Usage of sewage effluent in irrigation of some woody tree seedlings. Part 3: Swietenia mahagoni (L.) Jacq.

Abstract

A pot experiment was investigated to study the effect of sewage irrigation treatments (primary and secondary effluents) compared with tap water on the growth and chemical constituents of mahogany seedlings (Swietenia mahagoni (L.) Jacq.) as well as soil chemical properties. The experiment was conducted at a greenhouse in the nursery of Timber Trees Research Department of Sabahia, Horticultural Research Station in Alexandria, Egypt, from June 2003 to December 2004 for three irrigation periods (6, 12 and 18 months). The sewage effluent waters were taken from oxidation ponds located in New Borg EL-Arab city and used directly for irrigation.

The primary effluent treatment was superior than other treatments in improving the growth parameters (plant height, stem diameter, leaf area, leaves number, fresh and dry weights of leaves, shoots and roots and shoot/root ratio) and showed the highest concentration and total uptake of N, P, K, Cd, Ni, Pb and Fe in plant parts, followed by secondary effluent then tap water. The data revealed that soil salinity in terms of electrical conductivity of saturated paste (EC), CaCO₃%, organic matter% and soluble anions and cations were influenced significantly by primary or secondary effluent treatment. The data also showed that the use of sewage effluent for irrigation increased N, P, K and DTPA-extractable-heavy metals (Cd, Cu, Ni, Pb, Fe, Mn and Zn). The effects of sewage effluent on growth parameters and elements content in plant parts and treated soil were more pronounced as water treatments were used for long period.

The results suggested that the use of sewage effluent in irrigating mahogany trees grown on calcareous sandy loam soil was an important agriculture practice for improving soil properties, increasing fuel and timber production, and is an economic and safe way to dispose wastewaters.

1. Introduction

In arid and semi-arid countries, water is becoming scarce resource to consider any sources of water, which might be used economically and effectively to promote further development. Rapid increases in population and industrial growth have led to use low quality water such as drainage and saline water as well as wastewater for irrigation.

Irrigation of forest species with wastewater for fuel and timber production is an approach which helps to overcome health hazards associated with sewage farming. Establishment of the green belts around the cities by forest trees under wastewater irrigation also helps revive the ecological balance and improves environmental conditions by self-treatment of wastewater through the application of forest irrigation.

The use of primary and secondary effluent in irrigation can improve the quality of the soil and plant growth because they are considered as natural conditioners through their nutrient elements and organic matter. However, the direct application of wastewater on agricultural land is limited by the extent of contamination with heavy metals, toxic organic chemicals and pathogens (EL-Nennah et al., 1982; Abulroos et al., 1996; Salem et al., 2000; Sebastiani et al., 2004; EL-Sayed, 2005; Singh and Bhati, 2005; Ali et al., 2010).

Swietenia mahagoni (L.) Jacq. (mahogany) is a hardwood tree belongs to family Meliaceae. Mahogany is a large tropical tree, with a height of more than 40–60 ft, a diameter at breast height (dbh) ranged from 30 to 105 cm and wood density is 560–720 kg/m³.

Mahogany is one of the true Mahoganies that has a heavy trunk, and is considered as one of the most valuable timber trees. Mahogany is extremely strong, hard, stable and decay resistant. The color of mahogany ranges from pale pink to dark reddish brown. It is rated among the top 12 timber woods in the world. It is readily available in large widths, thick thicknesses and lengths of timber. Its rich warm color and mellow texture finishes, stains and polishes to a beautiful natural luster. This wood is used for making fine jewelleries, decorative veneers and interiors and pattern-making mahogany is used in shipbuilding and for fine boat interiors.

At times, new model automobiles are originally carved, full size, entirely out of mahogany. Once the various parts – front bumper, dashboard, the drive shaft, back to the lock on the trunk are first fashioned with this beautiful stable wood. Once the thousands of parts have been refined to fit perfectly together, they are then used as patterns to make the moulds (Mathew, 1994).

