Potential nutritional value of olive-mill wastewater applied to irrigated olive (Olea europaea L.) orchard in a semi-arid environment over 5 years

Scientia Horticulturae 241 (2018) 218–224

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Potential nutritional value of olive-mill wastewater applied to irrigated olive (Olea europaea L.) orchard in a semi-arid environment over 5 years

T

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Isaac Ziporia, , Arnon Daga, Yael Laorb, Guy J. Levyc, Hanan Eizenbergd, Uri Yermiyahua, Shlomit Medinab, Ibrahim Saadib, Arkadi Krasnovskib, Michael Ravive a

Agricultural Research Organization (ARO), Institute of Plant Sciences, Gilat Research Center, 85280, Israel Agricultural Research Organization (ARO), Institute of Soil, Water and Environmental Sciences, Newe Ya'ar Research Center, Ramat Yishay 30095, Israel c Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization (ARO), The Volcani Center, Bet Dagan 15159, Israel d Agricultural Research Organization (ARO), Institute of Plant Protection, Newe Ya'ar Research Center, Ramat Yishay 30095, Israel e Agricultural Research Organization (ARO), Institute of Plant Sciences, Newe Ya'ar Research Center, Ramat Yishay 30095, Israel b

A R T I C LE I N FO

A B S T R A C T

Keywords: Waste recycling Plant nutrition Soil potassium Soil phosphorus Olive production

Controlled spreading of olive mill wastewater (OMW) on cultivated soils is a low-cost disposal method of an otherwise problematic pollutant, with potential recycling of plant nutrients. The nutritional value of successive OMW applications was examined in an intensive olive orchard grown on sandy loam soil in a semi-arid region. Application at 50–150 m3 ha−1 y−1 for 5 years had no negative effects on tree vegetative growth, fruit yield or oil quality. OMW application did not increase N content in the soil or plants; yet, it caused a consistent increase in soil P and K contents and significantly affected diagnostic leaf P and K concentrations. It also led to a significant increase in exchangeable potassium percentage (EPP) already from the first application, and soluble K migration to deep soil layers after 3 years of successive applications. Soil tillage after OMW application did not affect N, P or K dynamics in the soil or uptake of these nutrients by plants. Controlled application of OMW to intensive olive orchards can be a significant source of K and P and thus save on fertilizers without negatively affecting tree performance.

1. Introduction Olive oil extraction systems generate large amounts of olive mill wastewater (OMW) in a relatively short time during the olive harvest season. Roig et al. (2006) estimated a volume of 1–1.6 m3 of OMW per ton of processed olives in the three-phase extraction systems and 0.2 m3 of OMW in the two-phase systems. The physicochemical properties of OMW depend on the characteristics of the processed olives and on the system used for olive oil extraction and its local operation conditions (Aviani et al., 2012). Improper treatment and/or uncontrolled disposal of OMW might cause severe environmental problems (Peikert et al., 2014). Typically, OMW is characterized by high organic load (up to 100,000 and 220,000 mg L−1 biochemical oxygen demand [BOD] and chemical oxygen demand [COD], respectively), high salinity (5–12 dS m−1), oil residues (1–23 g L−1) (Azbar et al., 2004; Roig et al., 2006), and a highly phytotoxic nature (Aviani et al., 2009, 2012). Together with the fact that large OMW volumes are produced in a short period during the year, OMW discharge into domestic wastewater treatment plants is prohibited, as it may result in their collapse. Over

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Corresponding author. E-mail addresses: [email protected], [email protected] (I. Zipori).

https://doi.org/10.1016/j.scienta.2018.06.090 Received 14 January 2018; Received in revised form 3 May 2018; Accepted 29 June 2018 0304-4238/ © 2018 Elsevier B.V. All rights reserved.

