Abstract: A field study was designed primarily, to observe the effect of phosphorous and potassium application in terms of improving water use efficiency and nitrogen fixation capacity of mash bean and secondly, on the nitrate content of soil. The experiment was carried out during summer 2003 on a silty loam soil (Typic Ustochrepts) at Research Farm of University of Arid Agriculture, Rawalpindi with phosphorous and potassium fertilization at recommended rates separately as well as in combination. The highest water use efficiency (1.04 kg mm-1 ha-1, 8% above control) and total fixed nitrogen (57.24 kg ha-1, 41.28% over control) was observed with treatment P + K fertilization at the rate of 80 and 40 kg ha-1, respectively. NO3-N content of soil were significantly higher at 60-90 cm depth, highest (24.34 kg ha-1) being with phosphorous application at the rate of 80 kg ha-1.
INTRODUCTION
Mash bean (Vigna mungo L.) holds a key position in the rainfed agriculture of northern Punjab. It is considered important due to its nitrogen fixation capacity and richness in protein content. Mash bean can fix atmospheric nitrogen up to 30-70 kg ha-1 in one season[1]. Therefore development and adoption of mash bean in set rotations with wheat can enhance the soil nitrogen fertility and cereal yields. The economic product of mash bean is seed grain, which is a good source of dietary protein (25-32%). Mash bean can be successfully grown on marginal lands where other crops perform poorly[2].
Water use efficiency act as the indicator to check the ability of a crop to convert available water to economic yield. Increasing the efficiency of water use by crop continues to escalate as a topic of concern because of the increasing demand for water use and improved environmental quality by human population[3]. Therefore, it is highly desirable to investigate the means to improve the water use efficiency of mash bean crop, especially in the rainfed agriculture. Phosphorous has been reported to improve water use efficiency in a number of crops. In low-P soils, the addition of phosphatic fertilizer increased the dry matter yield and WUE of pearl millet[4]. The significance of potassium pertains to the increased number of pods per plant, number of grains per pod, straw yield, total dry matter production and grain yield of legume crops[5]. Actually, the soil nutrient status affects the growth efficiency of crops, which leads to improved dry matter production relative to given amount of water used by the crop. These changes increase the water using efficiency.
The soil enriching property of mash bean is of greater value especially to our country, where soils are deficient in organic matter, nitrogen and other soil fertility parameters[6]. Being leguminous, mash bean meets most of its N requirements from N2 fixation and has little dependence on soil or fertilizer N[2]. In Swat, mash bean (black gram) has been reported to have fixed 47% of its N requirements from the air[7]. Hence, it can contribute towards increased soil fertility. The efficiency of inoculation for better results is influenced by the proper availability of phosphorus and potassium. As these elements are directly involved in nutrition of legumes and help to improve the root as well as shoot growth of the crop. Phosphorus has a profound effect on normal functioning of N2 fixing bacteria and also on number and weight of effective nodules on root system[8]. Mahadhkar and Saraf have reported the positive role of phosphorus in increasing the yield of mash bean crop[9]. Phosphorus fertilizer application alone did increase root weight but had negligible effect on number of nodules. However, when potassium applied the nodule numbers and size both were improved significantly[10].
The objective of the study primarily was to observe the effect of phosphorous and potassium application on water use efficiency and nitrogen fixation capacity of mash bean and secondarily, on the nitrate content of soil.
MATERIALS AND METHODS
A field experiment was conducted on a silty loam soil (Typic Ustochrepts) at the Research Farm, University of Arid Agriculture Rawalpindi during summer 2003. The experiment was laid out in Randomized Complete Block Design with four replications. Treatment plot size was 5x4 m. The crop was sown with the help of single row hand drill with row spacing of 30 cm. The mash bean seed were inoculated with Bradyrhizobium japonicum. The treatments of the experiment were T1: Control; T2: P-fertilized (80 kg P2O5 ha-1); T3: K-fertilized (40 kg K2O ha-1); T4: P-K fertilized (80 and 40 kg ha-1).
Soil analysis: Physical and chemical properties of composite soil samples taken before sowing from the experimental field were determined, i.e., soil texture, pH, electrical conductivity, available phosphorus, NO3-N, extractable potassium and organic matter (Table 1). The soil samples at planting and harvest were collected by using king tubes for nitrate nitrogen and soil moisture content from 0-30, 30-60 and 60-90 cm depths. The soil nitrate nitrogen was determined using method given by Vendrell and Zupancic[11].
Assessment of N2 fixation by xylem solute technique: Nitrogen fixation was estimated by the xylem solute collected at the pod fill stage by vacuum extraction method, then concentrations of ureide, nitrate and amino N were determined by prescribed methods to calculate the relative abundance of ureide (RUN%) by the following formula:
| Table 1: | Physico-chemical properties of experimental field |
After getting the value of RUN %, the proportion of plant N derived from N fixation (Ndfa %) was estimated at the time of sampling the xylem sap.
| Table 2: | Rainfall and average temperature data during growing season (mm) |
| Total rainfall during the growing period: 1383.6 mm | |
The legume N was derived from the measure of biomass accumulation and tissue N-content.
The amount of nitrogen fixed by legume was regulated by two factors, the amount of N accumulated during growth and the production of that N derived from symbiotic N2 fixation Nfda% ).
*1.5 factor is used to include an estimate for contribution by below ground N.
Water use efficiency of grain yield: Water use efficiency of grain yield were calculated by combining grain yield with data on water use[13]
Where, e is grain yield (kg ha-1), f and g are soil water contents (mm) to 120 cm depth measured at planting and at harvest, respectively. h is growing season precipitation.