Available data about the use of sewage for irrigating forest trees in Egypt soils are limited, therefore, this work is aimed to study the effects of irrigation with sewage effluent on the vegetative growth and chemical composition of S. mahagoni (L.) Jacq. (mahogany), as well as the soil chemical properties.

2. Materials and methods

This study was carried out at a greenhouse at the nursery of Timber Trees Research Department of Sabahia, Horticultural Research Station in Alexandria, Egypt. The study lasted for 18 months from June 2003 to December 2004, to investigate the effects of irrigation with different treatments of sewage effluent on the vegetative growth and chemical composition of S. mahagoni (L.) Jacq. and soil chemical properties for three periods (6, 12 and 18 months).

Two types of sewage effluent were used for irrigation, primary and secondary treated wastewater that were taken from oxidation ponds of sewage effluent treatment station in New Borg El-Arab City, Alexandria. The sewage effluent contains a mixture of domestic and industrial sources. Tap water was used as a control treatment. The analysis of the used water in irrigation is shown in Table 1. Trace elements in samples were analyzed using Atomic Absorption Spectrophotometer. Soluble N was determined by kjeldahle method (Page et al., 1982). Soluble P was determined by the ascorbic acid molybdenum blue method (Watanabe and Olsen, 1965). Dissolved oxygen (DO) was determined by the azide modification of winkler method and chemical oxygen demand (COD) by dichromate oxidation method. Five days biochemical oxygen demand (BOD₅) was determined by the amount of oxygen lost after incubation for 5 days in the dark at 20 °C (APHA, 1995).

Table 1.

Average composition of water used in irrigation treatments in the experiment.

	Sewage effluent
Parameter	Primary treatment	Secondary treatment	Tap water	Limits of wastewater for agric. reuse FAO (1992)
pH	6.82	7.56	6.80	6.50–8.40
E.C. ds/m	1.60	2.96	0.68	3.00–7.00
Soluble cations (meq/l)
Ca2+	2.83	3.34	1.10	–
Mg2+	2.21	3.31	1.90	–
K+	0.23	0.26	0.20	–
Na+	11.95	16.75	2.60	–
Soluble anions (meq/l)
${\text{CO}}_3^{-}$	–	–	–	–
${\text{HCO}}_3^{-}$	4.63	5.00	2.00	1.50–8.50
Cl−	8.41	9.34	3.80	–
DO (mg/l)	0.00	2.90	–	–
BOD5 (mg/l)	220	100	–	40–500
COD (mg/l)	402	311	–	80–600
TSS (mg/l)	1024	1894	–	–
Soluble N (ppm)	1.25	1.08	0.26	–
Soluble P (ppm)	0.38	0.33	0.01	–
Total heavy metals (ppm)
Cd	0.02	0.01	0.007	0.01
Cu	0.14	0.19	0.009	0.20
Mn	0.06	0.05	0.014	0.20
Ni	0.02	0.01	0.002	0.20
Pb	0.25	0.24	0.02	5.00
Zn	1.86	1.07	0.09	2.00
Fe	12.5	8.60	0.26	5.00

Dissolved oxygen (DO), biochemical oxygen demand (BOD₅) and chemical oxygen demand (COD).

One-year-old seedlings of S. mahagoni were used; seedlings averaged 28 cm in height and 3.6 mm in diameter (at 5 cm from the soil surface). The seedlings were planted on 1st March 2003 in earthenware pots (50 cm in height and 50 cm in diameter), filled with 25 kg of sandy loam soil (one seedling/pot). The physical and chemical properties of the used soil are shown in Table 2. All seedlings were irrigated with tap water for three months until adaptation on 1st June 2003 after that started the treatments with sewage effluent. The tree seedlings were irrigated to the field capacity to standardize the irrigation rate for the three treatments.