the last decade, much work has been devoted to the search for efficient treatment processes with the aim of reducing organic load and overall toxicity, as well as to exploiting potentially valuable substances (Arvanitoyannis et al., 2007; Komilis et al., 2005; Roig et al., 2006). To date, the economic feasibility of various proposed methods is questionable as they seem to be barely supported by olive growers or olive mill owners. As a result, the most common solution for the disposal of OMW is still spreading it on the soil surface. In most olive oil-producing countries, there are regulations regarding the permissible annual amount of OMW application per unit area. Many researchers (Buchmann et al., 2015; Piotrowska et al., 2006; Rinaldi et al., 2003; Saadi et al., 2007; Sierra et al., 2007) have found that under conditions of controlled spreading in the range of several tens of cubic meters per hectare and even as high as 400 m3 ha−1; (Chartzoulakis et al., 2010), most of the toxic effects of OMW are short term and virtually disappear after a period of weeks to months. Mekki et al. (2006) and Saadi et al. (2007) reported complete recovery from phytotoxicity symptoms in soil 3 months after OMW application at 72 m3 ha−1. On the other hand, OMW contains large amounts of organic matter, and potentially

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Table 1 Chemical properties of the olive mill wastewater (OMW) used in each year of the experiment, average values (Avg.) and standard deviations (SD). Empty cells are missing data. Property

Units

pH EC Cl Na Ca Mg N-NO3 N-NH4 total N soluble P total P K Fe Zn Mn Cu SAR TS Total COD Sol. COD BOD total BOD sol. Oils @ Grease

dS m−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 (meq L−1)0.5 g L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1

Halutza 2012 1 st appl.

Rahat 2013 2nd appl.

Rahat 2014 3rd appl.

Rahat 2015 4th appl.

Rahat 2016 5th appl.

Avg.

SD

4.8 12.1 1125 437 116 186 14.7 21 1160 181 283 5122 1.1 2.5 4 0.12 5.85 101 126758 83126 32250 28500 13598

4.2 11.2 1100 506 94 146 22.0

4.3 13.3 902 198 664 195.6 11.8 56 1454 242 368 6843 26 3.6 2.3 1.16 1.73 78 134540

4.6 10.7 774 78.2 400 122.7 9.7 46.0 1720 242.0 283.0 5040 31.2 2.3 2.5

27500

4.2 13.3 841 218 192 186 7.9 34 483 300 323 4570 50.5 3.4 2 < 0.25 2.71 54 85811 74,646 49880

5008

12270

12107

4.42 12.1 948 287.5 293 167.3 13.2 39.3 1103 222.5 291.6 5308 27.8 6.5 2.4 0.6 3.80 75.1 115765 67644 35826 21750 12177

0.27 1.2 157 178 240 31.3 5.52 15.2 513 59.2 61.6 884 17.7 7.9 1.0 0.7 2.90 20.1 18588 19929 10026 9546 4642

