*3.4. Fertilizer Leaching*

*3.4. Fertilizer Leaching*  Results of all simulation scenarios showed that nitrate was adsorbed in all soil types under all fertigation scenarios. Therefore, there was insignificant leaching of nitrate outside the soil domain, Results of all simulation scenarios showed that nitrate was adsorbed in all soil types under all fertigation scenarios. Therefore, there was insignificant leaching of nitrate outside the soil domain, with a leaching percentage below 1% for all fertigation strategies. No leaching of potassium, phosphorus, and ammonium took place due to high adsorption rates. Results of the amount of fertilizers leaving the simulation domain showed that SDI systems with a shallow emitter depth had the lowest leaching percentage as compared to DI and SDI systems with deep emitter depths. This is attributed to the fact that the fertilizers were applied in the zone of maximum root density, as the emitter was located close to this zone. Thereby, fertilizers could effectively be taken up by plant roots. The results of leaching (accumulated) percentages below the maximum root density (25 cm from the soil surface) are shown in Table 5.


**Table 5.** Percentage of potassium, phosphorus, and nitrogen accumulated below the maximum root density zone as a fraction of the total potassium, phosphorus, and nitrogen added, respectively.

The table shows that the potential leaching of fertilizers in sand soil is much greater than for loamy sand and sandy loam with DI. For the SDI, the cumulative fertilizers below the maximum root density zone were the lowest for sandy loam soil, due to the upward movement of water and fertilizers that occurred by capillary action. The limited infiltration capacity and the high adsorption characteristics of fine-textured soil particles increase the soil retention of water and fertilizers, making fine-textured soils less susceptible to leaching losses. It is noted that fertigation strategy had little effect on fertilizer leaching. For DI, the potential leaching was the lowest for strategy E and the highest for strategy B. However, for SDI, the leaching potential was the lowest for strategy B. These results concur with the results of Hanson et al. [5] and Cote et al. [15]. They found that fertigation strategy affects leaching to a small degree only. Ajdary et al. [17] found that fertigation strategy did not affect nitrogen leaching, except in the case of coarse-textured soil when applying fertilizers directly before ceasing of the irrigation event. In our study, gravity force dominated fertilizer movement in strategy E, where fertilizers entered the wetting zone and moved downward with the flow. On the other hand, capillarity dominated the movement of fertilizers in strategy B, where the fertilizers moved with water downward and upward by capillary action. Therefore, more fertilizers can be maintained near and above the emitter. Thus, strategy E is recommended to reduce the groundwater contamination risk for DI, and strategy B is recommended for SDI.

#### *3.5. Root Fertilizer Uptake*

Table 6 shows that slight differences in the amount of root fertilizer uptake (fertilizer use efficiency) occurred when comparing different fertigation strategies. In our study, fertilizer use efficiency (FUE) is defined as the ratio between the amounts of fertilizer uptake by plant roots to the total amount of fertilizer added to the simulation domain. For the DI, FUE varied in the three soil types from 9.0 to 15.5%, 11.0 to 15.4%, and 26.2 to 35.8% for potassium, phosphorus, and nitrogen, respectively. The largest FUE occurred in sand soil, while the lowest FUE occurred in sandy loam. The higher value of FUE in loamy sand soil as compared to the sandy loam soil may be attributed to higher fertilizer adsorption on sandy loam particles. For SDI with an emitter depth of 10 cm, FUE varied in the three soil types from 9.3 to 15.6%, 11.4 to 15.7%, and 27.2 to 36.4% for potassium, phosphorus, and nitrogen, respectively. For the SDI with an emitter depth of 20 cm, the potassium uptake varied from 9.0 to 14.6%, phosphorus uptake varied from 11.0 to 14.4%, and nitrogen uptake varied from 26.7 to 34.5% in the three soil types. The results show that there is an insignificant difference between root fertilizer uptakes under different fertigation strategies. Consequently, the fertigation strategy does not seem to have any effect on the fertilizer uptake by the plant roots. This result concurs with Gardenas et al. [16]. They reported that nitrate taken up by plant roots was independent of fertigation strategy. However, Hanson et al. [5] found that the best nitrogen uptake ratio occurred when using strategy E for DI, while fertigation strategy did not have any effect on nitrogen uptake for SDI. Results also showed that root nutrient uptake was higher for SDI with a shallow emitter depth as compared to other systems. This may be due to the fact that the emitter was located approximately in the middle of the zone of maximum root density.

This study did not include considerations of practical network effects such as mainline and manifold hydraulics, lag time, and mixing of chemical flow with motive flow (main flow of irrigation water in the lines). However, we still believe that our results display some general and universal results for DI and SDI irrigation system outlines. However, future studies would need to quantify the above network effects.


**Table 6.** Percentage of root uptake for potassium, phosphorus, and nitrogen as a fraction of the total potassium, phosphorus, and nitrogen added, respectively, for different fertigation strategy.

#### **4. Conclusions**

The present study investigated the effect of soil type, fertigation strategy, and adsorption behavior on fertilizer distribution, fertilizer uptake by plant roots, and the amount of fertilizers that can leach below the simulation domain and below the zone of maximum root density for DI and SDI with 10 and 20 cm emitter depths for tomato plants. Simulation results showed that fertilizer leaching is significantly affected by the soil type. Sandy soils were more susceptible to the risk of fertilizer leaching below the maximum root density zone than loamy sand and sandy loam soils. Fertilizer accumulation in sandy loam was also larger than for other soil types. In addition, fertilizer leaching below the simulation domain was affected by varying irrigation systems. SDI with shallow emitter depths had a lower amount of leaching as compared to other drip irrigation systems where water and fertilizers were effectively injected into the zone of maximum root density. Therefore, the amount of fertilizer uptake by roots was the highest for the SDI with a shallow emitter depth. Consequently, a shallow emitter is recommended as compared to deep emitters for SDI, as it reduces the potential risk of groundwater contamination, especially in sandy soil and plants with shallow roots.

Simulation results showed that it is best to conduct fertigation at the end of the irrigation event (strategy E) in the case of DI, and fertigation at the beginning of the irrigation event (strategy B) in SDI. Simulation results showed that nitrate adsorption behavior has a considerable impact on leaching, uptake by plants, and distribution within the soil domain. Logically, as the adsorption coefficient increases, the amount of solute leaching from the soil domain decreases. Additionally, as the emitter discharge and/or the amount of solute increases, the amount of solute leaching from the soil domain increases. Shekofteh et al. [25] used different emitter discharges, varying from 0.5 to 8 L h−<sup>1</sup> , and different amounts of potassium nitrate, varying from 950 to 2550 kg ha−<sup>1</sup> , while neglecting the adsorption coefficient of nitrate. They found that, as the emitter discharge and fertilizer increased, the amount of nitrate leaching increased. In any case, it is important to determine the adsorption coefficient for nitrate before planting, as it will help to precisely assign nitrate application rates (fertilizer application rate and duration). This will lead to improved nutrient uptake and minimal leaching.

**Author Contributions:** Conceptualization, T.S. and A.M.; Methodology, R.E. and T.S.; Software, R.E. and T.S.; Validation, R.E. and T.S.; Formal Analysis, R.E.; T.S.; and R.B.; Investigation, R.E. and T.S.; Resources, T.S. and A.M.; Data Curation, R.E.; T.S. and R.B.; Writing—Original Draft Preparation, R.E. and T.S.; Writing—Review & Editing, R.B.; Visualization, R.E.; T.S. and R.B.; Supervision, T.S. and A.M. Project Administration, T.S. and A.M.; Funding Acquisition, T.S. and A.M.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
