*3.8. Soil Analysis*

Deficient irrigation may cause accumulation of salts, especially when it combines fertigation. In addition, when applying fertilizers that contain highly mobile components such as nitrates, one should consider leaching and possible contamination hazards. As mentioned before, shallow sub-drip irrigation with low discharge can minimize these hazards. However, there is a need to leach the accumulated salts, especially from the top 30 cm of the soil profile (i.e., the potato root zone). Therefore, it is important to examine the soil profile as well as crop parameters.

The soil profile was first sampled after 100 mm sprinkler irrigation was supplied, following germination and initial seedling establishment ('Initial' in Figures 7 and 8). Based on a saturated soil water content of 0.36 (v v−1) this amount of water is equivalent to about 1.4 pore volumes that flow through the top 20 cm of the soil. Theoretically, for non-reactive solute transport, two pore volumes of saturated water flow should fully displace the native salts in this layer.

Even at the lowest irrigation regime (40%, 226.6 mm), approximately 1.6 pore volume of the 20–60 cm layer is displaced and this is enough to leach non-reactive solutes such as Cl− from the active root zone (Figure 7). Clearly, then, the higher irrigation regimes are able to continuously leach solutes from the soil profile to a depth of 60 cm and deeper. At the end of the growing season, before the harvest and soil sampling, the fertigation was stopped in order to 'kill' the foliage (i.e., plant water uptake and transpiration are negligible), but irrigation with reduced water salinity (due to the lack of fertilizer) continued for a further 21 days, allowing for efficient solute leaching and peel formation. As already mentioned, the dripper was buried to 5 cm and therefore, under continuous fertigation, the top soil accumulates salts continuously due to evaporation and capillary rise. Sprinkler or surface irrigation can achieve removal of these salts between growing seasons when there is no interference of the foliage.

During the harvest, soil samples in proximity to a dripper were taken until a depth of 60 cm. The salinity (EC and Cl−) in the top 5 cm was similar to that at the beginning of the season and behaved similarly in all the treatments. Contrary to the top layer, the rest of the profile was washed.

The EC and Cl− distributions shown in Figure 7 show that accumulated topsoil salts were displaced by approximately 20 cm and this is in broad agreement with the solute transport/pore volume relationship mentioned above. The top soil layer EC and Cl−<sup>1</sup> values (3.52 dS m−<sup>1</sup> and 422 mg L−<sup>1</sup> respectively) obtained from 1:1 (soil:DDW) extracts indicate a chemical equilibrium was being established in this system.

Figure 7 also shows the DOC concentration in the soil profile. The DOC sources are organic amendments and plant litter. DOC is an available carbon source for microbial activity, which in turn can increase the nitrogen demand and consumption by microorganisms [37]. It can be seen that the DOC concentrations are similar between the treatments. Similar to Cl−, DOC was also washed but with lower impact. This can be explained due to higher retardation of DOC in the soil because of the many DOC functional groups. From the C/N ratio (DOC/TN—data not shown), we observed that with higher fertigation and water dose we have more nitrogen in the soil profile and thus a lower C/N ratio. A low C/N ratio implies high nitrogen concentrations in the soil profile and thus can be interpreted as both a waste of fertilizer and an increased hazard for groundwater contamination.

With regard to the nitrogen, we examined the inorganic and organic fractions. The inorganic nitrogen includes NO3 − and NH4 +. The organic fraction is the dissolved organic nitrogen (DON) calculated by subtraction from the total nitrogen (TN): DON = TN − N-NO3 <sup>−</sup> − N-NH4 +.

**Figure 7.** Mean values of soil sample analyses (1:1 soil:water) for EC, Cl−, and DOC at four different depths (5, 20, 40, and 60 cm) according to the irrigation regime. The dashed line ('Initial') represents the sampling at the beginning of the season. (**a**,**b**,**c**) F0%; (**d**,**e**,**f**) F50%; (**g**,**h**,**i**) F100%.

**Figure 8.** Mean values of soil sample analyses (1:1 soil:water) of TN, NO3 −, NH4 <sup>+</sup> and DON at four different depths (5, 20, 40, and 60 cm) according to the irrigation regime. The dashed line ('initial') represents the sampling at the beginning of the season. (**a**–**d**) F0%; (**e**–**h**) F50%; (**i**–**l**) F100%.

It can be seen (Figure 8) that the TN was mainly compromised of NO3 −, except for the F100% treatment that also had a higher fraction of DON. In general, NH4 <sup>+</sup> concentrations were significantly low or negligible for all treatments, both at the beginning and at the end of the season. TN was leached and/or consumed mainly in the F0% and F50% dose treatments, thus minimizing waste and N contamination of groundwater. More importantly, the F50% treatment was able to sustain profitable potato yield and quality. As for the F100% treatments, some accumulation of DON occurred in the top 5 cm. A similar trend was observed for all the irrigation doses with some higher values in the W100%F100% treatment, which showed significantly higher accumulation in this top layer. A possible explanation for this observation is the higher concentration of the mineral nitrogen as reflected by the lower C/N ratio (data not shown). Specifically, since mineral nitrogen was available for both plants and microbial activity, the DON was not consumed and thus accumulated in the root zone.

The soil NO3 − concentration was most affected by fertigation due to its addition as the main source of nitrogen. The NO3 − trend at the beginning and end of the season was similar to that of the TN. It can be seen that for the F0% and F50% doses, most of the nitrogen was consumed, while for the 100% treatment nitrate accumulated throughout the soil profile, with higher concentrations found in the top 5 cm.

As mentioned previously, the DON fraction was significantly higher for the F100% dose and exhibited a similar trend to the NO3 −. As a result, when applying sprinkler irrigation between seasons when using the F100% treatments, the farmer may leach nitrogen components below the root zone, reducing the salinity but with possible groundwater contamination.