700 148 201 4966 30.2 20.6 1.2 7.65 67 113471

42500

0.90 118246 45159 27000 15000 17900

2. Materials and methods

available plant nutrients: nitrogen [N], phosphorus [P], potassium [K]. Therefore, controlled application of OMW to agricultural soils can result in an increase in soil organic matter (Barbera et al., 2013; Regni et al., 2017) and can contribute to soil nutritional status (Belaqziz et al., 2016; Chaari et al., 2014; Chartzoulakis et al., 2010; Sierra et al., 2007). Piotrowska et al. (2006) reported that soil P and K concentrations increased as a result of OMW application of 40 and 80 m3 ha−1, whereas the toxic effects diminished after a relatively short time (42 days). Moreover, Magdich et al. (2012) reported an increase in fruit yield in rain-fed olives in Tunisia as a result of annual application of 100 m3 ha−1 OMW over 6 successive years. Undesired leaching of OMW components may be minimized by soil tillage after OMW application. Laor et al. (2011) observed for a clay soil greater leaching of dissolved organic carbon (DOC) and phenolic compounds after surface spreading of OMW, compared to surface spreading followed by tillage. Levy et al. (2017) who studied the experimental plots described in the present study, observed higher increase in OC content in the tilled plots and suggested that it might be caused by the better interactions between the added organic matter and soil particles upon tillage. Moreover, the observed effect of OMW spreading on soil hydrophobicity (water drop penetration time) in this experimental platform was not visible in tilled plots (Steinmetz et al., 2015). Most of the studies reporting the effect of OMW application on soil and tree nutritional status have been carried out in rain-fed, non-intensive olive orchards. Under such conditions, total biomass production is quite limited and therefore, the demand for nutrients is lower than that in intensive orchards. Moreover, in non-intensive orchards, the available soil volume per tree is relatively high and tree nutrient demand can be satisfied even when soil nutrient concentrations are low. Thus, the objectives of the present study were to examine the impact of controlled OMW application to an intensive, irrigated olive orchard, with and without subsequent soil tillage on (i) tree performance, (ii) tree nutritional status, and (iii) soil nutritional potential, including the dynamics of OMW-borne P and K in the soil. These objectives were addressed over 5 years of successive OMW application at varying rates.

The experimental field platform used in this study is located at the Gilat Research Center, in the northern Negev of southern Israel (31°20′N 34°40′E). The region is characterized by a semi-arid Mediterranean climate with a cool winter and a warm dry summer (average annual minimum of 14.2 °C and maximum 27.7 °C). The rainy season is between November and April and the average annual rainfall during the period of the experiment was 290 mm, with most of it, about 75% occurring prior to OMW application. The soil was classified as a sandy loam (Calcic Haploxeralf), composed of 50% sand, 35% silt and 15% clay with a pH of 8.2 and 11.5% calcium carbonate. Soil organic matter content was around 0.5%. The experimental platform was managed for 5 years (2012–2016) in a 7-year-old olive orchard, where trees (cv. Leccino, mainly cultivated for olive oil production) were planted at 3.5 m × 7 m distance. The trees were drip irrigated with a single drip line per row, with 2 L h−1 drippers every 50 cm. Irrigation was generally performed from March to October with a Kc of 0.55 relative to Penman ET0, which resulted in an annual amount of 650 mm. The orchard was fertilized commercially from 2004 to 2011 with 15, 8 and 30 kg ha−1 of N, P2O5 and K2O, respectively. No fertilizers were applied during the 5 years of the OMW application. Every four trees formed a plot, with the two central ones being the measured trees and the other two - border trees. Each treatment had five replicates in a completely randomized block design (a total of 25 plots for the five treatments). To avoid underground migration of OMW between plots and to restrict the root zone to the targeted treatment, vertical 0.2-mm thick plastic partitions were buried along the rows, in the center between rows, at a depth of 1 m. This resulted in a plot size of 7 m wide by 14 m long. Five OMW treatments were studied: (i) no application of OMW (control), (ii–iv) annual application of OMW at rates of 50, 100 or 150 m3 ha−1, respectively, and (v) annual application of 100 m3 ha−1 followed by superficial soil tillage with a hand rototiller to a depth of 5 cm, 3 weeks after OMW application. In treatments (ii)–(v), the OMW was applied shortly after the end of the olive milling season (January–February). The OMW was taken from threephase medium-size olive mills (Halutza mill in 2012 and Rahat mill in 219

