1. Introduction
Modern agriculture focuses on the necessity to increase crop productivity, without increasing (or even by decreasing) mineral fertilization rates in soils [
1]. Natural or synthetic inorganic amendments have been used in agriculture as a tool to: (i) improve soil properties; (ii) enhance crop productivity; (iii) restore/remediate polluted, saline, and acid soils and decrease metal uptake; (iv) reduce the potential environmental and climate impacts arising from excessive nutrient availability (mainly N and P) in soil [
2,
3,
4,
5,
6,
7]. Zeolites, which are among the most common inorganic amendments, are suitable for decreasing nutrient leaching, thus having beneficial and ecological effects [
8,
9].
Τhe high NH
4+ adsorption capacity of zeolite is of particular importance in agricultural soils receiving high amounts of N fertilization, which may regulate ammonia release and, therefore, nitrification activity, N availability to crops, and N losses to the environment (either in the form of nitrates or in the aerial forms NH
3 or N
2O). Indeed, certain benefits from the combined use of N (organic or inorganic) and zeolite have been recorded, indicating higher N uptake, crop growth, and yields [
10,
11,
12]; enhanced N use efficiency [
13,
14]; and low N losses via nitrate leaching [
14,
15] or via denitrification/volatilization processes [
12,
16].
Apart from the effect of zeolite on soil N availability, zeolite has been found to influence other nutrients’ availability (mainly exchangeable cations, such as K), due to its inherent adsorptive properties and high nutrient (especially K) content [
17,
18], which significantly influences soil fertility and crop nutrition [
5,
19]. According to Gholamhoseini et al. [
20], the addition of zeolite to composted manure decreased P leaching, probably reflecting the zeolite’s high adsorption capacity. Besides its effects on nutrient availability, zeolite has also been used for other agricultural purposes; according to Eroglu et al. [
21], more than 40 naturally occurring zeolites have been used: recent findings have supported their role in stored pest management as inert dust applications, pesticide and fertilizer carriers, soil amendments, animal feed additives, mycotoxin binders, and food packaging materials. It was also found that natural zeolite, together with symbiotic microorganisms, showed a positive effect on the root growth of olive plants, as well as on olive cuttings’ rooting [
22]. In addition, zeolite, in a mixture with perlite or other materials, has been used as a soilless growing medium for gerbera under alkaline stress conditions [
23].
Vermiculite is a clay-type material found in nature; its magnesium form consists of two layers of silicon tetrahedra, in which silicon is partially replaced by aluminum, and a layer of OH− groups and magnesium ions, which form a strongly bound mica stack [
24]. Due to its nature, vermiculite, as zeolite, has been a subject of investigation in soil-based or composting systems focusing mainly on vermiculite’s effect on NH
4+ and/or nutrient availability (e.g., P, K, and Mg) [
6,
25], on rate of metal adsorption [
24], and on decrease in N emissions, in the form of ammonia [
26,
27] or N
2O [
26]. However, there is much less information available due to the limited number of studies; thus, the unanswered questions regarding the fate on nutrients and potential effects on soil processes still remain.
Despite the agronomic uses of zeolite and vermiculite, a few things are known with regard to their use and ability to support crop nutrition (instead of using chemical fertilizers or in combinational application with other inorganic/organic fertilizers, in terms of decreasing their excessive rates), sustain yields, and promote sustainability. Glisic et al., (2009) [
2], who compared (in a two-year study) the treatments (i) NPK + manure and (ii) zeolite + manure on soil properties, in strawberry and blackberry crops, found that similar yields for both species were obtained between the two treatments, i.e., zeolite produced similar agronomic effects as the inorganic fertilizer 15-15-15. However, in their study, no plant nutrition data (foliar nutrient concentrations, nutrient uptake) were included. Aainaa et al. [
28] studied the ability of clinoptilolite zeolite (CZ) to act as a supplementary fertilizer (together with mineral P fertilizers) for
Zea mays L. and found that CZ inclusion in the fertilization program was beneficial, in terms of reducing excessive use of chemical fertilizers due to the decrease in fertilizers’ usage by 25%. In a pot experiment, it was concluded that zeolite and biochar, combined with appropriate NP fertilizer rates, can improve plant growth and provide a good source of nutrients for ryegrass establishment [
29]. Other researchers, such as Kocaturk-Schumacher et al. [
30], investigated the ability of zeolite enriched with digestate nutrients clinoptilolite (CZ) to supply N when used as fertilizer for ryegrass, while Mehrab et al. [
14] found that the NH
4+-enriched zeolite increased N uptake by wheat plants, compared to the raw zeolite (control). Similarly, Paskovic et al., [
31] evaluated Zn
2+-containing zeolite as fertilizer and quoted that it provided favorable conditions for Zn uptake in calcareous soils.
