**3. Results and Discussion**

## *3.1. Distribution of Fertilizers*

Figure 2 visualizes the progress of the fertilizer distribution at an observation point located on the top left corner of the simulation domain for DI, considering fertigation strategy C with different soil types (a: sand, b: loamy sand, and c: sandy loam). It can be observed that the fertilizer concentration increased at the end of fertigation events and then decreased between irrigation events, due to root uptake and adsorption to soil particles. For irrigation events free of fertilizers, fertilizer concentration decreased at the observation point, due to the movement of fertilizers with irrigation water. Potassium concentration increased at the end of the first fertigation event (*t* = 0.31 day), and then it decreased after ceasing irrigation. The same trend occurred throughout the simulation period, but with a significant decline in potassium concentration at 6.31, 20.31, and 34.31 days. These represent irrigation events without fertilizers, as potassium was applied three times a week. Phosphorus followed the same trend as potassium during the first 21 days of simulation. It then decreased with time until the end of simulation, as phosphorus was only applied during the first 3 weeks of simulation. For nitrate, the concentration increased at the end of the first fertigation events (*t* = 0.31 days). After that, nitrate concentration increased due to the nitrification of ammonium to nitrate and negligible denitrification, as the soil was not saturated between irrigation events. As nitrate was applied twice a week, nitrate concentration decreased during the second irrigation event. By applying the second fertigation, associated with the third irrigation event, nitrate concentration increased, but did not reach the concentration it had after the first fertigation event, due to adsorption, root uptake, and movement with irrigation water. Maximum nitrate concentration occurred at *t* = 16 days, due to the application of two subsequent fertigation events. After that and until the end of the simulation period, nitrate concentration decreased and increased but did not reach the maximum value. Ammonium concentration, on the other hand, increased by the end of fertigation events and decreased between fertigation events, due to nitrification to nitrate, adsorption, and root uptake. It is worth mentioning that approximately the same trend occurred in other fertigation strategies. The concentration of fertilizers around the emitter was the lowest for strategy B as compared to other strategies, due to the movement of fertilizers with irrigation water, while it was the highest for strategy E, as a result of high soil moisture content before fertigation. At the end of the simulation, fertilizer concentrations at the observation point for fertigation strategy C were 82.4, 1.1, 21.4, and 88.3 mg/L for sand; 79.7, 1.2, 20.3, 87.4 mg/L for loamy sand; and 76.5, 1.31, 18.9, and 85.5 mg/L for sandy loam for potassium, phosphorus, ammonium, and nitrate, respectively. For fertigation strategy B, they were 49.3, 1.1, 9.3, and 48.7 mg/L for sand; 50.4, 1.2, 9.1, and 50.4 mg/L for loamy sand; and 51.1, 1.3, 8.9, and 52.8 mg/L for sandy loam, respectively. For fertigation strategy M, they were 64.7, 1.1, 14.4, and 66.9 mg/L for sand; 63.7, 1.2, 13.6, and 67.1 mg/L for loamy sand; and 62.8, 1.3, 12.9, and 68.0 mg/L for sandy loam, respectively. However, for fertigation strategy E, they were 133.1, 1.2, 40.6, and 149.3 mg/L for sand; 124.9, 1.3, 38.8, and 144.6 mg/L for loamy sand; and 115.6, 1.4, 35.0, and 135.8 mg/L for sandy loam, respectively. It is worth mentioning that, in order to save space, only the aforementioned results are discussed herein.