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years 2013–16, Table 1). During application, the specified dose of OMW was evenly distributed manually over the entire plot, from the treetrunk line to the plot borders (using a container and a submersible pump). Application of OMW was repeated in this manner for five consecutive seasons. A sample of the applied OMW was taken every year for analysis of selected chemical properties (Table 1). Soil samples were taken at depths of 0–10, 10–30 and 30–60 cm twice a year, 1 to 2 months before OMW application (autumn) and 1 to 2 months after application (spring). Soil samples were taken at a perpendicular distance of 1.5 m from the tree line. Content of nitrate and soluble K was determined in soil-saturated paste extract, and available P was determined by extraction with sodium bicarbonate according to Olsen et al. (1954). In addition, soil cation exchange capacity (CEC) was determined by sodium acetate extraction (Rhoades, 1986), and exchangeable potassium by ammonium acetate extraction (Thomas, 1986); the exchangeable potassium percentage (EPP) was then calculated. Although soil samples were taken from the 0–10 cm layer separately from the 10–30 cm layer, in most cases the results refer to the 0–30 cm layer. In these cases, the analysis was carried out on a soil sample composed of a 1:2 wt ratio of the 0–10 cm layer and the 10–30 cm layer, respectively. Trunk circumference was measured once a year, in March, to assess vegetative development of the trees. Diagnostic leaves were sampled in July and taken for N, P, and K analyses. The leaves were rinsed and dried at 70 °C. After reaching a constant weight, the leaves were milled to a fine powder and a sample of 0.1 g was taken for heated digestion with concentrated sulfuric acid and 30% hydrogen peroxide. N and P concentrations were determined in the solution with an automated discrete photometric analyzer (Gallery Plus, Thermo Fisher Scientific). K was measured with an atomic absorption spectrometer (Perkin Elmer Precisely AAnalyst 200). Yield was assessed by harvesting each measured tree separately, using electric combs (Olivium, Pellenc, France), and weighing the yield. A 3 kg subsample was taken to the laboratory for further analyses: oil and water content were determined by means of near infrared spectrometer (OliveScan, Foss, Denmark) and oil was extracted with a laboratory-scale olive mill (Abencor, mc2 Ingenieria y Sistemas, Seville, Spain). Free fatty acid content was determined according to IOOC protocol and polyphenol level was determined colorimetrically, by the Folin-Ciocalteau method, using tyrosol as a standard for calibration. The presented data are averages of five replicate field plots for each treatment. Comparison between mean values of the treatments for a given attribute was based on post-hoc Tukey–Kramer honestly significant difference test at P ≤ 0.05 (JMP software Vers. 5.0.1; SAS, 2002).

Table 2 Potential annual contribution of olive mill wastewater (OMW) to soil nutritional status (kg ha−1), compared to the commercially recommended levels for irrigated olives. OMW applied (m3 ha−1)

N-NO3

N-NH4

Total N

Soluble P (expressed as P2O5)

Soluble K (expressed as K2O)

50 100 150 Commercial recommendation

0.7 1.4 2.1 150

1.9 3.7 5.6

55.2 110.4 165.6

25 50 75 80

318 636 954 300

Fig. 1. Effects of olive mill wastewater (OMW) application rate on soil nitrate-N (NO3) concentration in the 0–30 cm soil layer (obtained from a saturated paste [SP]) during five successive annual applications. NS = non-significant differences at P ≤ 0.05 between treatments for a given sampling date. Legend indicates annual application rates in m3 ha−1.

3.2. Soil parameters Concentration of nitrate-N in the soil following OMW application at the various rates is presented in Fig. 1. There were no significant differences in nitrate-N concentration between the treatments. Soil nitrateN concentrations increased after OMW application and decreased towards the subsequent one but the maximum values fluctuated strongly in all treatments (including the control and the tilled treatment) over the 5 years of the experiment and did not follow any clear trend. The effect of the OMW application levels on soil soluble K concentrations is presented in Fig. 2. In the upper soil layer (0–30 cm, Fig. 2a), there was a sharp increase in soil K concentration after each OMW application, whereas soil soluble K concentration before the subsequent application was much lower. Soil tillage after OMW application did not result in a significant difference between this treatment and the non-tilled parallel treatment. The general trend of soil K concentrations indicates an increase with time resulting from five successive annual OMW applications. The data in Fig. 2b indicate that soluble K concentration in the 30–60 cm layer was nearly one order of magnitude lower than that in the 0–30 cm layer (Fig. 2a). Furthermore, following the first two applications of OMW, K concentration was similar for all treatments, suggesting that it did not migrate into the soil layers below 30 cm. Increases in soil K concentration relative to the control in the 30–60 cm layer appeared for the first time after the third OMW application, and generally seemed to coincide with the OMW amounts applied (Fig. 2b). Furthermore, in this soil layer, no significant differences were found between the 100 m3 h−1 treatment and the parallel, tilled one. The differences in soil K concentrations between treatments in the 30–60 cm soil layer were not significant, even after the fifth application. Yet, the observed trend of