Table 2.

Physical and chemical analysis of the used soil.

Parameter	Mean
Practical size distribution
Sand%	70.00
Silt%	20.00
Clay%	10.00
Soil texture	Sandy loam
pH	8.31
E.C. ds/m	2.42
CaCOs%	32.04
Organic matter%	0.62
Soluble cations (meq/l)
Ca2+	5.58
Mg2+	6.15
Na+	14.25
K+	0.74
Soluble anions (meq/l)
${\text{CO}}_3^{-}$	–
${\text{HCO}}_3^{-}$	8.30
Cl−	9.10
${\text{SO}}_4^{-}$	9.42
Available P (ppm)	4.60
Available N (ppm)	7.28
DTPA-extractable-heavy metals (ppm)
Cd	0.00
Cu	0.77
Mn	1.44
Ni	1.11
Pb	2.13
Zn	0.89
Fe	3.10

2.1. The experiments design

A complete randomized design was used for the experiment as described by Snedecor and Cochran (1968). The three treatments were replicated three times, and each repetition contained four seedlings. The means among all used all treatments were compared by Duncan’s Multiple Range Test, according to Snedecor and Cochran (1968).

At the end of each period (6, 12 and 18 months), three seedlings for each treatment were chosen randomly to determine the following parameters:

2.1.1. Vegetative growth

Plant height (cm), stem diameter (mm), leaves number/plant, leaf area (cm²)/leaf, root length (cm), fresh and dry weights for leaves, shoots and roots (g) and shoot/root ratio.

2.1.2. Chemical composition of the different plant parts (leaves, shoots and roots)

N and P were measured colorimetrically according to Evenhuis (1976) and Murphy and Riley (1962), respectively. K and Na were measured against standard using a flame photometer (Page et al., 1982). Cd, Ni, Pb and Fe (ppm) were determined by Perkin Elmer, 3300 Atomic Absorption Spectrophotometer. Also, the total uptake of these elements in plant was calculated.

2.1.3. Soil analysis

At the end of each period, soil samples were taken from each treatment to determine their chemical properties according to Page et al. (1982). Heavy metals (Cd, Fe, Ni, Pb, Cu, Mn and Zn) were extracted by DTPA and measured in the solution by Atomic Absorption Spectrophotometer (Lindsay and Norvell, 1978).

3. Results and discussion

3.1. Vegetative growth

Results of vegetative growth parameters (plant height, stem diameter, leaves number, leaf area, leaves fresh and dry weight, shoots fresh dry weights) as affected by sewage effluent treatments are presented in Table 3.

Table 3.

Effect of sewage effluent on vegetative growth parameters of Swietenia mahagoni during 18 months (2003–2004).

Treatments	Periods
	6 months	12 months	18 months	6 months	12 months	18 months
	Plant height (cm)			Stem diameter (mm)
Tap water	53.00c	87.75c	211.25c	6.13c	12.33c	19.40c
Primary effluent	89.75a	144.75a	338.75a	14.23a	20.82a	35.75a
Secondary effluent	69.00b	117.25b	302.50b	9.33b	16.93b	30.00b
	Leaves number/plant			Leaf area/leaf (cm2)
Tap water	11.50c	11.66c	44.25c	188.47c	202.64c	265.71c
Primary effluent	22.75a	39.00a	138.25a	257.19a	285.94a	337.65a
Secondary effluent	17.00b	26.00b	116.50b	211.65b	239.51b	296.29b
	Leaves fresh weight (g/plant)			Leaves dry weight (g/plant)
Tap water	5.61c	13.99c	86.83c	1.00c	6.03c	36.91c
Primary effluent	34.34a	116.53a	282.14a	7.53a	35.60a	108.46a
Secondary effluent	17.05b	48.03b	239.62b	4.29b	16.97b	94.98b
	Shoots fresh weight (g/plant)			Shoots dry weight (g/plant)
Tap water	9.06c	37.38c	162.62c	1.71c	12.11c	82.03c
Primary effluent	50.33a	192.75a	659.75a	17.27a	88.35a	343.00a
Secondary effluent	28.80b	98.34b	472.28b	7.18b	43.03b	238.44b

Means followed by a similar letter within a column are not significantly different at the 0.05 level probability by Duncan’s Multiple Range Test.

