Collective efforts to fight mineral nutrient malnutrition in humans require consideration of soil fertility management practices (SFMP) in vegetable production. This study aimed at establishing the relationship between SFMP and vegetable nutrient concentration for human health in farming systems of Tanzania. Soil and vegetable samples collected from vegetable growing areas in Kilombero and Dodoma were analyzed for chemical properties and mineral nutrient concentration. Descriptive statistics, analysis of variance and correlation analysis were employed. The results showed that soil pH in Kilombero ranged from 6.04 to 6.8 and in Dodoma ranged from 6.23 to 8.58. The organic C was low, ranged from 0.10% to 1.87%. All soils studied had sufficient Zn (0.45 to 29.3 mg/kg), Cu (0.71 to 3.23 mg/kg), Fe (3.70 to 171.7 mg/kg) and Mn (2.84 to 41.38 mg/kg). Zinc concentration in all vegetables ranged from 12.57 to 134.54 mg/kg, 14% of vegetables had low Zn (<20 mg/kg) for human health. The Cu concentration in vegetables ranged from 0.07 to 52.37 mg/kg, and vegetables from Kilombero had very low Cu (<0.10 mg/kg) for plant and human nutrition. Vegetable Fe and Mn concentration ranged from 152.95 to 1780 mg/kg and 35.10 to 321.82 mg/kg, respectively. The SFMP used did not affect mineral micronutrients concentration in vegetables, but affected soil Zn, Cu, Fe and Mn concentrations. Soil pH, Zn, and CEC correlated with vegetable Cu, K, Mg, Zn, P and Fe concentrations, and differed among soils. Therefore, soil properties differed with SFMP, and both determined mineral concentrations in vegetables for human health.
Mineral micronutrient malnutrition in humans is the major cause of hidden hunger, affecting about 50% of the world population [
In Tanzania, all cereal-, roots- and tubers-based stable foods are usually eaten along with vegetables in most average and low income families. These diets are dominated by staple crops such as maize, sorghum, millet, cassava, rice, sweet potatoes and green banana. Most of these staples are of inherently low concentration of mineral nutrients, but relatively inexpensive [
Prevalence of hidden hunger micronutrient deficiencies in developing countries, including Tanzania, is partly attributed to the lack of a comprehensive and multi-sector approach to address micronutrient malnutrition that includes agriculture [
Human beings require essential mineral micronutrients such as Cu, Zn, Fe and Mn for physiological processes and insure good health [
The objective of this study was to assess the potential contribution of soil fertility management on vegetable mineral nutrient quality under small scale vegetable farming systems of humid and semi-arid areas of Tanzania for improved human health. Specifically, i) to determine the physical-chemical properties of vegetable growing soils under different management practices in humid and semi-arid areas, ii) to establish the mineral micronutrient profiles of common vegetables grown in Kilombero and Dodoma municipality and deduce whether they are sufficient, deficient or toxic for both plant and human health, and iii) to determine the relationship between soil fertility management and mineral nutrient concentration of vegetables.
This study was conducted in Kilombero district and Dodoma Municipality, Tanzania. Kilombero district is located in eastern Tanzania between Longitude 8˚15'00"E and Latitude 36˚25'00"S, while Dodoma Municipality is located in central Tanzania between Longitude 36˚00'00"E and Latitude 6˚00'00"S. The two areas have contrasting climate; Kilombero district has warm and humid tropical climate with annual minimum and maximum temperature of 26˚C and 32˚C, respectively. The annual mean rainfall of Kilombero ranges from 1200 mm to 1400 mm. The dominant soils of Kilombero are alluvial and Mang’ula area, where the studies were undertaken, is gleyic Cambisol (loamy) soil type [
Soil and vegetable samples were collected from Mang’ula and Mgudeni sites in Kilombero district (
Site | Farm & Soil sample ID | Vegetable sample collected | Eastings | Northings | Fertility and soil-water management |
---|---|---|---|---|---|
Mgudeni | Mgudeni 1 | Chinese cabbage (Brassica chinensis L.) | 07˚50'08.7"E | 036˚53'15.6"S | FYM, irrigated with stream water |
Mgudeni 2 | Sweet potato leaves (Ipomea batatas L.) | -do- | -do- | FYM, irrigated with stream water | |
Mgudeni 3 | Amaranths (Amaranthus hybridus L.) | -do- | -do- | Compost + rice husks, irrigated with stream water | |
Mang'ula | Mang’ula 1 | Kale (Brassica oleracea L.) | 07˚05'08.8"E | 036˚53'15.6"S | FYM + Urea, irrigated with stream water |
Mang’ula 2 | Amaranths (Amaranthus hybridus L.) | -do- | -do- | FYM + Urea, irrigated with stream water | |
Site | Farm & Soil sample ID | Vegetable sample collected | Eastings | Northings | Fertility and soil-water management |
FYM = Farm Yard Manure; ID = identification.