Regarding vermiculite, much less information is available; however, current knowledge indicates a positive effect on crops. Indeed, in a field experiment, the combined application of vermiculite with rock phosphate or medical stone resulted in an increase in wheat (
Triticum aestivum L.) yield [
6]. Furthermore, vermiculite application in organic (compost) and/or inorganic N-amended soils enhanced growth parameters, NPK uptake, and wheat yield [
32]. Similarly, application of vermiculite under different organic amendments increased NPK uptake by straw and grain of barley plants [
33]. Despite the available knowledge so far, little is known regarding the zeolite and/or vermiculite effects (including the combinational application of these materials with manure) on the growth, yields, nutrition, and physiology of vegetable crops (and especially on
Solanum lycopersicon L.). According to our knowledge, only Baninasab [
8] studied the effects of natural zeolite application on the growth and nutrient status of radish (
Raphanus sativus L.) and Paskovic et al. [
9] on radicchio mineral composition.
The hypothesis of our study was based on the premise that zeolite and vermiculite application (applied independently or in co-application with goat manure) could be used as potential, alternative, soil amendments (instead of chemical fertilizers) to sufficiently satisfy tomato crop nutritional needs. Thus, the objectives of this study were to investigate the effects of zeolite and vermiculite application (independently or in combination with goat manure) on soil fertility, tomato (Solanum lycopersicon L., cv. ‘Mountain Fresh’) growth, nutrition, and photosystem II (PSII) activity. The cultivar “Mountain Fresh” was chosen for the study, since it produces high quality firm and smooth fruits, with a good uniform red color.
4. Discussion
Potassium, Ca, and Mg contents were significantly higher in zeolite than in vermiculite (
Table 1); the significantly higher K content in zeolite led to the significantly higher soil exchangeable K (more than 15 times higher, compared to vermiculite) and to an impressive boosting in soil K, compared to the control (
Table 2). The beneficial effects of zeolite application as soil amendments (especially on the increase in exchangeable K and afterwards on the increase in Ca) have also been referred to by other researchers [
47,
48]. Apart from the increase in exchangeable K, zeolite application significantly increased foliar K; indeed, the highest leaf K concentration was found in zeolite (5.93% dw), followed by the concentrations in zeo + ver and manure + zeo treatments (5.13 and 4.83% dw, respectively) (
Table 5). Our results for the significant increase in K uptake after zeolite addition are in agreement to those of Assimakopoulou et al. [
19] for pepper plants and to those of Doostikhah et al. [
5] for tomato plants. Optimum foliar K levels in tomato plants should be within the range from 3.0 to 6.0% dw [
49]; therefore, in all the treatments of our experiment, leaf K concentrations were within the range of sufficiency (
Table 4). These data for K are of high agronomic importance, since zeolite contains approximately 15,000 mg kg
−1 K (
Table 1); thus, it is a very good source of K for crop nutrition, which could substitute inorganic K fertilization inputs.