3. Results 3.1. OMW characteristics Chemical properties of the OMW used over the 5 years of the experiment are presented in Table 1. General wastewater properties such as chemical oxygen demand (COD), biological oxygen demand (BOD), total suspended solids (TSS), and oil residues (determined by the OG (oil & grease) method, using hexane (SM 5520A; APHA, 1999)(were within the previously reported range of values (Aviani et al., 2012; Regni et al., 2017). With respect to plant nutrients, average values were used to assess the potential annual contribution of OMW compared to the commercially recommended levels for irrigated olives (Table 2). Total N concentrations averaged 1.1 g L−1 whereas mineral N concentrations were two orders of magnitude lower, averaging 13.2 and 39.3 mg L−1 nitrate-N and ammonium-N, respectively. Soluble K concentration was 5.3 g L−1. Concentrations of total and soluble P were 291 and 223 mg L−1, respectively. 220

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Fig. 4. Effects of olive mill wastewater (OMW) application rate on available P concentration in the 0–30 cm soil layer during five successive annual applications. For a given sampling date, different letters indicate significant differences between treatments (P ≤ 0.05). NS = non-significant differences between treatments.

application. These effects were found only in the upper (0–30 cm) soil layer (results for deeper soil layers are not shown). Soil available P after OMW application or before the subsequent one (Fig. 4) did not show the same magnitude of fluctuations as that noted for K concentration (Fig. 2a). This is probably due to P fixation and adsorption. The sharp drop in soil available P before the fourth application (also noted for K, Fig. 2a) probably resulted from the relatively large amount of rainfall after the third application, causing most of the OMW-borne P to migrate into lower soil layers before P fixation could take place. Also in this case, the results obtained for the 100 m3 ha−1 treatment and the parallel tilled one did not differ significantly.

Fig. 2. Effects of olive mill wastewater (OMW) application rate on potassium (K) concentration in the soil saturated paste extract (SP). (a) 0–30 cm soil layer. (b) 30–60 cm soil layer. For a given sampling date, points labeled with the same letters do not differ significantly at P ≤ 0.05. NS = non-significant differences between treatments for a given sampling date. Legend indicates annual application rates in m3 ha−1.

increase in K concentration in the OMW treatments relative to the control may support the prediction that significant differences between treatments would have eventually developed had the experiment been continued for a longer period. Analysis of the adsorbed K in the 0–10 cm layer for the different treatments in the years 2014–2016 revealed an increase in EPP with increasing amount of applied OMW (Fig. 3). At the low application rate (50 m3 ha−1 year−1), EPP increased from 15.2% in 2014 to 22.1% in 2016. At the higher application rates, EPP values already reached high values (24%–30%) in 2014 and did not increase further with time or with application rate. The effect of OMW application on available P is presented in Fig. 4. Soil available P increased with increasing OMW application rates, with significant differences between treatments appearing before the third