Site | Farm & Soil sample ID | Vegetable sample collected | Eastings | Northings | Fertility and soil-water management |
---|---|---|---|---|---|
Bwawani | Bwawani 1 | Sweet potato leaves (Ipomea batatas L.) | 05˚56'57.9"E | 035˚58'16.6"S | FYM, irrigated with HIS irrigation canal water |
Bwawani 2 | Chinese cabbage (Brassica chinensis L.) | -do- | -do- | FYM, irrigated with HIS irrigation canal water | |
Bwawani 5 | Swiss chard (Beta vulgaris subsp vulgaris) | -do- | -do- | FYM, irrigated with HIS irrigation canal water | |
Bwawani 3 | Bwawani 3 | Kale (Brassica oleracea L.) | 05˚57'01.9"E | 035˚58'18.7"S | FYM + Urea, irrigated with HIS irrigation canal water |
Bwawani 4 | Chinese cabbage (Brassica chinensis L.) | -do- | -do- | FYM + Urea, irrigated with HIS irrigation canal water | |
Zepisa | Zepisa | Amaranths (Amaranthus hybridus L.) | 05˚56'27.8"E | 035˚55'17.0"S | FYM, irrigated with bore hole well water |
Bwawani | Bwawani 1 | Sweet potato leaves (Ipomea batatas L.) | 05˚56'57.9"E | 035˚58'16.6"S | FYM, irrigated with HIS irrigation canal water |
FYM = Farm Yard Manure; HIS ? Hombolo Irrigation scheme; ID = identification.
Each soil and vegetables sample was air dried in a dust free screen house. The air-dried soil samples were ground to pass through a 2-mm sieve for physical and chemical analysis. The air-dried vegetable samples were subsequently oven dried at 70˚C to constant weight. The oven-dried vegetable samples were then ground to fine powder using a plant grinder, and stored in clean plastic bags (zip
Site | Farm & Soil sample ID | Vegetable sample collected | Eastings | Northings | Fertility and soil-water management |
---|---|---|---|---|---|
Chididimo 1 | Chididimo 1 | Kale (Brassica oleracea L.) | 06˚09'16.7"E | 035˚54'03.8"S | FYM, irrigated with bore hole well water |
Chididimo 2 | Mnavu (Solanum nigram L.) | -do- | -do- | Urea, irrigated with bore hole well water | |
Chididimo 3 | Amaranths (Amaranthus hybridus L.) | -do- | -do- | Urea, irrigated with bore hole well water | |
Chididimo 4 | Swiss chard (Beta vulgaris subsp vulgaris) | -do- | -do- | FYM + Urea, irrigated with bore hole well water | |
Chididimo 5 | Chididimo 5 | Chinese cabbage (Brassica chinensis L.) | 06˚09'14.6"E | 035˚54'05.0"S | FYM + Urea, irrigated with bore hole well water |
Shuleni 1 | Shuleni 1 | Amaranths (Amaranthus hybridus L.) | 06˚10'07.5"E | 035˚52'51.3"E | FYM, irrigated with bore hole well water |
Shuleni 2 | Sweet potato leaves (Ipomea batatas L.) | -do- | -do- | FYM + Urea, irrigated with bore hole well water | |
Shuleni 4 | Chinese cabbage (Brassica chinensis L.) | -do- | -do- | FYM + Urea, irrigated with bore hole well water | |
Shuleni 3 | Shuleni 3 | Sweet potato leaves (Ipomea batatas L.) | 06˚10'17.9"E | 035˚52'45.2"S | FYM, irrigated with bore hole well water |
Shuleni 5 | Shuleni 5 | Kale (Brassica oleracea L.) | 06˚09'16.7"E | 035˚54'03.8"S | Urea + CAN with bore hole well water |
FYM = Farm Yard Manure; HIS ? Hombolo Irrigation scheme; ID = identification.
lock bags) at room temperature of about 25˚C for chemical analysis.