Apart from zeolite, goat manure was also a good source of K (2.54% dw), contributing to plant K nutrition, since together with zeolite, it had a strong synergistic action in boosting exchangeable K (it significantly increased soil K to 2433 mg kg
−1 in the manure + zeo treatment, compared to 1613 mg kg
−1 in the zeo and 1062 mg kg
−1 in the zeo + ver treatments, respectively) (
Table 2). This synergistic action of zeolite with goat manure in boosting K uptake was clearly apparent in tomato fruit K, since the highest K concentrations were found in the manure + zeo and zeo treatments (5.30 and 4.75% dw, respectively) (
Table 6). Potassium was found to affect the qualitative fruit characteristics [
50]; thus, it is quite reasonable to assume that with the suitable combinational fertilization with zeolite and goat manure, the optimum tomato fruit quality will be achieved; however, additional research (including also postharvest measurements for tomato fruits) should be included to verify this hypothesis. In the study of Glisic et al. [
2], it was found that the combinational supply of zeolite with manure provided similar fruit quality characteristics (soluble solids, total acids) with the co-application of the inorganic fertilizer 15-15-15 and manure. Similarly, the co-application of the inorganic fertilizer 15-15-15, calcium ammonium nitrate, and natural zeolite induced the highest levels of total phenolics, flavonoid content, and total antioxidant capacity (qualitative characteristics) in apricot fruits [
51]. Jami et al., [
52] concluded that two years after fertilization with farmyard manure, linoleic and oleic acid contents were increased and the quality of sunflower seed oil was improved. With regard to K uptake by
Solanum lycopersicon L. plants, significantly higher K content was found in the CRF, zeo, manure + zeo, and zeo + ver treatments, compared to the ver and manure + ver treatments (
Figure 1C). These findings clearly show the beneficial role of zeolite to obtain similar beneficial results with inorganic fertilization in enhancing K uptake by
Solanum lycopersicon L. plants.
After the application of zeo, and especially in co-application with manure, exchangeable Ca was significantly increased (2964 mg kg
−1 in the manure + zeo and 2550 mg kg
−1 in the zeo treatment), compared to the control soil (1711 mg kg
−1) (
Table 2). This increase in exchangeable Ca led to an improvement in foliar Ca, compared to the control (although the increase was nonsignificant in the manure + zeo treatment) (
Table 5). A similar tendency was also found for total Ca content, which was significantly higher in the CRF, zeo, and manure + zeo, compared to the ver treatments (ver, zeo + ver, manure + ver) (
Figure 1D). Foliar Ca concentrations varied from 2.66 to 3.86% dw (
Table 5). Bergmann (1986) [
49] quoted that leaf Ca sufficiency levels in tomato plants should vary from 3.0 to 4.0% dw. Based on this, foliar Ca in the CRF was below the optimum levels, while the zeo and ver treatments provided the best results. The highest fruit Ca concentrations were determined in the manure + ver (0.23% dw) and zeo (0.21% dw) treatments (
Table 5), which probably shows the beneficial effects of zeo, ver, and goat manure in improving tomato fruit Ca and its quality, since Ca affects cell wall structure and fruit quality [
50]. However, more research, including also postharvest measurements for tomato fruit qualitative characteristics, should be included in order to verify this hypothesis.
In contrast to the previous data for the role of zeo in boosting soil K and Ca, zeo did not have an impressive boosting effect in NO
3-N, NH
4-N, and P (
Table 2) because its nutritional effect, as a soil amendment, for N and P was low (zeo contained only 9.36 mg kg
−1 N and 1.87 mg kg
−1 P) (
Table 1). NO
3-N was impressively enhanced from 28 to 48 times only after manure application, compared to the control soil (
Table 2). This beneficial role of goat manure in increasing NO
3-N should be ascribed to the high N content of goat manure (2.80% dw). Foliar N among the treatments varied from 3.42 to 4.83% dw (
Table 5). According to Bergmann (1986) [
49], the optimum N in tomato leaves should vary from 4.0 to 5.5% dw. Based on this reference, it should be pointed out that only the leaf N concentrations in the CRF and in the zeo + ver treatments (4.83 and 4.24% dw, respectively) were within the optimum range of sufficiency, while in all the other treatments, they were either marginal or insufficient.