3.3. Plant parameters and olive oil quality The effects of OMW application on tree vegetative development, fruit yield, selected oil quality parameters and leaf N, P, and K concentrations at the beginning and end of the experiment are presented in Table 3. Tree vegetative development, as characterized by the change in trunk cross-sectional area between 2012 and 2016, was not affected by OMW application. Similarly, fruit yield and some oil-quality parameters were also not affected by 5 years of OMW application. Leaf N concentration decreased from 1.3% in 2013, the threshold value for N deficiency in olive (Freeman et al., 1994), to 1.01%, indicating the development of N deficiency with time despite OMW application. Leaf P concentration decreased significantly in the control between 2013 and 2016, reaching a value below the deficiency threshold for olives (Freeman et al., 1994). There was no significant decrease in leaf P concentrations in the treatments amended with OMW, and no significant effect of application rate on leaf P concentration (Table 3). Leaf K concentration in the control decreased significantly, from 1.2% in 2013 to 1.07% in 2016, whereas no significant differences were found between 2013 and 2016 in the OMW-amended treatments. However, in 2016, a significant difference in leaf K concentration was found between the control and the high OMW-application rate of 150 m3 ha−1 (Table 3). The data presented in Table 2 show the potential annual contribution of OMW at the different application levels to soil nutritional status. Its potential contribution to soil mineral N forms was more than two orders of magnitude lower than that recommended for commercial fertilization. Conversely, the contribution of K by OMW application already met the commercial fertilization recommendations at the lowest OMW application rate. Only the highest OMW application rate supplied an amount of P to the soil comparable to that supplied by commercial fertilization. All other OMW doses led to lower than

Fig. 3. Effect of olive mill wastewater (OMW) application rate on exchangeable potassium percentage (EPP) in the 0–10 cm soil layer, determined 1–2 months after OMW applications. Different letters indicate significant differences between treatments (P ≤ 0.05). 221

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Table 3 Effects of olive mill wastewater (OMW) application rate on tree vegetative development, yield, oil-quality parameters (free fatty acid [FFA] and polyphenol [PP]) and leaf N, P, K concentrations (expressed as % of dry matter [DM]) at the beginning (2013) and end (2016) of the experiment. Different lowercase letters indicate significant differences between treatments at P ≤ 0.05. Different uppercase letters indicate significant differences between 2013 and 2016 at P ≤ 0.05. OMW applied (m3 ha−1 year−1)

0 50 100 150 100+tilling

Cumulative vegetative development (% change in trunk cross-sectional area between 2016 and 2012)

52.7 57.5 55.7 55.2 58.9

a a a a a

Cumulative yield (kg tree−1)

55.6 37.0 43.0 51.7 41.3

a a a a a

Average FFA (%)

0.18 0.21 0.20 0.17 0.19

a a a a a

Average PP (mg kg−1 oil)