Application of sewage effluent treatments increased significantly all the studied vegetative growth parameters compared with control (tap water). Generally, vegetative growth parameters at primary effluent treatments were significantly higher than those found at secondary effluent treatments, while tap water treatments gave the lowest values.

The results are explained by many investigators, who found that sewage effluent had a stimulatory effect on vegetative growth of trees, provided the soil with plant nutrients and organic matter and improved the soil physical characteristics, that reflected on the growth by enhancing the cell elongation and division (Kaneker et al. (1993) on Acacia nilotica, Hassan (1996) on Acacia saligna and Leucaena leucocephala, Berbec et al. (1999) on poplar, Guo and Sims (2000) on Eucalyptus globulus, Abbaas (2002) on Casuarina glauca, Taxodium distichum and Populus nigra, Bhati and Singh (2003) on Eucalyptus camaldulensis, EL-Sayed (2005) on Ceratonia siliqua, A. saligna and Acacia stenophylla, Singh and Bhati (2005) on Dalbergia sissoo and Ali et al. (2010) on Tipuana speciosa).

The results indicated that the growth parameters at any given sewage effluent treatments increased gradually with irrigation period and reached a maximum after 18 months of irrigation period.

3.2. Root characters

Table 4 exhibited that roots fresh and dry weights of S. mahagoni were the highest at primary sewage effluent treatment followed by secondary effluent treatment, then tap water treatment with significant differences among them. This trend was observed at the three studied growth periods. If the seedlings were allowed to grow for a longer period (from 6 to 18 months), roots fresh and dry weights would be greater regardless of sewage effluent treatments. In contrast to the above results, tap water treatment gave the longest root, followed by primary sewage effluent treatment, then secondary effluent treatment. This could be explained by accumulation of soluble salts and heavy metals as a result of applying sewage effluent, which might adverse root elongation. These results are in harmony with those of Nessel et al. (1982) on pond cypress, Hopmans et al. (1990) on E. camaldulensis and Pinus radiata, Gogate et al. (1995) on Tectona grandis and Hassan et al. (2002) on Albizzia lebbek, T. distichum and T. speciosa.

Table 4.

Effect of sewage effluent on the root characters and shoot/root ratio of Swietenia mahagoni during 18 months (2003–2004).

Treatments	Periods
	6 months	12 months	18 months	6 months	12 months	18 months
	Roots fresh weight (g/plant)			Roots dry weight (g/plant)
Tap water	14.33c	18.97c	72.78c	3.69c	8.32c	39.73c
Primary effluent	50.31a	83.76a	243.82a	19.37a	28.31a	114.28a
Secondary effluent	32.19b	45.93b	152.93b	7.86b	16.37b	73.40b
	Root length (cm)			Shoot/root ratio
Tap water	85.75a	100.50a	120.50a	0.73b	2.18a	2.99a
Primary effluent	80.75a	91.75a	117.00a	1.28a	4.37a	3.95a
Secondary effluent	72.00a	85.50a	108.75a	1.45a,b	3.67a	4.54a

Means followed by a similar letter within a column are not significantly different at the 0.05 level probability by Duncan’s Multiple Range Test.

The data also indicated that the effect of sewage effluent on shoot/root ratio was slight and after 6, 12 and 18 months tap water treatment gave a lower value for shoot/root ratio compared with primary and secondary effluent treatments with significant difference between primary and secondary effluent treatments and tap water treatment after 6 months only.