At the end of the experiment, the highest total N content per plant was found in the CRF treatment (approximately 750 mg), while the lowest total quantities were determined in the manure treatments (approximately 100–200 mg) (
Figure 1A). The fact that the lowest plant N contents were found in the manure treatments (manure + ver, manure + zeo), despite the high NO
3-N levels existing in the amended soil (approximately 97 and 57 mg kg
−1, respectively) (
Table 2), could be possibly ascribed to the lower foliar, stem, and root N concentrations, as well as to the lower biomass of these tissues (
Table 4). Another possible explanation of the low N content in the manure treatments might include the co-effect of intensified N microbial immobilization and/or heterotrophic (nitrification-coupled) denitrification processes, induced by the presence of organic C and high nitrate availability in the manure treatments [
53,
54]. However, this hypothesis needs further investigation in the near future. In line with the previous hypothesis, the controlled release fertilizer or the single/mixed vermiculite and zeolite treatments used in the present study might have prevented the creation of excessive nitrates, enhancing plant growth, while also minimizing the N loss via leaching or denitrification.
With regard to P, the boosting effect of goat manure in soil P, from 1.44 to approximately 24–27 mg kg
−1 after its application (
Table 2), might be ascribed to the high P content of manure (0.13% dw). Significantly higher P content was absorbed by the plants in the CRF treatment (approximately 230 mg), followed by the contents taken up by the plants in the manure treatments (approximately 180–190 mg) (
Figure 1B). Thus, it seems that either CRF or manure application provided the most promising results with regard to P fertilization of the
Solanum lycopersicon L. plants. However, in all the treatments, foliar P concentrations were high (
Table 5), more than those quoted by Bergmann (1986) [
49] to be within the optimum range (0.40–0.65% dw).
Iron content of vermiculite (14.44 mg kg
−1) was about 28 times higher to that of zeolite (only 0.57 mg kg
−1) (
Table 1). However, both were very low compared to the Fe concentration of goat manure (1927 mg kg
−1)
, which seems to be a good source of Fe for plant nutrition. Similarly for Mn and Zn, goat manure was also a good source of these micronutrients, since their concentrations were high (367 and 81 mg kg
−1)
, compared to the Mn and Zn concentrations of zeolite and vermiculite, which were low (1.12–1.92 and 0.24–0.33 mg kg
−1, respectively) (
Table 1). However, after manure application only, soil Zn concentrations were increased (approximately 5–6.5 times, compared to the control soil), while those of Fe and Mn were decreased (
Table 3). This might be probably ascribed to the formation of stable Fe and Mn complexes with organic matter, which decreased Fe and Mn availability. In the study of Hamidpour et al., (2017) [
4], who managed to decrease Cd and Zn availability with vermicompost and zeolite application in a contaminated soil, it was found that the reduced soil availability was accompanied by a transformation/redistribution of plant-available forms to organic matter and metal oxide-associated fractions [
4]. Similarly for zeo and ver, their negative effect on soil Fe and Mn availability was clear after their application; especially for zeo, the most clear inhibitory effect was recorded on soil Mn, where its availability was decreased by approximately 50% after zeo application, compared to the control soil (
Table 3).
The highest Fe, Mn, and Zn accumulation was recorded in the CRF treatment, compared to the other ones (
Figure 2A–C); in the case of Mn, quite remarkable is the insignificant difference in total quantity recorded between the plants that received inorganic fertilization (CRF) and those that received only zeo (
Figure 2B), while in the case of Cu, the zeo-treated plants absorbed slightly higher Cu quantity, compared to those fertilized with CRF (
Figure 2D). This probably means that zeo application could be an alternative fertilization strategy for micronutrients (especially for Mn and Cu and afterwards, for Zn), being able to partially decrease the high chemical fertilization inputs in
Solanum lycopersicon L. crops. Indeed, previous studies have highlighted the potential role of zeolite to supply crops with micronutrients, such as Mn, Cu, and Zn [
55,
56]. However, in our study, it seems that the beneficial role of zeolite in boosting plant Mn and Cu uptake by tomato plants was probably owed to the enhanced retention capacity of zeolite [
57], which probably prevented micronutrients from leaching, rather than to supply tomato plants with Mn and Cu, since both zeolite and vermiculite were poor in Mn and Cu, compared to those of goat manure (
Table 1). For Fe and Zn, an alternative fertilization strategy, which could partially decrease inorganic fertilization inputs, might constitute a single application of zeo/ver, or a co-application of zeo and ver (
Table 5;
Figure 2A,C). In the study of Najafi-Ghiri and Rahimi [
58], it was found that application of zeolite and vermicompost significantly increased Zn uptake by spinach plants. According to Gholamhoseini et al. [
20] and Khodaei-Joghan et al. [
59], co-amending soil with manure and zeolite can be a beneficial approach for decreasing chemical fertilizer applications and improving the sustainability of agricultural systems. In addition, the co-application of a NPK fertilizer, cattle manure, and zeolite improved foliar micronutrient nutrition in apple trees [
60].
