Sewage sludge as a substitute for mineral fertilization of spinach (Spinacia oleraceae L.) at two growing periods

Abstract

Abstract.

Keywords:

• farmyard manure
• mineral fertilizer
• organic fertilizers

Introduction

Adequate and balanced fertilization has a positive influence on plant yield. Currently, excessive amounts of inorganic fertilizers have been applied to vegetables which have a short growing season, such as spinach and lettuce, in order to achieve a higher yield. This may cause problems for human health and the environment and, as an alternative, it may be advisable to use organic fertilizers instead of inorganic fertilizers. The most common organic fertilizer used in Turkey and in some other countries is farmyard manure, but this is not available in all areas and in all seasons for economic use. Therefore, sewage sludge, an alternative source of organic fertilizer, is gaining importance in agriculture because of its nutrient value.

Sewage sludge consists of multi-element organic wastes used commonly as organic fertilizer (Otabbong et al., 1997). The use of sewage sludge as a fertilizer is not only important for the environment – recycling or disposing of the sludge – but also for retaining the nutrients it contains for agricultural usage. Moreover, sewage sludge increases the crop yield and improves the chemical and physical structure of soil. Bozkurt et al. (2000) stated that sewage sludge had significant contribution to the N, P, Fe, Mn, Zn, and Cu requirements of plants, and found it to be very suitable, especially on alkaline soils, which are often deficient in micro-nutrient elements.

There is, however, a risk and disadvantage of using sewage sludge if the heavy metal contents (Zn, Cu, Cr, Co, Ni, Pb, Se, and Cd) are too high, and if it contains pathogenic organisms (McBride, 1995; Arcak et al., 2000a,b). Therefore, sludge application is subject to various guidelines and regulations, and these are usually based on the concentration of heavy metals in the sludge (Alloway and Jackson, 1991). In the EU, although there is variation among member countries, sewage sludge use in agriculture is influenced by some initiatives and some constraints (Anonymous, 2001a). For example, the main advantages of agricultural recycling of sludge are economic, and it is usually recycled at the lowest possible cost and used as a supply of low-cost fertilizer. However, concerns over food safety and public health become important constraints. In Turkey, sewage sludge application for agricultural use, especially in horticulture, is also regulated by The Ministry of Environment (Anonymous, 2001b). According to their regulations, sewage sludge for agricultural application must be analysed and then used only if it is below various threshold values, such as; Pb 1200 mg kg SS⁻¹ DW, Cd 40 mg kg SS⁻¹ DW, Cr 1200 mg kg SS⁻¹ DW, Cu 1750 mg kg SS⁻¹ DW, Ni 400 mg kg SS⁻¹ DW, Zn 4000 mg kg SS⁻¹ DW, and Hg 25 mg kg SS⁻¹ DW.

Heavy metal uptake varies according to plant species, and to the physical and chemical properties of soils. For example, soil with a high lime content decreases the heavy metal availability (Bozkurt et al., 2000). In various studies, the application of sewage sludge up to 30% of the growing medium had positive effects on plant growth (Bozkurt et al., 2000; Şensoy et al., 2000; Türkmen et al., 2001). In another study, using various combinations of topsoil, perlite, vermiculite, and sewage sludge, for tomato seedling production, there was no Zn, Cd, Pb, and Ni toxicity, and healthy seedlings were obtained from these combinations (Taşatar & Haktanir, 2000).

The objectives of the present study were to: 1) assess the suitability of sewage sludge to supply some essential plant nutrients (macro and micro nutrients), 2) determine an optimum application rate of sewage sludge, 3) evaluate and compare the effects of mineral fertilizers and sludge on the yield and the chemical composition of spinach.

Materials and methods

Field experiments were conducted at the research farm of Yüzüncü Yil University (YYU), located in the eastern part of Turkey. The laboratory analyses were completed at both YYU and Atatürk University from 2000 to 2003. Soil analyses of experimental plots are presented in Tables 1 and 2.

