Hydraulic conductivity and diffusivity of an Oxisol cultivated with sugarcane fertigated with nitrogen and potassium

This study had the objective to evaluate the effect of irrigation and fertigation (NK) in the hydraulic conductivity and diffusivity of an Oxisol cultivated with sugarcane. The experimental design comprised randomized blocks in a 5 × 2 factorial scheme, with four replications. Treatments consisted of five levels of water replacement (100, 75, 50, 25 and 0%), with and without fertirrigation (NK). The planting of sugarcane, cultivar RB85-5453, was performed in a double row (W-shaped), 8 m long, with 1.80 m spacing between the double rows, the distance between the crops in the double row was 0.40 m, with a total area of 52,8 m in each paddock. For treatments with water, replacement (WR) a drip tube was placed in the ground at a depth of 0.20 m among the furrows of the double row. The drip tube (DRIPNET PC 16150) comprised a thin wall, 1.0 bar pressure, nominal discharge 1.0 L h, and 0.50 m spacing between drippers. Nitrogen was applied by fertirrigation at a dose of 100 Kg ha, at 30-day intervals, with 10 applications throughout the development of the sugarcane culture. Potassium fertilization was done partially, in 30% of the furrows, and the remaining part was treated with the irrigation water. Nitrogen and potassium were spread only in the treatment with 0% water replacement. Was evaluated hydraulic conductivity and diffusivity versus logarithmic pressure head, at a depth of 10 cm, using RETC software. The hydraulic diffusivity for water replacement of 25 and 50% with fertigation was 160.3 and 14.9 cm days for the lower values of the logarithm of the pressure head.


Introduction
The hydraulic conductivity of the saturated soil indicates the ability of a soil to transmit water in the unsaturated soil, this conductivity varies with the amount of water present in its empty pores, that is, with its degree of saturation, consequently the hydraulic conductivity is extreme importance to the agricultural use (Gonçalves;Libardi, 2013;Sobrinho et al., 2018).
The saturated soil K depends on the characteristics of the soil matrix and the fluid present in the soil, it is important both in the flow process and of the transportation of contaminants in porous media, should also consider the soil K in the solution of problems related to irrigation and the drainage of farm soils (Delgado-Rodríguez et al., 2011).
A detailed understanding of Ks is critical in the assessment of irrigation practices, infiltration rates, runoff, groundwater recharge rates, and drainage processes, consequently sugarcane is expected to be an important factor that influences the hydraulic properties of soil by affecting its physical and chemical characteristics (Aimrun et al., 2004;Cookson et al., 2007;Breulmann et al., 2012, Hao et al., 2019. Many researchers have found that the hydraulic conductivity of soil is affected by many factors such as density, water contents, degree of saturation, void ratio, grain size distribution, and particle structure (Chung et al., 2018).
Different hydrological transport models have used hydraulic conductivity at saturation as a constant for chemical leaching risk assessment, water infiltration characterization, and surface runoff modeling, also has been used frequently to predict unsaturated soil hydraulic conductivity Ruy, 2009;Neyshabouri et al., 2013;Masís-Meléndez et al., 2014;Rahmati, 2017;Rahmati et al., 2019).
This study had the objective to evaluate the effect of irrigation and fertigation (NK) in the hydraulic conductivity and diffusivity of an Oxisol cultivated with sugarcane.

Methodology
The experiment (field study) (Lakatos and Marconi, 2003) was performed in the experimental area of the IFGoiano -Campus Rio Verde, GO Brazil, 17°48'28"S and 50°53'57"W, mean altitude 720 m, slightly rolling ground relief (slope 6%), red dystrophic Latissoil (LVdf) with mean texture 458, 150 and 391 g kg -1 sand, silt and clay, respectively, and chemical characteristics as shown in Table 1.  1.0 L h -1 , and 0.50 m spacing between drippers. On planting, all furrows of the plots were fertilized with 30 kg N ha -1 (urea), 120 kg P2O5 ha -1 (single superphosphate) and 80 kg K2O ha -1 (potassium chloride). Nitrogen was applied by fertirrigation at a dose of 100 Kg ha -1 , at 30-day intervals, with 10 applications throughout the development of the sugarcane culture. Potassium fertilization was done partially, in 30% of the furrows, and the remaining part was treated with the irrigation water. Nitrogen and potassium were spread only in the treatment with 0% water replacement.
Water demand was calculated by a 0.1 kPa puncture digital tensiometer. Tensiometric sensors were placed at a depth of 0.20, 0.40, 0.60 and 0.80 m, at a distance of 0.15, 0.30, 0.45 and 0.60 m from the drip tube, with daily readings of water tension in the soil. The soil's physical and water characteristics were determined by the water retention curve in the soil, with an available water capacity (AWC) of 100 mm. Soil was kept at field capacity in treatments with 100% WR. By the end of the experiment, the water supplemented to the soil was calculated to determine the volume of water provided (Table 2).  Mualem, 1976), and the soil diffusivity was determined according to Klute (1965) and Pauletto et al. (1988), according to Equation 1 to 3: where: θ -water contents, cm 3 cm -3 ; θr -the residual water contents, cm 3 cm -3 ; θs -the saturated water contents, cm 3 cm -3 ; m, n e α -empirical parameter. With m = 1-1/n; h -is the soil water pressure head (with dimension cm); t -is time (days); z -is soil depth (cm); K -is the hydraulic conductivity (cm days -1 ); Ks -is the saturated hydraulic conductivity (cm days -1 ); D(θ) -soil water diffusivity function (cm 2 days -1 ).
Results were analyzed by ANOVA. In significant cases, regressions of linear and quadratic were performed for water replacement levels. Nitrogen and potassium application means were compared using Tukey test at significance degree α = 0.05.

Results and Discussion
The hydraulic diffusivity (HD) minimum (approximately zero) it was verified in the logarithmic pressure head (LPH)  According to Cunha et al. (2015) the initial hydraulic diffusivity in the no-tillage system was 16% higher than the conventional crop diffusivity, this difference has not changed much with the increase of the hydraulic load.

Conclusion
The hydraulic diffusivity for water replacement of 25 and 50% with fertigation was 160.3 and 14.9 cm 2 days -1 for the lower values of the logarithm of the pressure head.
The hydraulic conductivity for the water replacement 50% with and without fertigation was 0.5 and 0.88 cm day -1 to the logarithm of the pressure head of -3 cm.