and 149.3 mg/L for sand; 124.9, 1.3,

(*t* = 0.31 days). After that, nitrate concentration increased due to the nitrification of ammonium to nitrate and negligible denitrification, as the soil was not saturated between irrigation events. As nitrate was applied twice a week, nitrate concentration decreased during the second irrigation event. By applying the second fertigation, associated with the third irrigation event, nitrate concentration increased, but did not reach the concentration it had after the first fertigation event, due to adsorption, root uptake, and movement with irrigation water. Maximum nitrate concentration occurred at *t* = 16 days, due to the application of two subsequent fertigation events. After that and until the end of the simulation period, nitrate concentration decreased and increased but did not reach the maximum value. Ammonium concentration, on the other hand, increased by the end of fertigation events and decreased between fertigation events, due to nitrification to nitrate, adsorption, and root uptake. It is worth mentioning that approximately the same trend occurred in other fertigation strategies. The concentration of fertilizers around the emitter was the lowest for strategy B as compared to other strategies, due to the movement of fertilizers with irrigation water, while it was the highest for strategy E, as a result of high soil moisture content before fertigation. At the end of the simulation, fertilizer concentrations at the observation point for fertigation strategy C were 82.4, 1.1, 21.4, and 88.3 mg/L for sand; 79.7, 1.2, 20.3, 87.4 mg/L for loamy sand; and 76.5, 1.31, 18.9, and 85.5 mg/L for sandy loam for potassium, phosphorus, ammonium, and nitrate, respectively. For fertigation strategy B, they were 49.3, 1.1, 9.3, and 48.7 mg/L for sand; 50.4, 1.2, 9.1, and 50.4 mg/L for loamy sand; and 51.1, 1.3, 8.9, and 52.8 mg/L for sandy loam, respectively. For fertigation strategy M, they were 64.7, 1.1, 14.4, and 66.9 mg/L for sand; 63.7, 1.2, 13.6, and 67.1 mg/L for loamy sand; and 62.8, 1.3, 12.9, and

**Figure 2.** Temporal variation in fertilizer concentration at observation points on the emitter for strategy C using a DI system and different soil types, (**a**) sand, (**b**) loamy sand, and (**c**) sandy loam. **Figure 2.** Temporal variation in fertilizer concentration at observation points on the emitter for strategy C using a DI system and different soil types, (**a**) sand, (**b**) loamy sand, and (**c**) sandy loam.

#### *3.2. Effect of Soil Type 3.2. E*ff*ect of Soil Type*

Figure 3 illustrates fertilizer distribution in the soil domain after the first fertigation event and at the end of the simulation period for strategy C, considering the three soil types and DI. Soil type affected the fertilizer distribution within the simulation domain. Potassium, phosphorus, ammonium, and nitrate reached soil depths of 48, 38, and 32 cm, 37, 30, and 26 cm, 15, 12, and 10 cm, and 68, 58, and 50 cm below soil surface in sand, loamy sand, and sandy loam, respectively. The figure shows that the fertilizers moved deepest in sandy soil as compared to other soil types. This is Figure 3 illustrates fertilizer distribution in the soil domain after the first fertigation event and at the end of the simulation period for strategy C, considering the three soil types and DI. Soil type affected the fertilizer distribution within the simulation domain. Potassium, phosphorus, ammonium, and nitrate reached soil depths of 48, 38, and 32 cm, 37, 30, and 26 cm, 15, 12, and 10 cm, and 68, 58, and 50 cm below soil surface in sand, loamy sand, and sandy loam, respectively. The figure shows that the

fertilizers in loamy sand and sandy loam was higher as compared to sand. This is due to the limited infiltration capacity in fine-textured soil as compared to coarse-textured soils, which led to less airfilled pore space. The adsorption behavior was larger in fine-textured soil as compared to coarsetextured soil. Similar results were obtained under different strategies and SDI (results not shown). It is pertinent to mention that the amount of fertilizers above the emitter in sandy loam soil was higher than for other soil types with SDI. This may be due to the capillary action that increases the upward

movement of water and fertilizers in sandy loam as compared to sand and loamy sand soils.