Table 1. Some physical properties of experimental soil

Depth (cm)	Sand (%)	Clay (%)	Silt (%)	Texture	pH	CaCO3 (mg kg^−1)	Total salt (%)	Organic matter (%)
0–20	26.8	34.0	39.2	Clay-silt	8.4	14	0.015	0.41
20–40	30.8	30.0	39.2	Clay-silt	8.5	12	0.015	0.30

Table 2. Some chemical properties of experimental soil

Depth (cm)	N (%)	P (mg kg^−1)	K (mg kg^−1)	Fe (mg kg^−1)	Zn (mg kg^−1)	Mn (mg kg^−1)	Cu (mg kg^−1)
0–20	0.060	3.6	285	2.7	0.6	2.6	1.1
20–40	0.050	6.0	285	0.6	0.6	1.6	1.0

Spinach (Spinacia oleraceae L. cv. Spinoza) seeds were sown for spring production on 30 April 2000, and for winter production on 26 September 2000. The spinach plants were harvested on 27 June 2000 and 10 May 2001, respectively.

Treatments consisted of sewage sludge (SS), mineral fertilizer (MF), and their combinations, as well as nil fertilizer and farmyard manure treatments, in a randomized block design with three replications. Sewage sludge and farmyard manure quantities were adjusted to equivalent mineral fertilizer. Treatments are summarized as follows: 1) Nil fertilizer; 2) 100% SS (60 tonnes ha⁻¹); 3) 75% SS+25% MF; 4) 50% SS+50% MF; 5) 25% SS+75% MF; 6) 100% MF (200 N kg ha⁻¹, 100 P₂O₅ kg ha⁻¹, 50 K₂O kg ha⁻¹); 7) 100% farmyard manure (BM) (60 tonnes ha⁻¹). Some properties of farmyard manure and sewage sludge are given in Table 3.

Table 3. Some properties of farmyard manure and sewage sludge

Properties	Farmyard manure	Sewage sludge
Organic matter (%)	36.5	25.0
pH (1/1 water)	5.94	6.06
Total N (%)	0.45	1.30
Total P (%)	0.17	0.59
Available P (mg kg^−1)	270.0	561.0
Total K (%)	0.27	0.41
Total Ca (%)	0.62	1.72
Total Mg (%)	0.38	1.76
Total Fe (%)	0.098	1.86
Total Zn (%)	0.060	0.19
Total Mn (mg kg^−1)	220.0	402.0
Total Cu (mg kg^−1)	28.0	74.0
Total Co (mg kg^−1)	2.6	13.2
Total Ni (mg kg^−1)	Trace	12.0
Total Cr (mg kg^−1)	Trace	51.0
Total Cd (mg kg^−1)	Trace	0.73

For mineral content analysis, plant samples were oven-dried at 68^oC for 48 h, and then were ground. Potassium, Ca, and Mg were determined after wet digestion of dried and ground sub-samples in a H₂SO₄-Se-salicylic acid mixture. In the diluted digests, P was measured spectrophotometrically by the indophenol-blue method and after reaction with ascorbic acid (AOAC, 1990). Potassium and Ca⁺² were determined by flame photometry; Mg⁺², Fe, Mn, Zn, Cu, B, Mo and Cd by an atomic absorption spectrometry method (AOAC, 1990).

Statistical analysis was performed using the analysis of variance procedure; means were compared using least significant differences (LSD) and were numbered according to Duncan's multiple range tests using the SAS statistics programme.

Results and discussion

Seasonal variation of spinach yield (kg ha⁻¹) and dry matter (%)

Winter production gave higher spinach yields than did spring production (Table 4). In winter production, the treatment with 100% mineral fertilizer produced the highest spinach yield. In spring production, the combination of 50% SS and 50% MF, and the combination of 75% SS and 25% MF, gave higher yields than those of other treatments. There was no significant difference between spring and winter production for dry matter percentages (Table 4). Farmyard manure application had the highest dry matter in both seasons.