The soil samples were analyzed for pH, electrical conductivity (EC), organic C, total N, available P, exchangeable K and the micronutrients Cu, Fe, Zn and Mn. Soil pH and EC was determined in 1:2.5 soil:water ratio by electrode method using pH meter and EC meter, respectively [
One gram of ground vegetable samples were weighed in crucibles and digested by the dry ash method by heating the samples at 600˚C in a muffle furnace for 2 hrs. The ashed samples were extracted with 10 ml of 6 N HCl and made to 50 ml with distilled water in a volumetric flask [
Descriptive statistics were used to determine the levels of mineral elements in soils and vegetables. Correlation analysis was also employed to determine relationships between the soil and vegetable mineral contents across all fertility management practices. The influence of soil properties (pH, OC, macro- and micro-nutrient concentrations) on the availability/uptake of nutrients in the vegetables was determined by simple correlation analysis. Analysis of variance using mixed model was used to determine effects of soil fertility management practices on soil properties and vegetable nutrient concentrations in each site, separately. Farms were considered random while SFMP was considered a fixed effect. All statistical analyses were carried out using SAS software version 9.00 [
Soil chemical properties and nutrient contents in soils are essential in determining the nutrient availability to plants. The soil pH of vegetable growing soils of Kilombero ranged from 6.04 to 6.85 while soil pH in Hombolo ranged from 7.24 to 8.58 and that of Ihumwa ranged from 6.23 to 8.53 (
Site | Garden | Soil pH (H2O) | EC dS/m | OC (%) | Avail. P mg/kg | Exch K cmolc/kg | Zn | Cu | Fe | Mn |
---|---|---|---|---|---|---|---|---|---|---|
mg/kg | ||||||||||
Kilombero | Mgudeni 1 | 6.85 | 0.01 | 0.20 | 78.80 | 0.72 | 11.20 | 2.73 | 62.70 | 20.3 |
Mgudeni 2 | 6.09 | 0.13 | 0.33 | 131.47 | 0.60 | 19.27 | 2.90 | 171.70 | 41.4 | |
Mgudeni 3 | 6.04 | 0.17 | 0.30 | 133.83 | 0.66 | 29.30 | 3.23 | 116.48 | 28.2 | |
Mang’ula 1 | 6.45 | 0.15 | 0.17 | 61.97 | 0.38 | 4.63 | 2.53 | 58.27 | 38.8 | |
Mangula 2 | 6.72 | 0.06 | 0.20 | 52.90 | 0.78 | 4.23 | 2.07 | 44.3 | 28.6 | |
Hombolo | Bwawani 1 | 8.04 | 0.29 | 0.63 | 3.79 | 0.72 | 2.31 | 1.07 | 9.42 | 18.8 |
Bwawani 2 | 8.47 | 0.20 | 0.89 | 5.57 | 1.89 | 0.47 | 1.37 | 3.70 | 11.7 | |
Bwawani 3 | 8.58 | 0.35 | 1.34 | 22.07 | 0.98 | 4.36 | 1.19 | 11.05 | 13.5 | |
Bwawani 4 | 8.34 | 0.35 | 1.22 | 9.86 | 1.00 | 3.59 | 1.10 | 11.15 | 16.0 | |
Bwawani 5 | 8.31 | 0.29 | 0.92 | 21.71 | 2.26 | 1.79 | 1.10 | 4.98 | 11.0 | |
Zepisa | 7.24 | 0.18 | 1.28 | 17.90 | 1.37 | 3.76 | 1.15 | 19.53 | 9.86 | |
Ipala | 7.33 | 0.04 | 0.96 | 22.88 | 1.52 | 2.31 | 1.75 | 22.18 | 12.2 | |
Ihumwa | Chididimo 1 | 8.53 | 0.23 | 1.35 | 12.15 | 0.07 | 1.21 | 1.43 | 6.25 | 6.7 |
Chididimo 2 | 7.97 | 0.20 | 1.03 | 3.78 | 0.12 | 1.34 | 1.14 | 5.84 | 6.9 | |
Chididimo 3 | 8.10 | 0.32 | 0.74 | 9.60 | 0.11 | 1.49 | 1.14 | 5.13 | 7.7 | |
Chididimo 4 | 7.93 | 0.31 | 1.87 | 27.58 | 0.19 | 3.16 | 1.02 | 7.38 | 10.0 | |
Chididimo 5 | 8.17 | 0.17 | 1.09 | 17.31 | 0.37 | 2.84 | 0.78 | 4.72 | 9.0 | |
Shuleni 1 | 7.17 | 0.46 | 0.64 | 8.87 | 0.21 | 1.63 | 1.12 | 7.17 | 11.2 | |
Shuleni 2 | 6.84 | 0.18 | 1.17 | 17.89 | 0.35 | 2.86 | 0.90 | 7.88 | 10.7 | |
Shuleni 3 | 7.43 | 0.15 | 0.95 | 7.57 | 0.37 | 2.66 | 0.75 | 6.76 | 10.9 | |
Shuleni 4 | 7.60 | 0.08 | 0.06 | 3.36 | 0.33 | 0.45 | 0.71 | 3.70 | 6.0 | |
Shuleni 5 | 6.23 | 0.15 | 0.10 | 15.48 | 0.33 | 1.30 | 0.75 | 10.85 | 2.8 |
*EC = electrical conductivity; OC = Organic carbon; Avail. P = available phosphorus; Zn = extractable zinc; Cu = extractable copper; Fe = extractable iron; Mn = extractable manganese.