because of the low field capacity of sand as compared to other soil types. The lateral spreading in sand, loamy sand, and sandy loam was 28, 30, and 30 cm, 20, 21, and 22 cm, 10, 11, and 11 cm, and *3.2. Effect of Soil Type* 

fertilizers moved deepest in sandy soil as compared to other soil types. This is because of the low field capacity of sand as compared to other soil types. The lateral spreading in sand, loamy sand, and sandy loam was 28, 30, and 30 cm, 20, 21, and 22 cm, 10, 11, and 11 cm, and 38, 41, and 41 cm for potassium, phosphorus, ammonium, and nitrate, respectively. The downward vertical extent of fertilizers was larger than the lateral extent for all soil types. This can be attributed to the gravity force that dominated during solute transport movement. Lateral movement of fertilizers in loamy sand and sandy loam was higher as compared to sand. This is due to the limited infiltration capacity in fine-textured soil as compared to coarse-textured soils, which led to less air-filled pore space. The adsorption behavior was larger in fine-textured soil as compared to coarse-textured soil. Similar results were obtained under different strategies and SDI (results not shown). It is pertinent to mention that the amount of fertilizers above the emitter in sandy loam soil was higher than for other soil types with SDI. This may be due to the capillary action that increases the upward movement of water and fertilizers in sandy loam as compared to sand and loamy sand soils. and 68, 58, and 50 cm below soil surface in sand, loamy sand, and sandy loam, respectively. The figure shows that the fertilizers moved deepest in sandy soil as compared to other soil types. This is because of the low field capacity of sand as compared to other soil types. The lateral spreading in sand, loamy sand, and sandy loam was 28, 30, and 30 cm, 20, 21, and 22 cm, 10, 11, and 11 cm, and 38, 41, and 41 cm for potassium, phosphorus, ammonium, and nitrate, respectively. The downward vertical extent of fertilizers was larger than the lateral extent for all soil types. This can be attributed to the gravity force that dominated during solute transport movement. Lateral movement of fertilizers in loamy sand and sandy loam was higher as compared to sand. This is due to the limited infiltration capacity in fine-textured soil as compared to coarse-textured soils, which led to less airfilled pore space. The adsorption behavior was larger in fine-textured soil as compared to coarsetextured soil. Similar results were obtained under different strategies and SDI (results not shown). It is pertinent to mention that the amount of fertilizers above the emitter in sandy loam soil was higher than for other soil types with SDI. This may be due to the capillary action that increases the upward movement of water and fertilizers in sandy loam as compared to sand and loamy sand soils.

**Figure 2.** Temporal variation in fertilizer concentration at observation points on the emitter for strategy C using a DI system and different soil types, (**a**) sand, (**b**) loamy sand, and (**c**) sandy loam.

Figure 3 illustrates fertilizer distribution in the soil domain after the first fertigation event and at the end of the simulation period for strategy C, considering the three soil types and DI. Soil type

ammonium, and nitrate reached soil depths of 48, 38, and 32 cm, 37, 30, and 26 cm, 15, 12, and 10 cm,

*Water* **2018**, *10*, x FOR PEER REVIEW 9 of 14

**Figure 3.** Fertilizer distribution after the first fertigation event (*t* = 0.31 days) and at the end of simulation period (*t* = 40 days) for strategy C for different soil types with DI (**a**: potassium, **b**: phosphorus, **c**: ammonium, and **d**: nitrate, units: mg cm<sup>−</sup>3). **Figure 3.** Fertilizer distribution after the first fertigation event (*t* = 0.31 days) and at the end of simulation period (*t* = 40 days) for strategy C for different soil types with DI ((**a**): potassium, (**b**): phosphorus, (**c**): ammonium, and (**d**): nitrate, units: mg cm−<sup>3</sup> ).

#### *3.3. Effect of Irrigation System 3.3. E*ff*ect of Irrigation System*

strategies and soil types (results not shown).

ammonium, and **d**: nitrate, unit: mg cm<sup>−</sup>3).

