Roots and Nutrient Distribution under Drip Irrigation and Yield of Faba Bean and Onion

Drip irrigation proved to efficiently provide irrigation water and nutrients to the roots of plants, while maintaining high yield production. This research was established to study the root and nutrient distribution under drip irrigation. Faba bean and onion plants were cultivated in the experimental farm of the Faculty of Agriculture of Suez Canal University in Ismailia city with the application of normal fertilizers to soil. The data showed that soil moisture content in the soil planted with faba bean increased with the horizontal distance between drippers, contrariwise moisture content decreased with horizontal distance with the soil planted with onion. The data showed the vertical distribution of root length, root length density and specific root length of faba bean and onion decreased with increasing soil depth. The data showed that ammonium and nitrate pattern at the soil planted with the both plants increased between drippers and laterals. The peak concentration was recorded 35 mg/kg at 60 80 cm soil depth for faba bean and onion, indicating that the NO3-N leaching was low by drip irrigation. Available phosphorus was higher at the surface layer than the subsurface layer at the soil planted with faba bean and onion. Available potassium tended to move both horizontally and downward under drip irrigation.


Introduction
Irrigation is the artificial application of water which aims to maintain the soil moisture required for an optimum in plant growth. The crop response to irrigation methods was often different [1], and the effect of irrigation development of crop root systems also differed from irrigation technique to another because of differences in soil water regimes. The changes of soil moisture, which is related to irrigation method, not only significantly affect the spatial distribution of crop roots and the efficiency of nutrition and water adsorption, but also directly affect the biomass of shoots [2].
Drip irrigation proved to efficiently provide irrigation water and nutrients to the roots of plants, while maintaining high yield production. Modern drip irrigation has arguably become the world's most valued innovation in agriculture since the invention of the impact of the sprinkler, which replaced flood irrigation. This is because high water application efficiencies are often possible with drip irrigation, since there is reduced surface evaporation, less surface runoff as well as minimal deep percolation [3]. Very significant differences in root growth exist among crops and effective rooting depths from 0.2 m to more than 1 m have been reported [4]. It is important to know the root growth of the vegetables and crops when trying to optimize the nutrients use efficiency in vegetable production both at the crop level and at the crop rotation level.
Information on root growth can also be used to design crop rotations. By placing deep-rooted crops at points in the rotation where available nutrients present in deeper soil layers, total losses can be reduced significantly. By placing shallow rooted crops only where little nutrients are available in deeper soil layers, nutrients losses after these crops can be reduced [4].
The aim of the present work was to study the distribution of roots and nutrients under drip irrigation system and growth parameters of faba bean and onion.

Materials and Methods
The study was conducted at the experimental site of Agricultural College of Suez Canal University during the winter season (2014/2015) at 30˚37'15.09''N and 32˚16'1.43''E. The soil of the experimental site was sandy in texture, very low in organic matter (0.3%) with pH (7.2) and EC (2.4 dSm −1 ). The average climate conditions of the site of the experiment during the growing season were illustrated in Table 1. The available N, P and K were 35, 7, 70 mg/kg, respectively before the initiation of the experiment according to [5]. The faba bean (Giza 716) seeds were sowed and twenty-eight day old seedling of onion ( Giza 20) were transplanted in double rows to the main field on 18/11/2014, with the spacing of 20 cm between the rows and 30 cm between the plants in a row. The experiment was laid out in a randomized complete block design having three replicate 3 × 3 m in a plot. Normal fertilizers applied to soil with drip irrigation.
The levels of fertilizers adopted in the study were 47. 6  dose of the fertilizers added to the soil after one month after germination and then every two weeks until the maturity of the crops.
To study moisture content and nutrients distribution in the soil, samples were collected according to [6] method using tube auger from the experimental area.
Soil moisture was determined gravimetrically. Samples were air dried ground and sieved through a 2 mm screen and analyzed for available Nitrogen, phosphorus and potassium using standard procedures described by [5].
To study root distribution, root length, root length density and specific root length in the field, soil cores were removed using a 20 cm tall auger with an internal diameter of 10 cm. The cores were washed to collect the deep to 80 cm and one every 20 cm. Samples were collected at the end of the experiment. A similar method was obtained by [7].
Parameters assessments: After physiological maturity, ten plants from each of faba bean and onion were taken at random select for measuring plant height