Data of the root characters are in agreement with those of Sebastiani et al. (2004) on poplar and EL-Sayed (2005) on C. siliqua.

3.3. Chemical composition

Generally, irrigation with primary effluent gave the highest concentrations of N, P, K, Cd, Ni, Pb and Fe in leaves, shoots and roots of S. mahagoni followed by secondary effluent treatment, then tap water (Tables 5 and 6).

Table 5.

Effect of sewage effluent on leaves (L), shoots (S) and roots (R) N, P and K percentage and total uptake of Swietenia mahagoni during 18 months (2003–2004).

Treatments	Periods
	6 months			12 months			18 months			6 months			12 months			18 months
	L	S	R	L	S	R	L	S	R	L	S	R	L	S	R	L	S	R
	N (%)									Total N uptake (g/plant)
Tap water	0.92c	0.52c	0.57c	0.94c	0.68c	0.66c	0.59b	0.59c	0.54b	0.04c			0.19c			0.92c
Primary effluent	2.09a	1.49a	1.51a	1.82a	1.49a	1.23a	1.92a	1.14a	1.15a	0.71a			2.31a			7.31a
Secondary effluent	1.77b	1.24b	1.24b	1.22b	0.88b	0.96b	1.57a	0.93b	0.95a	0.26b			0.74b			4.41b
	P (%)									Total P uptake (g/plant)
Tap water	0.34b	0.32c	0.26c	0.22b	0.17b	0.19b	0.21b	0.18b	0.22c	0.03c			0.05c			0.31c
Primary effluent	0.52a	0.54a	0.57a	0.46a	0.22a	0.30a	0.76a	0.52a	0.54a	0.24a			0.44a			3.23a
Secondary effluent	0.47a	0.41b	0.45b	0.37a	0.20a,b	0.19b	0.45b	0.36a	0.40b	0.08b			0.18b			1.58b
	K (%)									Total K uptake (g/plant)
Tap water	0.87b	0.75b	0.42c	0.79b	0.63b	0.43c	0.70b	0.48b	0.46c	0.04c			0.16c			0.83c
Primary effluent	1.39a	0.87a	0.70a	1.16a	0.80a	0.76a	1.23a	0.83a	0.73a	0.39a			1.33a			5.02a
Secondary effluent	0.97b	0.79b	0.56b	0.90	0.73a,b	0.58b	0.95b	0.57b	0.60b	0.14b			0.56b			2.70b

Means followed by a similar letter within a column are not significantly different at the 0.05 level probability by Duncan’s Multiple Range Test.

Table 6.

Effect of sewage effluent on leaves (L), shoots (S) and roots (R) Cd, Ni, Pb and Fe percentage and total uptake of Swietenia mahagoni during 18 months (2003–2004).