Meteorological data: Rainfall data and average temperature data (Table 2) necessary for interpreting the results was obtained from Regional Agro-met Center located at University Farm.
Statistical analysis: The data collected for various characteristics were analyzed statistically by analysis of variance technique and the means obtained were compared by LSD at 5% level of significance[14].
RESULTS AND DISCUSSION
Effect of phosphorous and potassium on percent nitrogen derived from atmosphere (Ndfa%): The impact of phosphorous and potassium on the proportion of nitrogen derived by mash bean through the N2 fixation (Nfda%) at pod fill stage (Table 3) reveals that the average Ndfa% values observed were 24, 35, 28 and 44% with T1, T2, T3 and T4 treatments, respectively. The results were statistically significant. The highest value was obtained with the combined use of phosphorous and potassium i.e., 44% (46% over control) followed by T2, T3 and T1. The higher values for Ndfa% obtained with phosphorous and in combination with potassium indicates that mainly phosphorous enhanced nitrogen fixation and potassium stimulated it. The results were in conformity with those given by Shehzadi et al.[15] who suggested that phosphorous application enhanced Ndfa%.
| Table 3: | Effect of phosphorous and potassium on % Ndfa, Total N fixed, WUE and grain yield of mash bean |
| *Means with different letter (s) differ significantly according to LSD (p = 0.05) | |
Effect of phosphorous and potassium on total N fixed (kg ha-1) by mash bean: In Table 3 total fixed N2 obtained were 35, 36, 33 and 57 kg ha-1 for T1, T2, T3 and T4, respectively. The maximum amount of fixed N2 was obtained for treatment where phosphorus and potassium were combined and was significantly higher than that for control and P or K alone treatments. The P or K alone treatments did not produce significantly higher N2 fixed than the control treatment.
The reason for greater amount of total fixed nitrogen in T4 was that the availability of phosphorus along with potassium probably enhanced the microbial activity and in return increased amount of nitrogen fixed by mash bean. The similar findings have been reported by Khan et al.[16] on N2 fixation in Sesbania aculeate.
Effect of phosphorous and potassium on water use efficiency of mash bean: Table 3 presents significantly higher water use efficiency of mash bean with combined application of phosphorous and potassium. Maximum value of water use efficiency i.e., 1.04 kg mm-1 ha-1 (8% above control) was obtained in T4 (P + K at the rate of 80 kg ha -1 and 40 kg ha-1) followed by T2 (P at the rate of 80 kg ha-1). The lowest value, 0.90 kg mm-1 ha-1, was obtained in T1 (Control). The findings are in line with the observations of Payne et al.[4] who reported that the addition of phosphorous increased the dry matter yield and water use efficiency of pearl millet.
Effect of phosphorous and potassium on grain yield of mash bean: The grain yield was 1.26, 1.37, 1.40 and 1.47 t ha-1 under T1, T2, T3 and T4 treatments, respectively (Table 3). When the treatments were compared with each other it was revealed that the highest grain yield (1.47 t ha-1) was obtained by T4 while T1 produced the lowest grain yield of 1.26 t ha-1. The results were statistically significant. The increase in yield may be attributed to adequate supply of phosphorous by improving seed development as it is essential for seed formation and potassium stimulated it. The percent increment over control observed was 8.03, 10 and 14.3% for T2, T3 and T4, respectively.
The results were in agreement with Sharar et al.[17] who reported that improved grain yield of legumes by phosphorous application.
| Table 4: | Effect of phosphorous and potassium on residual soil nitrate content at different depths |
| *Means with different letter (s) differ significantly according to LSD (p = 0.05) | |
Effect of phosphorous and potassium on residual soil nitrate content: Mash bean being a nitrogen fixer, has effect on soil nitrate nitrogen content. After harvesting the crop, the NO3-N contents were found to be 24.2, 23.5, 24 and 24.3 kg ha-1 under T1, T2, T3 and T4, respectively. The data indicated the non-significantly difference among the treatments at 0-30 cm depth. However, the highest soil NO3-N contents (24.3 kg ha-1) were observed with P+K fertilized at the rate of 80 kg and 40 kg ha -1 treatment and the lowest nitrate nitrogen (24 kg ha-1) was found in T3 where only potassium was applied at the rate of 40 kg ha-1 (Table 4).
At 30-60 cm depth, the soil N03-N contents were 20.8, 23, 20.3 and 22 kg ha-1 with treatment under T1, T2, T3 and T4, respectively. The differences were statistically non significant. However, maximum amount was observed in T2 where only phosphorus was applied at the rate of 80 kg ha-1.
At 60-90 cm depth, the soil NO3-N contents after mash bean harvest were 16.37, 24.34, 16.09 and 19.66 kg ha-1 under treatments T1, T2, T3 and T4, respectively. Results obtained were statistically significant at this depth. The highest nitrate content of 24.34 kg ha-1 was observed in P fertilized treatment and the lowest contents (16 kg ha-1) were observed in K fertilized treatment (Table 4).
The higher nitrate content at lower soil depths (60-90 cm) may be result of leaching of nitrate to lower depths with rain water. The results also indicated that effect of mash bean on NO3-N content of soil was better by using phosphorous only. This suggests that phosphorous is beneficial for increasing NO3-N at different soil depths. The results were in conformity with those of Nimje and Seth who reported that phosphorous application improves total nitrogen content of soil. High plant densities will more rapidly deplete soil nitrate and favour earlier establishment of a functioning symbiosis[18].
ACKNOWLEDGMENTS
Financial support from the Pakistan Science Foundation is gratefully acknowledged.