low organic C (<2.0%) according to [
Soil pH and SOM are the major determinants of micronutrient availability in plants, and could contribute to micronutrient elements contents in vegetables. The soil pH range of the vegetable garden farms in Kilombero is adequate for good growth and yields of most vegetable. This is because the pH range of 6.5 to 6.8 does not pose limitations to availability of nutrients in terms of solubility of nutrients and will not cause plant root injury [
High available P in the soils of Kilombero vegetable gardens is due to use of DAP fertilizer [
Adequate levels of essential micronutrients in soils are the primary source of micronutrients in vegetables and for human health. The Zn content in all vegetable-growing soils studied ranged from 0.45 to 29.3 mg/kg, Cu ranged from 0.71 to 3.23 mg/kg, Fe ranged from 3.70 to 171.7 mg/kg, while Mn ranged from 2.84 to 41.38 mg/kg (
The micronutrient concentration of a soil is a function of inherent soil parent materials, chemical properties and management practices. The differences in Zn, Fe and Cu concentrations between Kilombero and Dodoma farms can be explained by the differences in soil types and soil properties. Kilombero soils have higher Zn, Cu, and Fe concentrations than Dodoma soils due to soil parent materials and lower pH. However, the fact that the literature reported low Zn in Kilombero soils due to continuous cropping for long time without Zn fertilization [
Zinc is one of the essential mineral elements for human health that are supplied through vegetables in diets. Across all sites and vegetable types, vegetable zinc contents ranged from 12.57 to 134.54 mg/kg (
The vegetables from Kilombero and Ihumwa are capable of supplying the recommended maximum allowable Zn intake of 20 mg/day for adults [
Site | Farm ID | Vegetable type | Zn | Cu | Fe | Mn |
---|---|---|---|---|---|---|
mg/kg | ||||||
Kilombero | Mgudeni 1 | Chinese cabbage (Brassica chinensis L.) | 70.14 | 0.08 | 1230.78 | 36.06 |
Mgudeni 2 | Sweet potato leaves (Ipomea batatas L.) | 12.57 | 0.08 | 254.64 | 35.10 | |
Mgudeni 3 | Amaranths (Amaranthus hybridus L.) | 65.67 | 0.08 | 970.06 | 72.93 | |
Mang’ula 1 | Kale (Brassica oleracea L.) | 49.53 | 0.08 | 1780.60 | 42.30 | |
Mangula 2 | Amaranths (Amaranthus hybridus L.) | 34.32 | 0.07 | 319.57 | 51.87 | |
Mean | 46.45 | 0.08 | 911.13 | 47.65 | ||
Hombolo | Bwawani 1 | Sweet potato leaves (Ipomea batatas L.) | 29.20 | 14.17 | 158.85 | 22.18 |
Bwawani 2 | Chinese cabbage (Brassica chinensis L.) | 13.14 | 6.67 | 569.86 | 87.77 | |
Bwawani 3 | Swiss chard (Beta vulgaris subsp vulgaris) | 25.08 | 11.17 | 276.84 | 90.64 | |
Bwawani 4 | Kale (Brassica oleracea L.) | 18.26 | 4.87 | 152.95 | 44.68 | |
Bwawani 5 | Chinese cabbage (Brassica chinensis L.) | 30.55 | 5.92 | 324.04 | 92.32 | |
Zepisa 1 | Amaranths (Amaranthus hybridus L.) | 51.02 | 7.27 | 254.20 | 89.44 | |
Ipala | Chinese cabbage (Brassica chinensis L.) | 36.45 | 5.77 | 184.41 | 102.13 | |
Mean | 29.10 | 7.97 | 274.45 | 75.59 | ||
Ihumwa | Chididimo 1 | Kale (Brassica oleracea L.) | 91.90 | 12.58 | 714.202 | 137.17 |
Chididimo 2 | Mnavu (Solanum nigram L.) | 100.43 | 37.36 | 861.27 | 122.78 | |