80.0 68.0 75.0 71.0 75.0

a a a a a

Leaf analysis (% in DM) 2013

Leaf analysis (% in DM) 2016

N

N

1.32 1.30 1.30 1.24 1.31

P aA aA aA aA aA

0.10 0.09 0.09 0.09 0.09

K aA aA aA aA aA

1.20 1.17 1.18 1.18 1.18

aA aA aA aA aA

1.08 1.11 1.08 1.17 1.10

P aB aB aB aB aB

0.08 0.09 0.09 0.10 0.09

K aB aA aA aA aA

1.02 1.19 1.12 1.24 1.17

aB abA abA bA aA

The amounts of soluble K applied with the OMW were high, and could potentially fully satisfy the recommended amount for irrigated olives, even at the lowest OMW-application rate of 50 m3 ha−1 (Table 2). Soil K concentrations increased consistently and significantly with time, reaching higher values with increasing application rates. Similar results have been reported by Piotrowska et al. (2006); Mekki et al. (2006), and Chartzoulakis et al. (2010). Application of OMW resulted in an immediate increase in soluble soil K concentration (Fig. 2a), which in turn led to a significant increase in the percentage of adsorbed K relative to the total CEC of the soil, i.e., the EPP (Fig. 3). Levy et al. (2017) calculated that adding 150 m3 ha−1 of OMW can theoretically lead to adsorbed K occupying ∼50% of the exchange capacity of the upper 0.1 m of a sandy loam soil. In our study, the EPP at the 0–10 cm layer was < 30%. These results suggest that the efficiency of K adsorption in our soil was ∼50%. It is postulated that this limited adsorption efficiency of K may explain the observed trend of leaching of soluble K to the 30–60 soil layer following OMW application (Fig. 2b). The phenomena of K adsorption and soluble K leaching to deeper soil layers could have led to the relatively low K concentrations in the soil prior to the subsequent OMW application. The fluctuations in K concentration occurred until the level of adsorbed K (i.e., the EPP) reached a certain equilibrium state with the soluble K concentration. Once this equilibrium was attained, soluble K concentrations no longer dropped to low values before the subsequent OMW application (Fig. 2a, before the fifth application). Simultaneous with this process, leaching of soluble K with the rain water between OMW applications into deeper soil layers started and K began to accumulate there only at a relatively late stage (Fig. 2b). OMW application had a significant effect on leaf K concentrations (Table 3). While in the control treatment, leaf K concentration decreased significantly from 1.2% in 2013 to 1.07% in 2016, similar values were maintained in the OMW treatments during this period. This is a strong indication that OMW application can be a good substitute for K fertilization in olive orchards. The fact that leaf N, P and K concentrations in the control treatment did not drop to lower values, although no fertilization was applied during the duration of the experiment, can be attributed to the low fruit yields obtained in all treatments (Table 3). Olive fruits are a strong sink for nutrients and contain large amounts of N and K and considerable amounts of P (Dag et al., 2009b; Chatzissavvidis et al., 2004). When fruit yields are low, relatively small amounts of these nutrients are removed from the orchard and their depletion is accordingly. The low fruit yields are attributed to the insufficient chilling for the cv. Leccino, the cultivar used in this experiment, resulting in low flowering intensity under the prevailing climatic conditions. Tree performance was not affected negatively by OMW application (Table 3). Similar results have been reported by Barbera et al. (2013) and by Chartzoulakis et al. (2010), also in an irrigated orchard. Increased fruit yield in response to OMW application was reported by Magdich et al. (2012), who attributed the yield increase to improvement in the nutritional status of the trees, initially grown in poor soils

recommended P levels (Table 2).

4. Discussion According to Israeli regulations, the maximum permissible amount of OMW that can be added to agricultural fields annually is 50 m3 ha−1 (Laor, 2016). This value is similar to the permissible levels of OMW application in other countries (Rinaldi et al., 2003; Sierra et al., 2007). Referring to this dose, OMW in the present work could potentially provide less than 2% of the recommended annual amount of N when only mineral N content is considered. When total N content is considered, assuming future N mineralization, OMW could potentially provide almost 30% of this amount (Table 2). However, organic N becomes immobilized due to an increase in microbial activity after OMW application (Bengtson and Bengtson, 2005; Saadi et al., 2007; Sierra et al., 2007; Steinmetz et al., 2015). This immobilization is reversible (Sierra et al., 2007), but the re-mineralized N will first be taken up by both plants and microorganisms and can easily leach down the soil profile. The data presented in Table 2 indicate that OMW application could potentially contribute a certain (but unknown) amount of available N to the soil. However, the leaf analysis results presented in Table 3 indicate that these amounts were insufficient and the trees, which were not fertilized in this experiment, became significantly Ndeficient, even at the highest application rate of 150 m3 ha−1 there were no significant differences in leaf N concentration between the treatments. Our results are in agreement with those of Piotrowska et al. (2006), who also reported no increase in soil total N or mineral N as a result of OMW application to soil at a dose of 80 m3 ha−1. For many years, P application to olive trees was considered of secondary importance, due to the symbiosis between olive roots and mycorrhiza (Dag et al., 2009a). However more recently, Erel et al. (2013, 2016) emphasized the importance of P in olive tree nutrition. Consequently, P fertilization has become common practice in intensive olive orchards. In the present study, soil available P increased consistently and significantly as a result of OMW application (Fig. 4), similar to results reported by Sierra et al. (2007) and Piotrowska et al. (2006). The amounts of soluble P applied with the OMW were relevant with respect to the recommended amounts; at an OMW application rate of 50 m3 ha−1, about 30% of the required P could already come from the OMW (Table 2). The leaf analysis data in Table 3 support the calculated values presented in Table 2, as leaf P concentration in the control decreased significantly between 2013 and 2016, whereas no significant decrease was observed in the OMW-amended treatments. Leaf P concentration did not increase with the increase in OMW application rate. This lack of response of leaf P concentration to the different OMW application rates might be a result of the immobility of P in the soil profile, especially in calcareous, high pH soils like the sandy loam soil in this work, and the prevailing semi-arid conditions. Note that under these climatic conditions, coupled with the use of drip irrigation, most of the soil volume is dry for approximately 6–8 months per year and the trees cannot absorb nutrients from this soil volume. 222