Table 4. Effects of sewage sludge in compensation for mineral fertilizer on the yield, dry matter and nutrient content in spinach

			Treatements^1
Traits	LSD^2	Season	1	2	3	4	5	6	7	Mean	LSD^4
Yield (kg ha^−1)	6800	Spring	1710 c^3	7740 ab	10600 a	10670 a	10420 a	9330 ab	5180 ab	7950 B^5	3430
	12730	Winter	12290 c	28020 b	28770 b	34350 ab	37610 a	40770 a	15130 c	28480 A
Dry matter (g 100 g plant ^−1)	3.02	Spring	10.30 ab	11.23 b	9.00 b	8.10 b	8.07 b	9.10 b	12.80 a	9.80	NS
	3.08	Winter	10.32 b	9.86 a–c	9.21 a–c	6.83 bc	7.59 bc	6.61 c	11.91 a	9.91
N, (g 100 g plant ^–1 Dw)	0.13	Spring	3.86 e	4.58 a	3.97 d	4.26 c	4.43 b	4.61 a	3.96 d	4.24 A	0.05
	0.15	Winter	3.55 e	4.31 a	3.86 c	3.90 c	4.15 b	4.29 a	3.75 d	3.97 B
P, (g 100 g plant ^−1 DW)	0.04	Spring	0.20 e	0.41 c	0.28 d	0.40 c	0.46 b	0.49 a	0.22 e	0.35 B	0.02
	0.05	Winter	0.26 d	0.48 a	0.33 c	0.44 b	0.50 a	0.52 a	0.27 d	0.40 A
K, (g 100 g plant ^−1 DW)	0.19	Spring	3.39 d	4.72 a	4.42 b	4.82 a	4.83 a	4.81 a	4.04 c	4.43 A	0.07
	0.19	Winter	3.21 f	4.43 bc	4.05 d	4.34 c	4.53 b	4.68 a	3.76 e	4.14 B
Ca, (g 100 g plant ^−1 DW)	0.11	Spring	0.52 b	0.63 a	0.65 a	0.68 a	0.64 a	0.65 a	0.64 a	0.63 A	0.05
	0.13	Winter	0.42 b	0.64 a	0.59 a	0.61 a	0.64 a	0.58 a	0.61 a	0.58 B
Mg, (g 100 g plant ^−1 DW)	0.07	Spring	0.41 c	0.42 c	0.56 a	0.53 a	0.52 ab	0.48 b	0.41 c	0.47 A	0.03
	0.04	Winter	0.33 e	0.40 c	0.44 b	0.47 a	0.45 ab	0.41 c	0.36 d	0.41 B
S, (g 100 g plant ^−1 DW)	0.04	Spring	0.12 d	0.34 a	0.20 c	0.26 b	0.33 a	0.34 a	0.13 d	0.24 A	0.01
	0.02	Winter	0.14 c	0.24 a	0.19 b	0.20 b	0.23 a	0.23 a	0.16 c	0.20 B
Fe, (mg kg plant^−1 DW)	8.61	Spring	54 g	194 a	87 e	122 d	152 c	177 b	72 f	123 B	3.89
	13.1	Winter	63 g	221 a	108 e	139 d	164 c	209 b	85 f	141 A
Mn, (mg kg plant^−1 DW)	9.89	Spring	23 e	76 ab	50 c	70 b	81 a	81 a	35 d	59 A	2.79
	5.34	Winter	28 e	61 a	46 c	56 b	60 a	61 a	38 d	50 B
Zn, (mg kg plant^−1 DW)	4.73	Spring	27 d	40 b	36 c	43 b	42 b	47 a	34 c	38 A	1.60
	4.33	Winter	21 d	35 a	25 c	29 b	33 a	34 a	21 d	28 B
Cu, (mg kg plant^−1 DW)	2.73	Spring	6.53 e	14.00 a	7.10 de	9.13 bd	9.67 bc	11.00 b	7.57 c–e	9.29 A	0.73
	2.92	Winter	7.77 bc	9.90 ab	7.67 bc	7.00 c	8.23 bc	11.20 a	6.43 c	8.41 B
B, (mg kg plant^−1 DW)	4.22	Spring	12 d	30 a	19 c	22 bc	24 bc	25 b	13 d	19 A	1.26
	2.83	Winter	11 d	17 ab	14 c	17 ab	18 a	16 bc	11 d	15 B
Mo, (mg kg plant^−1 DW)	0.10	Spring	1.01 f	1.25 b	1.1 de	1.18 c	1.16 cd	1.33 a	1.05 ef	1.17 A	0.04
	0.07	Winter	0.65 d	0.86 a	0.75 c	0.79 bc	0.84 ab	0.85 a	0.63 d	0.77 B
Cd, (mg kg plant^−1 DW)	0.04	Spring	0.12 e	0.30 a	0.20 d	0.22 cd	0.24 bc	0.25 b	0.13 e	0.21 A	0.01
	0.03	Winter	0.10 g	0.29 a	0.14 e	0.17 d	0.19 c	0.24 b	0.12 f	0.18 B
^1:1=Nil, 2=SS, 3=75% SS+25% MF, 4=50% SS+50% MF, 5=25% SS+75% MF, 6=MF, 7=BM. LSD values (P<0.01) of spring and winter production: ^2,3 in groups LSDs and values followed by the in the lines are significantly different:^4,5 among groups LSDs and values followed by the different capital letter in the lines are significantly different. Italicized values are P<0.05, NS: not significant.