Treatments	Periods
	6 months			12 months			18 months			6 months			12 months			18 months
	L	S	R	L	S	R	L	S	R	L	S	R	L	S	R	L	S	R
	Cd (ppm)									Total Cd uptake (mg/plant)
Tap water	0.27b	0.30c	0.57c	0.05b	0.03b	0.06c	0.06b	0.21b	0.22b	0.01c			0.01b			0.03c
Primary effluent	2.83a	2.23a	3.37a	2.60a	1.93a	2.70a	1.96a	1.70a	1.90a	0.13a			0.34a			1.01a
Secondary effluent	0.90b	1.10b	1.87b	0.80b	1.00ab	1.06b	0.56b	1.33a	0.90b	0.03b			0.07b			0.44b
	Ni (ppm)									Total Ni uptake (mg/plant)
Tap water	12.33c	8.67b	15.33c	14.33c	11.33c	11.67c	10.33c	9.33b	11.66c	0.08c			0.32c			1.61c
Primary effluent	43.33a	37.33a	46.67a	134.33a	117.00a	136.00a	151.33a	126.33a	159.00a	1.87a			18.97a			77.91a
Secondary effluent	31.67b	28.33a	35.33b	98.33b	95.66b	92.67b	114.33b	102.33a	119.66b	0.62b			7.30b			44.04b
	Pb (ppm)									Total Pb uptake (mg/plant)
Tap water	38.00c	31.00c	39.00c	43.66c	36.33c	36.66c	42.00c	24.33c	31.67b	0.23c			1.01c			4.80c
Primary effluent	122.33a	114.33a	170.00a	131.33a	114.33a	147.00a	150.67a	135.66a	155.33a	6.19a			18.94a			80.62a
Secondary effluent	90.67b	95.33b	111.67b	105.66b	84.67b	81.33b	102.33b	77.33b	162.67a	1.95b			6.77b			40.10b
	Fe (ppm)									Total Fe uptake (mg/plant)
Tap water	195.67c	147.67c	246.67b	148.67c	95.33b	130.00c	106.67c	93.33c	111.67c	1.36b			3.13c			16.03c
Primary effluent	405.00a	398.67a	530.00a	356.67a	323.33a	370.00a	359.33a	303.33a	346.67a	20.26a			51.74a			182.63a
Secondary effluent	232.33b	242.33b	320.00b	223.33b	174.33b	240.00b	280.00b	213.33b	250.00b	5.25b			15.22b			95.81b

Means followed by a similar letter within a column are not significantly different at the 0.05 level probability by Duncan’s Multiple Range Test.

The concentrations of N, P and K in leaves were much higher than those of shoots and roots. The increase of N, P, K, Cd, Ni, Pb and Fe in plant parts might be attributed to an increase in the occupancy root zone by applying sewage effluents that reflected on their uptake by roots. The results agree with the findings of Singh and Bhati (2005) who found that concentrations of N, P and K were greater in foliage compared to the other plant parts. In contrary heavy metals (Cd, Ni, Pb and Fe) tended to accumulate in root more than that in leaves and shoots with few exceptions. When irrigation time increased from 6 to 18 months the uptake of N, P, K, Cd, Ni, Pb and Fe in the whole plant was increased due to progressive increase of vegetative growth. Also EL-Sayed (2005) found that irrigation with secondary effluent increased N, P, K, Ca, Mg, Na, Fe, Zn, Mn, Cu, Pb, Cd, Cr and Ni in leaves, stems and roots of tree species (C. siliqua, A. saligna and A. stenophylla) compared with tap water.

The magnitude of increase of the studied heavy metals in the whole plant due to primary effluent treatment compared with tap water after 18 months ranged from 2 to 9 times for all elements and can be arranged in the following order: Cd > Ni > Pb > Fe.

3.4. General soil characterization

Data in Table 7 revealed that the soil salinity in terms of electrical conductivity (EC) of saturated paste, CaCO₃%, organic matter% and soluble anions and cations was influenced by either primary or secondary effluent treatment where, pH value was slightly changed by irrigation with sewage effluent. EC values of soil treated with sewage effluent were corresponding to EC values of irrigation water treatment; therefore, EC of soil irrigated with tap water was decreased to 1.65 and 1.55 ds/m after 12 and 18 months, respectively, of irrigation periods. However EC of soil treated with secondary and primary effluent was ranged between 2.78 and 3.19, respectively. Meantime, the soluble cations and anions followed the same trend of EC of soil except for Ca²⁺ and ${\text{SO}}_4^{-}$.

Table 7.

Effect of sewage effluent on the used soil properties during 18 months (2003–2004) under Swietenia mahagoni plantation.