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under rain-fed conditions. Despite the increase in leaf P and K concentrations reported in the present study as a result of OMW application, no improvement in the trees' vegetative or reproductive performance was found. It seems that the low P and K values measured in the control leaves were still above the threshold values below which there is a reduction in vegetative growth and yield (Freeman et al., 1994). OMW application can increase the soil's nutritional potential, but this potential was only partially exploited by the trees under the specific conditions of the present experiment. The whole soil volume between tree rows is wet only during the winter—the rainy period but also the period of low tree activity. During the very dry, rainless summers, this soil volume dries out and water uptake by the trees is limited to the soil volume wetted by the drippers that are located under the trees. This means that most soil nutrients, either soluble in the soil solution or adsorbed to soil particles, are not taken up by the trees during the dry summer. The effect of soil water content (caused by differential irrigation levels) on macro-element availability in olive orchards has been described by Zipori et al. (2015). The situation would be different in regions with summer rainfall. Soil tillage after OMW application did not affect N, P or K dynamics in the soil or uptake by plants. While this treatment emerged as the most effective treatment in maintaining higher soil aggregate stability compared with the other OMW treatments (Levy et al., 2017), it had no effect on soil chemical attributes beyond those presented in this study for OMW application with no tillage. Based on the data presented in Table 2, and taking into account the permissible OMW application rate of 50 m3 ha−1 y−1 in Israel, the potential economic value of soil amendment with OMW can be estimated. As this work showed that the contribution of OMW to plant N nutrition is negligible, only P and K are considered. The cost of 1 kg of P2O5 and of K2O, originating from fertilizers, is $2.5 and $1.7, respectively (prices were provided by Israel Chemicals Ltd., ICL). Taking into account the potential contribution of OMW application at the permissible rate of 50 m3 ha−1, it was estimated that a saving of over $600 ha−1 y−1 on fertilizers can be achieved and can account for about one-third of the recommended application rate for P and the whole recommended amount for K. An additional benefit of controlled OMW application is its direct and indirect economic value. Apart from enabling the grower to save on fertilizers, there are also environmental considerations. P and K fertilizers originate from exhaustible resources and, as a consequence, worldwide fertilizer prices are constantly rising. P and K supplied via OMW applications represent a renewable resource that meshes with the sustainable agriculture concept, where an attempt is made to recycle materials within the orchard as much as possible.

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5. Conclusions OMW application to the soil surface in an olive orchard at 50–150 m3 ha−1 year-1 over 5 successive years had no negative effects on the trees' vegetative growth, fruit yield or oil quality. OMW met the K requirements for the orchard and at least partially satisfied the trees' P requirement, but its value as a source for N was poor, at least under the experimental growing conditions. Under certain other growing conditions, e.g., rain-fed orchards and higher rainfall, the contribution of OMW application to olive orchard N nutrition might be significant. Acknowledgements This research was funded by the Middle East Research Cooperation (MERC) program, grant No. M31-019. The financial support of MERC is gratefully acknowledged. References APHA (American Public Health Association), 1999. Standard Methods for the

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