*3.4. Fertilizer Leaching* 

Figure 4 shows fertilizer distribution in sandy soil at the end of the simulation period for strategy C, using DI and SDI systems with emitters at 10 and 20 cm depths. It is noted that the fertilizer distribution depends mainly on the location of the emitter. For the DI, potassium, phosphorus, ammonium, and nitrate reached 48, 37, 15, and 68 cm depth below soil surface, respectively. In SDI with an emitter depth of 10 cm, potassium, phosphorus, ammonium, and nitrate reached depths of 50, 41, 24, and 68 cm, respectively. In SDI with an emitter depth of 20 cm, potassium, phosphorus, Figure 4 shows fertilizer distribution in sandy soil at the end of the simulation period for strategy C, using DI and SDI systems with emitters at 10 and 20 cm depths. It is noted that the fertilizer distribution depends mainly on the location of the emitter. For the DI, potassium, phosphorus, ammonium, and nitrate reached 48, 37, 15, and 68 cm depth below soil surface, respectively. In SDI with an emitter depth of 10 cm, potassium, phosphorus, ammonium, and nitrate reached depths of 50, 41, 24, and 68 cm, respectively. In SDI with an emitter depth of 20 cm, potassium, phosphorus, ammonium,

**Figure 4.** Fertilizer distribution at the end of the simulation period (*t* = 40 days) for strategy C in sand with DI and SDI systems and emitter at 10 and 20 cm depths (**a**: potassium, **b**: phosphorus, **c**:

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,

ammonium, and nitrate moved down to depths of 59, 49, 32, and 77 cm depths. As expected, the downward movement of fertilizers increased as the emitter depth increased. Shallow emitter depths *3.3. Effect of Irrigation System* 

and nitrate moved down to depths of 59, 49, 32, and 77 cm depths. As expected, the downward movement of fertilizers increased as the emitter depth increased. Shallow emitter depths allowed the fertilizers to reach the soil surface and spread more horizontally as compared to the deeper emitters. Thus, large emitter depth may increase the potential risk of groundwater contamination as well as decreasing fertilizer uptake. Similar results were obtained for the other strategies and soil types (results not shown). ammonium, and nitrate moved down to depths of 59, 49, 32, and 77 cm depths. As expected, the downward movement of fertilizers increased as the emitter depth increased. Shallow emitter depths allowed the fertilizers to reach the soil surface and spread more horizontally as compared to the deeper emitters. Thus, large emitter depth may increase the potential risk of groundwater contamination as well as decreasing fertilizer uptake. Similar results were obtained for the other strategies and soil types (results not shown).

50, 41, 24, and 68 cm, respectively. In SDI with an emitter depth of 20 cm, potassium, phosphorus,

**Figure 3.** Fertilizer distribution after the first fertigation event (*t* = 0.31 days) and at the end of simulation period (*t* = 40 days) for strategy C for different soil types with DI (**a**: potassium, **b**:

Figure 4 shows fertilizer distribution in sandy soil at the end of the simulation period for strategy C, using DI and SDI systems with emitters at 10 and 20 cm depths. It is noted that the fertilizer distribution depends mainly on the location of the emitter. For the DI, potassium, phosphorus,

phosphorus, **c**: ammonium, and **d**: nitrate, units: mg cm<sup>−</sup>3).

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**Figure 4.** Fertilizer distribution at the end of the simulation period (*t* = 40 days) for strategy C in sand with DI and SDI systems and emitter at 10 and 20 cm depths (**a**: potassium, **b**: phosphorus, **c**: ammonium, and **d**: nitrate, unit: mg cm<sup>−</sup>3). **Figure 4.** Fertilizer distribution at the end of the simulation period (*t* = 40 days) for strategy C in sand with DI and SDI systems and emitter at 10 and 20 cm depths ((**a**): potassium, (**b**): phosphorus, (**c**): ammonium, and (**d**): nitrate, unit: mg cm−<sup>3</sup> ).
