchers including [31] [32] [33] made similar observations. However, further application of N did not further increase this trend, rather it showed the decreasing trend though it was not significant reduction. Both the increase over 60 kg N ha⁻¹ and relative yield (Figure 4) showed the highest value at 90 kg N ha⁻¹ which revealed that it could be the suitable and more economical level of N for colocasia under the prevailing soil and climatic conditions for the given variety.