Gouia et al. reported that high salt stress results in slower growth and even death of seeding cotton, lower dry matter accumulation, less roots, softer and darker leaves, shorter period of blooming, and even death of seedling cotton. Ashraf showed that under high salt stress, the growth rate of cotton was significantly reduced with less fruiting branches. The appearance of square, flowers and boll was significantly retarded; the blooming stage was shorter and the shedding also increased. Nevertheless, our experimental results also indicated that the adverse effects of soil salinity on cotton growth could be alleviated by fertilizer application, but over fertilization with N might contribute to soil salinization and increase the negative effects of soil salinity on plant performance. Villa-Castorena et al. concluded that over fertilization during the early plant development might contribute to higher soil salinity and decreased pod yield of chile pepper, while salt-stressed chile performed well when they were adequately fertilized.Similarly, Kaya and Higgs reported that supplementary urea could overcome the effects of high salinity on fruit yield and whole plant biomass in pepper plants. The effects of soil salinity and N application on cotton above-ground dry mass were dominated by the soil salinity and could be correlated with root development. The root dry mass also decreased linearly with soil salinity, but at a less pronounced rate . Reduction in dry matter production with increasing salinity level was noticed by a number of researchers .

The effect of soil salinity on the root distribution may be closely related to the dynamic salt distribution corresponding to the irrigation and root water uptake . During an irrigation,plastic planters wholesale the salts around the emitters were transported with water to farther distance and to deeper soil layers. During the intervals between irrigations, the salts may be transported toward the soil surface and roots due to the root water uptake and evaporation. Our measurements showed that the lowest salinity in the soil profile was around the emitter and the highest salinity in the soil profile was found in the top 0–10 cm soil layer 15– 20 cm from the emitter. The average soil salinity at different depths is illustrated in Fig. 6. Under the low soil salinity treatment of S0, the soil salinity in the 30–50 cm soil layers was significantly higher than that in the 0–20 cm soil layers. In contrast, under the high soil salinity treatment , the soil salinity in bottom layers was similar with that in the top 0– 20 cm layer. During the growing period, cotton may adjust its growth pattern to avoid high salt stresses . As a result, more roots were distributed in bottom layers under the high soil salinity treatment than under the low soil salinity treatment. The uptake of nitrate is known to compete with that of Cl, a major ion in saline soil . Such an interaction results in diminished N uptake and decreased plant growth at increased Cl concentrations. Our experimental results illustrated that at low soil salinity level, increasing the N application rate could significantly enhance the N uptake. Under moderate soil salinity level, proper use of N fertilizer was necessary. Over fertilization did not benefit the N uptake. Under the high soil salinity, salt was the dominated factor governing the cotton growth and N uptake. The inhibition could not be alleviated by fertilizer application. The findings agree with those observed by Shenker et al. , who demonstrated that increasing the application of N fertilizer could increase N uptake, hence the cotton yield.

Our study indicated that N uptake and the cotton yield were not affected by high N application rate, and the N uptake and the cotton yield increased as the soil salinity decreased. The accumulation of N in different plant parts generally followed a decreasing order of seed > leaf > bur > stem > shed > root, but varied with the soil salinity level . Under the high salinity treatment , the N in the cotton seed accounted for about only 8% of the total N uptake, while under the other three salinity treatments, it accounted for as much as 31% of the total N uptake. Consequently, more N was accumulated in the leaf and stem parts under high salinity level. The results indicated that there was a nutrient imbalance under the high salinity treatment. The nutrient was mainly used for vegetative growth rather than for reproductive growth. The concentrations of Na, Ca, and Cl in various plant parts of cotton increased as soil salinity increased. The concentrations of these ions among different plant parts followed in a decreasing order: leaf > stem > bur > seed. This kind of distribution pattern of salt ions among different plant tissues was thought to be one of the major salt tolerance mechanisms of cotton . Leidi and Saiz demonstrated that the Na concentration in cotton leaves was closely related to plant growth. Fortmeier and Schubert compared the effect of NaCl and Na2SO4 on cotton growth and concluded that the toxicity of salts on cotton was due to Na, not due to C1, or combination of Na and C1. The accumulation of Na may result in decreased uptake of K, and Ca . Our experimental results showed that the Na concentration of the cotton roots, leaves and stems increased with the soil salinity, the uptake of K decreased with soil salinity, while the uptake of Ca increased with soil salinity.