With regard to the influence of soil amendments on plant growth, significantly lower dry weight of total plant biomass was found in the vermiculite treatments (ver, ver + manure), and manure + zeo, compared to the CRF (
Table 4). Similarly to our results, Milosevic et al. [
61] found that the combined application of manure and zeolite, as well as zeolite supply alone, induced the lowest blackberry growth and shoot bearing characteristics. In the ver and manure + zeo treatments of our experiment, total plant growth was negatively influenced by vermiculite and manure application, probably due to the low N uptake by plants that occurred in these cases (
Figure 1A). Low N negatively influences plant growth and vegetation flush [
62]. According to Boussadia et al. [
63], the total biomass of two olive cultivars (“Meski” and “Koroneiki”) was strongly reduced (mainly caused by a decrease in leaf dw) under N deprivation, which confirms our results. In the study of Basri et al. [
47], it was found that the highest kenaf growth was observed in a co-application of an inorganic NPK fertilizer, biochar, and zeolite, compared to the single application of biochar or zeolite [
47]. The ratio leaf + stem/root was significantly higher in the zeo treatments (zeo, zeo + ver, and manure + zeo), compared to the manure + ver, where the lowest value was recorded (6.24) (
Table 4), which coincided with the lowest N content (
Figure 1A) and the lowest foliar N concentration (3.42% dw) (
Table 5). Reddy and Matcha [
64], who worked with
Ricinus communis L. plants, found that the ratio shoot/root decreased under N deficiency, something which is also confirmed by our results.
Similarly to total plant biomass and the ratio shoot/root, the minimum mean fruit weights were also found in the manure + ver and ver treatments (
Figure 3); these minimum values coincided with the minimum fruit N and Zn concentrations (
Table 6). Low Zn concentrations are usually responsible for small leaf and fruit formation, since Zn plays a crucial role in IAA (auxin) synthesis (a direct influence of Zn on plant growth and biomass production); IAA concentration is significantly decreased in vegetative tissues with low Zn content [
62]. The maximum mean fruit weight was recorded in the CRF, followed by the values in the zeo + ver and zeo treatments (
Figure 3). Assimakopoulou et al. [
19], who studied the growth, yield, and nutrient status of pepper plants grown on a soil substrate with olive mill waste (OMW) sludge (0, 2.5, and 5%) and natural zeolite application (0, 2.5, and 5%), found that the plants grown under 5% zeolite and 2.5% OMW showed higher fruit biomass, compared to the plants grown under 2.5% zeolite and 2.5% OMW; in contrast, when the plants were grown under 5% OMW, with whatever zeolite level, they produced the lowest yields [
19]. In contrast, Litaor et al., (2017) [
48] concluded that (i) compost application significantly increased potatoes’ yield and the number of large tubers and (ii) zeolite addition had no impact on yield. Similarly, it was found that cattle manure application promoted the highest apricot fruit weight [
51].
The influence of the kind of soil amendment on photosystem II activity was quite interesting. The optimum and significantly higher performance of PSII (as described by the parameters F
v/F
m, F
v/F
0, and Performance Index) was recorded in the ver treatment, compared to the other ones; in contrast, the lowest PSII activity was found in the zeo and manure + ver treatments, for all the three parameters determined (
Figure 4A–C). These differences in PSII activity among the treatments should be mainly ascribed to the differences in foliar Mn nutrition and to the optimum Mn level found in ver (138 mg kg
1) (
Table 5). Manganese plays a key role in photosystem II of photosynthesis and particularly in the reactions liberating O
2 [
62].