Nutrient and heavy metal content of spinach

Growing season and fertilizer application had significant effects on plant N content. The N content (4.24 g 100 g plant⁻¹) of spinach grown in spring production was higher than that of winter production (3.97 g 100 g plant⁻¹ DW) (Table 4). Sewage sludge application (100%) yielded the highest nitrogen contents in both production seasons.

Growing season and fertilizer treatments significantly affected the P content of spinach leaves (Table 4). The P content (0.40 g 100 g plant⁻¹ DW) was higher in winter production than in spring production (0.35%). The highest P contents in both production seasons were obtained from 100% MF application.

The K content of spinach leaves was also significantly affected by growing season and fertilizer treatments (Table 4). The K content (4.43 g 100 g plant⁻¹ DW) of winter production was higher than that of spring production (4.14 g 100 g plant⁻¹). While sewage sludge, mineral fertilizer and their combination generally gave higher K contents in spring production, mineral fertilizer alone gave the highest K content in winter production.

Fertilizer treatments and growing season significantly affected the Ca content of spinach leaves (Table 4). The amounts of Ca in winter treatments were higher than those of spring production. While the nil treatment had the lowest Ca content, there was no significant difference among the other treatments in both growing seasons.

Similarly, both fertilizer treatments and growing season significantly affected the Mg content of spinach leaves (Table 4). The amounts of Mg in the winter production season were also higher than those grown in spring production. While the combination of 75% SS and 25% MF gave the highest Mg content in spring production, the combination of 50% SS and 50% MF gave the highest Mg content in winter production.

Both fertilizer treatments and growing season significantly affected the S content of spinach leaves (Table 4). Spring production with 0.24 g 100 g plant⁻¹ DW had a greater S content value than did winter production. In both production seasons, 100% SS, MF and the combination of 25% SS and 75% MF treatments had higher S contents than the other treatments.

The Fe content of spinach leaves was also significantly affected by growing season and fertilizer treatments (Table 4). The Fe content (141 mg kg⁻¹ DW) for winter production was higher than that of spring production (123 mg kg⁻¹ DW). The highest Fe contents in both production seasons were obtained from 100% SS application.

Both fertilizer treatments and growing season significantly affected Mn contents of spinach leaves (Table 4). Spring production with 59 mg kg⁻¹ DW of Mn content had greater Mn than did winter production. While the mineral fertilizer alone, and the combination of 75% SS and 25% MF, gave the highest Mn content (81 mg kg⁻¹ DW) in spring production, similar treatments and sewage sludge alone gave the highest Mn content in winter production (60 to 61 mg kg⁻¹ DW).

Similarly, Zn content of spinach leaves was significantly affected by growing season and fertilizer treatments (Table 4). The Zn content (38 mg kg⁻¹ DW) of spring production was higher than that of winter production (28 mg kg⁻¹ DW). The mineral fertilizer treatment alone gave the highest Zn content (47 mg kg⁻¹ DW) in spring production. Mineral fertilizer alone, sewage sludge alone, and the combination of 25% SS and 75% MF gave the highest Zn content (33 to 35 mg kg⁻¹ DW) in the winter production.

The Cu content of spinach leaves was significantly affected by fertilizer treatments and growing season (Table 4). Spring production with 9.29 mg kg⁻¹ DW of Cu content had a greater value than winter production. While sewage sludge alone gave the highest Cu content (14.0 mg kg⁻¹ DW) in spring production, mineral fertilizers alone gave the highest Cu content (11.2 mg kg⁻¹ DW) in winter production.