Moisture Distribution
The distribution of the water in the soil occurs along the hydraulic gradient between the wet and the dry soil, laterally by means of capillary action and vertically due to gravitation. In sand soil, the water moves more vertically than horizontally [9]. Drip irrigation system should apply water uniformity so that each part of the irrigated area receives the same amount of water. Wetting pattern in the soil and the spatial distribution of soil water depend on soil hydraulic properties, drip discharge rate, spacing and their replacement, irrigation amount and frequency, crop water uptake, rates and root distribution pattern [10].
The moisture distribution of the soil planted with faba bean was increased with the horizontal distance between drippers at the surface layer 0 -20 cm. The peak value of the soil moisture was 12% at the 60 cm horizontal distance at 40 -60 cm depth. The moisture distribution at depths 0 -20, 20 -40, 40 -60, 60 -80 cm closed to be uniform, and the same trend observed at the vertical distance illustrated at Table 2 and Figure 1.
For the soil planted with onion, the soil moisture content decreased with horizontal distance (2.9% to 2.0%) at 60 cm and fluctuated with depth but it could be described as the moisture content remain constant with depth. At the vertical distance, the percentage of soil moisture increased from 1.30% to 1.7% at 30 -60 cm ( Table 3, Figure 2). These results compatible with [11] who found that Surface drip irrigation allows water to move faster both vertically and horizontally and produced a wide surface wetted area at the top of the soil.

Root Distribution
The data in the Table 4 showed the vertical distribution of root length density and specific root length of faba bean and onion under drip irrigation system.    The root length density and specific root length decreased with increasing soil depth and the spatial distribution of roots was the basis for determining soil moisture changes at different soil depths. This results because of the close asso-ciation of crop root growth with soil water. Similar reports were reported by [12] [13] [14] [15] [16].
Under drip irrigation, 72% of root length density of faba bean was found at 0 -20 cm layer and 28% of root length density was at 20 -40 cm and below this depth, the roots disappeared from the 40 -60 cm depth. Specific root length in the upper layer 0 -20 cm was higher (18.41 cm•g −1 root dry weight) than the subsurface layer (20 -40 cm).
With regard to the root length density of onion at the top layer was 71% of the total root length density with the soil depth. In this respect, [17]

Nutrient Distribution
Irrigation introduces water rate into the soil along with salts dissolved in it.
During irrigation, water with micro and macro elements is spread on the surface of the field. At drip irrigation, water is applied in one spot [19], and nutrient distribution in the soil depends on the form of nutrient ions, the moisture content of the soil and other reacting ions present in the soil solution, crop water uptake rates, and root distribution patterns. The availability of nutrients at the root zone of the crops influences the uptake and yield of the crop. Leaching, volatilization, and fixation of nutrients in the soil are some of the factors that affect the availability of soil nutrients [6].