The high uptake of Ca may be attributed to the soil salinity treatment in which 1:1 of CaCl2 and NaCl was used. Consequentially, the Ca in the soil was significantly increased, thus the uptake of Ca by cotton plant. K and Ca play key roles in several physiological processes, while Na does not function as a macro-nutrient. Thus, the substitution of K/Ca by Na may lead to nutritional imbalances .Micro-irrigation has become the optimal standard for irrigation and fertigation of horticultural crops in Australia, due to increased water scarcity and higher costs of fertilizers over the last decade. Intensive fertigation schedules have been developed to increase yield and quality of many permanent horticultural crops, including mandarin. This combines drip irrigation and fertigation to deliver water and nutrients directly to the roots of the crop,vertical grow rack with the aim of synchronizing the applications with crop demand and maintaining the desired concentration and distribution of ions and water in the soil . The overall aim of these interventions is to develop an irrigation and nutrient management program that increases yield and fruit quality, while reducing leaching. The fundamental principle of drip fertigation is to apply water and nutrients regularly to a small volume of soil at a low application rate and at a high frequency to closely meet crop demand . However, the potential for movement of water and mineral nutrients, especially nitrogen , below the root zone and into the ground- and then surface-waters using these approaches is still high. This is due to a number of factors: amount and intensity of precipitation, the large amounts of water and nutrients being applied, the limited capacity of roots to take up these nutrients, and to the ability of irrigators to manage drainage and hence leaching. Citrus is one of the important horticultural crops being grown under intensive fertigation systems in Australia. The vast majority of citrus plantings are oranges , with the rest split between mandarins , lemons and limes , and grapefruit . About 75% of the Australian citrus industry is located in the Murray-Darling Basin, utilising the lighter-textured free-draining soils adjacent to the Murray, Darling and Murrumbidgee rivers, and thus potential off-site effects of poorly managed fertigation may have wider implications. Irrigated horticulture has, in general, been identified as the major source of nitrogen in drainage waters in the Murray Darling Basin .

A significantly high nitrate level has been reported in drainage water and soil solution under grapevines in the Murray Darling Basin. These values are significantly higher than the Australian environmental trigger value for nitrate . Leaching of nitrates from soils under perennial horticulture may pose a potential threat to groundwater. The main sources of nitrate in mandarin production are mineral fertilizers. Nitrate is removed from the soil by plant uptake or through decomposition by micro-organisms in the process of denitrification. In well-aerated soils typical of this region, denitrification is often negligible because of a lack of favourable conditions . Nitrate, being an anion, moves freely in these mineral soils, and hence has the potential to leach into groundwater and waterways if fertigation is not well scheduled . Several researchers have reported substantial leaching of applied N in citrus cultivation under field conditions . Syvertsen and Jifon found that N leaching was higher under weekly fertigated orange trees than under daily or monthly fertigated trees. Syvertsen and Sax reported that increasing the number of fertigation events could significantly reduce N leaching. However, they observed 38–52% leaching of N from fertilizer, and the nitrogen use efficiency ranging between 25% and 44% in Hamlin orange trees. Other researchers have reported that nitrate accumulates toward the boundary of the wetted volume for most combinations of drip emitter discharge, input concentrations, and volumes applied. These studies suggest that there is a need for efficient tools, capable of describing and quantifying nitrate leaching, as well as nitrate uptake by crops, which in turn would help in designing and managing drip irrigation systems and achieving a high N fertilizer use efficiency, thereby limiting the export of this nutrient as a pollutant to downstream water systems. In addition to nitrate leaching, salinity is also an important factor influencing the sustainability of the citrus production worldwide, as citrus species are relatively salt sensitive. The reported value of the average threshold electrical conductivity of saturation extract and slope for oranges are 1.7 dS m 1 and 16%, respectively . Salt damage is usually manifested as leaf burn and defoliation, and is associated with accumulation of toxic levels of Na+ and/or Cl in leaf cells. Under drip irrigation there are many factors influencing the distribution of soil water and salts, and hence the water use efficiency , such as water quality, dripper discharge rate , irrigation water depth , and irrigation frequency . Simulation models have been valuable research tools in studies involving complex and interactive processes of water flow and solute transport through the soil profile, as well as the effects of management practices on crop yields and the environment . HYDRUS-2D has been used extensively in evaluating the effects of soil hydraulic properties, soil layering, dripper discharge rates, irrigation frequencies, water quality, and timing of nutrient applications on wetting patterns and solute distribution . Although these studies demonstrate well the importance of numerical modelling in the design and management of irrigation and fertigation systems for various crops, most studies involving salinity and nitrate leaching are based on either an analysis of hypothetical scenarios, or are carried out for annual crops. Hence, there is a need to carry out modelling studies for perennial horticultural crops such as mandarin, using experimental results from field studies involving modern irrigation systems such as drip. The objectives of the present investigation were to evaluate water, salt , and nitrate movement in soil below young mandarin tree using HYDRUS-2D, and to evaluate various irrigation and fertigation strategies for controlling deep drainage and nitrate leaching, whilst maintaining soil salinity below the threshold for mandarin. This approach will help us understand the best irrigation and fertigation management practices to be adopted in future practical applications, with the goal to increase root water and nutrient uptake.The field experiment was conducted at the Dareton Agricultural and Advisory Station , located in the Coomealla Irrigation Area, 3 km from Dareton and 10 km from Went worth in New South Wales . The research station forms part of the Sunraysia fruit growing district of NSW and Victoria located in the Murray Darling Basin. An experimental site with an intensive fertigation system, consisting of various mandarin varieties budded onto a number of root stock varieties , was established in October 2005. The trees were planted at a spacing of 5 m – 2 m. The actual monitoring and measurements were initiated in August 2006.