The B content of spinach leaves was significantly affected by the growing season and the fertilizer treatments (Table 4). The B content (19 mg kg⁻¹ DW) of spring production was higher than that of winter production (15 mg kg⁻¹ DW). While the highest B content in spring production was obtained from 100% sewage sludge application, the highest B content in winter production was realised from a combination of 25% SS and 75% MF.

Both the fertilizer treatments and the growing season significantly affected Mo contents of spinach leaves (Table 4). Spring production with 1.17 mg kg⁻¹ DW of Mo content had a greater value than winter production. Mineral fertilizer alone gave the highest Mo content (1.33 mg kg⁻¹ Dw) in spring production. Mineral fertilizer alone, and sewage sludge alone had the highest Cu content (0.85 and 0.86 mg kg⁻¹ DW in winter production, respectively).

The Cd content of spinach leaves was also significantly affected by the growing season and fertilizer treatments (Table 4). The Cd content (0.21 mg kg⁻¹ DW) of winter production was higher than that of spring production (0.18 mg kg⁻¹ DW). The highest Cd contents (0.29 to 0.30 mg kg⁻¹ DW) in spring and winter production seasons were obtained from the 100% SS application.

Spinach yields of all treatments in winter production were higher than those grown in spring production. This is because spinach is a long-day plant for vegetative growth, and in spring production there are not enough short day conditions to fulfil its vegetative growth requirements. This is in line with the study previously carried out in the region (Görgün, 2001).

Macro and micro nutrient elements, and heavy metal contents, were generally higher in spring production than in winter production. In the spring production season, plants are grown for a relatively shorter period than those in the winter production season. Therefore, in spring production, plants have absorbed the required elements, but this has not been reflected in yield.

The reasons for higher N, P, and K contents of spinach leaves grown with mineral fertilizers alone were the higher available forms of this nutrient in mineral fertilizers. For macro elements, mineral fertilizer alone was followed by sewage sludge alone, and combinations of sewage sludge and mineral fertilizer. The fact that sewage sludge alone performed better than did the combinations was due to the essential micro-nutrient content of sewage sludge, which increased the N, P, and K contents.

This demonstrates the possible applicability of sewage sludge in spinach production. Sewage sludge studies performed with other plant species supported the use of sewage sludge in spinach (Reed et al., 1991; Gilmour & Skinner, 1999; Bozkurt et al., 2000; Şensoy et al., 2000; Türkmen et al., 2001; Cogger et al., 2001).

However, according to Jones et al. (1991), there were inadequate amounts of K, Ca and Mg in spinach leaf samples of all treatments. The uptake of these elements might be affected by antagonistic effects among minerals or high cation exchange capacity in soil with high clay content (Aktaş, 1994). Iron, Mn and Zn contents of all treatments except the nil treatment were adequate; Cu was adequate in all treatments in both seasons, but B in winter production was inadequate (Jones et al., 1991). Molybdenum and Co contents of spinach were at acceptable levels (Schachtschabel et al., 1989).

As a result, sewage sludge could be a very important alternative to mineral fertilizers in spinach production, especially on alkaline soils where its high macro and micro nutrient content, and low heavy metal content, should be of benefit. However, because it gave lower yield, a combination with mineral fertilizers should be considered. The combination of 25% SS and 75% MF, or the combination of 50% SS and 50% MF, could be especially recommended for spinach production.

Additional information

Notes on contributors

Önder Türkmen *

Türkmen, Ö., Sensoy, S., Dursun, A. and Turan, M. (Department of Hoticulture, Yüzüncü Yil University, TR-65080-Van, Turkey and Departments of Horticulture, and Soil Science, Agriculture Faculty, Atatürk University, TR-25240-Erzurum, Turkey). Sewage sludge as a substitute for mineral fertilization of spinach (Spinacia oleraceae L.) at two growing periods.

Notes

Türkmen, Ö., Sensoy, S., Dursun, A. and Turan, M. (Department of Hoticulture, Yüzüncü Yil University, TR-65080-Van, Turkey and Departments of Horticulture, and Soil Science, Agriculture Faculty, Atatürk University, TR-25240-Erzurum, Turkey). Sewage sludge as a substitute for mineral fertilization of spinach (Spinacia oleraceae L.) at two growing periods.