Available Nitrogen
Nitrogen is the most important determinant nutrient for plant growth and crop yield. The behavior of N in the soil system is complex, yet an understanding of the basic N processes, is essential for a more efficient N management program.
Major N processes in the soil are: mineralization, immobilization, denitrification and nitrification, and leaching. The most efficient way is to understand processes that contribute to N losses in soil and how can mineralization and nitrification be harnessed to improve N content in the soil. In coarse-textured soil, leaching is a dominant process that results in N losses. Nitrate-nitrogen (NO 3 -N) is soluble and moves readily with soil water becoming a potential source of ground water pollution. Ammonium Nitrogen is less subjected to leaching from the soil compared to nitrate because of its adsorption in the Cation Exchange Capacity.
However, losses of ammonium nitrogen through leaching occur in a coarsetextured soil with a low Exchange Capacity. Leaching is major N loss mechanism in coarse textured soil. Therefore, a proper understanding of N movement in coarse-textured soils can reduce N losses through leaching in the soil [20] [21].
At the end of the season, ammonium availability was studied at the soil planted with faba bean under drip irrigation (Table 5 & Figure 3). The  concentration was increased up to 30 cm at all depths at the horizontal distance.
With regard to the soil planted with onion, the ammonium distribution illustrated in Table 6 and  respectively. The data in Table 7 & Figure 5 showed that the nitrate move slowly at the soil depth and horizontally between the drippers and this may be due to the root distribution of faba bean at the surface layer consumes the nitrate from the soil solution. At the vertical distance between the laterals, the − 3 NO -N content increased from 0 -30 to 30 -60 cm at depths from 0 -60 cm. The peak concentration of − 3 NO -N was found 35 mg/kg at 60 -80 cm depth. It could be concluded that the − 3 NO -N leaching was low by drip irrigation. Nitrate movement at soil planted with onion showed at Table 8 and Figure 6.

Available Phosphorus
Phosphorus is one of the most important nutrients for plant growth so in most cases applied P fertilizer can lead to higher yield. Phosphorus transport in both vertical and lateral directions was too slow for the average rate of root growth into the soil [22]. der the drip irrigation system. This result was in agreement with [6].
Phosphorus pattern at the soil planted with onion is illustrated in Table 10 and Figure 8. Available phosphorus decreased with soil depth at horizontal distance 0, 30, 60 cm. Higher phosphorus was recorded (20.6 mg/kg) at 0 -20 cm below the dripper at the horizontal distance (0 cm). Available phosphorus decreased at 30 cm (11.8 mg/kg) then increased at 60 cm (16.8 mg/kg) in 0 -20 cm soil depth. The same result was observed at the other depths. Available phosphorus increased with vertical distance 0 -30 cm to 30 -60 cm and decreased with soil depth. [22] found that phosphate movement is not directly proportional to water movement. Phosphate transport in both vertical with depth and lateral directions was too slow for the average rate of root growth into the soil.

Available Potassium
At the end of the experiment, potassium availability was determined at the soil planted with faba bean under drip irrigation. Table 11 and Figure 9 showed that the available potassium at the top layer 0 -20 cm increased with the distance between drippers at the horizontal distance of 30 and 60 cm with the depth of 0 -60 cm. The potassium availability was increased at 60 cm from 100 to 115 mg/kg at the horizontal distance. This may be to the leaching of potassium from the soil      [22] who found that the available potassium throughout the profile tended to move with water toward the edge of the witting front. Many studies such as [23] [24] found the same results.
Potassium pattern at the soil planted with onion under drip irrigation showed at Table 12 and Figure 10 that the lowest concentration of available potassium was 108 mg/kg at the horizontal distance 30 cm (between drippers) in the 0 -20 cm depth of soil, then it increased to 112 mg/kg at 60 cm across the lateral. In 20 -40 cm, the available K concentration decreased with the horizontal distance from 112 mg/kg to 106 mg/kg. The same trend was found at 40 -60 cm of soil depth. These results stand in good agreement with [23] who found that regular application of irrigation water has taken down the soluble potassium to the middle layer. Across lateral, available potassium raised with vertical distance 0 -30 cm and 30 -60 cm in surface and subsurface layer (0 -20 cm and 20 -40 cm, respectively). A similar finding was reported by [17] who found that the shallow rooted plants such as onion left on average 80 kg•ha −1 in the soil at harvest.  Figure 10. Potassium distribution of the soil planted with onion under drip irrigation system.

Plant Growth Parameters
Data presented in Table 13 & Table 14

Conclusion
The  soil planted with faba bean was higher at the surface layer under drip irrigation but at the soil planted with onion, the highest concentration of available phosphorus was at surface irrigation. The potassium of the soil planted with faba bean and onion moves both horizontally and downward under drip irrigation.
At the soil planted with onion, potassium decreased with increasing in soil depth.