Parameter	6 months			12 months			18 months
	Tap water	Primary effluent	Secondary effluent	Tap water	Primary effluent	Secondary effluent	Tap water	Primary effluent	Secondary effluent
pH	8.19	8.37	8.00	8.14	8.08	8.05	8.08	8.30	8.33
E.C. ds/m	2.51	2.60	3.14	1.50	2.76	3.40	1.44	2.95	3.48
CaCO3%	30.26	30.26	31.15	28.48	32.04	30.26	30.26	31.15	31.15
O.M%	0.64	0.89	0.84	0.67	1.08	0.94	0.67	1.20	0.98
Ca2+ (6.25)	5.15	6.60	3.60	3.90	3.96	3.24	5.88	3.60	3.60
Mg2+ (7.88)	4.40	4.65	2.18	3.13	3.52	3.39	4.22	3.96	3.00
Na+ (12.50)	15.00	20.00	8.75	20.15	27.50	8.75	20.00	28.50	23.50
K+ (0.58)	0.64	0.93	0.48	0.68	0.68	0.30	0.54	0.54	0.62
${\text{CO}}_3^{-}$ (–)	–	–	–	–	–	–	–	–	–
HCO3 (3.32)	2.32	3.98	3.36	2.98	3.65	3.98	4.81	5.81	5.15
Cl− (11.47)	12.59	18.51	5.10	15.94	18.20	6.92	14.01	19.11	18.20
${\text{SO}}_4^{-}$ (12.42)	12.28	9.69	6.65	8.79	13.81	4.78	11.73	11.68	7.97
Available P (ppm)	0.80	6.30	4.40	1.80	8.50	5.20	1.00	9.20	5.90
Available N (ppm)	7.00	13.40	11.20	7.30	21.20	15.12	5.40	22.52	17.60
	DTPA-extractable-heavy metals ppm
Cd	–	0.07	0.03	–	0.09	0.05	–	0.05	0.01
Fe	3.10	2.98	3.64	3.28	3.40	3.44	3.10	3.12	4.66
Ni	1.11	1.52	6.70	5.10	1.62	6.21	4.10	1.92	5.87
Pb	2.13	2.50	7.96	6.87	2.42	6.50	6.54	2.21	6.32
Cu	0.77	0.57	0.71	0.69	0.66	0.68	0.66	0.59	0.54
Mn	1.44	1.44	1.8	1.62	0.81	0.72	0.66	0.66	0.52
Zn	0.89	0.72	1.54	1.16	0.74	1.50	1.10	0.68	1.42

It is clear, from Table 7, that irrigation with sewage effluent decreased the CaCO₃ content from 32.04% to 28.48% and 27.59% for primary and secondary effluent treatments, respectively, after 18 months of irrigation period. This is probably due to the fact that same CaCO₃ was dissolved by the organic acids present in sewage and leached down in soil.

Furthermore, organic matter content increased as irrigation period with sewage effluent increased, with higher values for primary effluent compared with secondary. Available P and N were also increased more by primary effluent compared to secondary effluent, while tap water gave the lowest values. It is clear that available P and N were accumulated more as period of irrigation increased.

The results agree with those of EL-Nennah et al. (1982) who found that the use of sewage effluent in irrigation resulted in remarkable change of organic matter, available P and total and soluble N which might have been added to soils upon irrigation.

3.5. Extractable-heavy metals

Results given in Table 7 show the DTPA-extractable Cd, Cu, Fe, Mn, Ni, Pb and Zn as affected by irrigation with primary and secondary effluent compared with tap water treatment. The results indicated that DTPA-extractable Cd, Cu, Ni and Pb increased as irrigation period increased by sewage effluent treatments compared to tap water with greater values for primary effluent treatment. Whereas, extractable Fe, Mn and Zn as affected by treatments were not in a consistent trend.

Many investigators stated that heavy metals accumulated in soil resulted from continuous irrigation with sewage effluent (EL-Nennah et al., 1982; Abulroos et al., 1996; Salem et al., 2000). This increment of heavy metals in soil and consequently in edible parts of field crop plant should be considered, which adversely affect human and animal health through food-chain. It would be of a great advantage to grow forest trees such as S. mahagoni in heavily polluted areas or soil irrigated with sewage effluent without serious problems.