Chididimo 3 | Amaranths (Amaranthus hybridus L.) | 134.54 | 32.85 | 1660.29 | 117.99 | |
Chididimo 4 | Swiss chard (Beta vulgaris subsp vulgaris) | 85.86 | 30.60 | 581.86 | 151.56 | |
Chididimo 5 | Chinese cabbage (Brassica chinensis L.) | 106.47 | 52.37 | 655.39 | 143.17 | |
Shuleni 1 | Amaranths (Amaranthus hybridus L.) | 110.09 | 20.84 | 1395.59 | 135.97 | |
Shuleni 2 | Sweet potato leaves (Ipomea batatas L.) | 57.78 | 18.59 | 905.39 | 321.82 | |
Shuleni 3 | Sweet potato leaves (Ipomea batatas L.) | 58.14 | 14.83 | 660.29 | 289.45 | |
Shuleni 4 | Chinese cabbage (Brassica chinensis L.) | 119.26 | 12.58 | 1655.39 | 140.77 | |
Shuleni 5 | Kale (Brassica oleracea L.) | 118.91 | 13.33 | 1429.90 | 188.73 | |
Mean | 98.44 | 24.60 | 1051.96 | 174.94 |
while Zn ingestion of 4000 to 8000 mg Zn is toxic to humans [
The Cu content in leafy vegetables studied ranged from 0.07 to 52.37 mg/kg (
The Cu concentrations of vegetables from Kilombero are insufficient for vegetable growth and yield and may contribute to low vegetable yields in these areas. Similarly, balanced Cu nutrition in humans requires a Cu intake of at least 2.4 mg/day for a net Cu gain, while intake of < 0.8 mg/day results in net losses of Cu [
A study conducted in Morogoro region a decade ago reported Cu contents in vegetables ranging from 8.85 to 13.5 mg/kg [
Iron contents of vegetables grown in Dodoma and Kilombero ranged from 152.95 to 1780 mg/kg, and averaged 911.13 mg/kg (
All vegetables contain sufficient Fe for human diets, with recommended nutrient intake (RNI) for Fe for adults being 27.4 mg/day for males to 58.8 mg/day for females [
Vegetables are a dependable source of Fe especially in poor families, due to higher Fe contents and availability of Fe from vegetables. Many vegetables contain ascorbic acid which enhances Fe bioavailability in the gastrointestinal tract [
Manganese contents in the vegetables studied ranged from 22.18 to 321.82 mg/kg (
Manganese is an essential nutrient for both plants and humans. The Mn contents in vegetables in this study are adequate for plant growth and yield. While no established recommended dietary intake of Mn has been reported due to insufficient information, the MAC for Mn is 11 mg/kg for adults and 6 mg/day for children below 13 years old [
Soil fertility management practices influence soil properties and hence nutrient uptake by plants, which eventually determine the quality of vegetables in terms of their nutrient contents. In this study, the common soil fertility management practices in vegetable production in Kilombero include use of farm yard manure alone (FYM), FYM and inorganic fertilizers (usually Urea and CAN) and use of FYM and compost from available organic materials such as rice husks. The results show that different fertility management practices used in the study areas resulted in significant differences in soil properties, especially soil TN, available P, Zn and Cu in Kilombero and Hombolo soils (
Source of Variation | df | Soil N | Soil P | Soil K | Soil Zn | Soil Cu | Soil Fe | Soil Mn |
---|---|---|---|---|---|---|---|---|
p-value | ||||||||
Kilombero-Fertility management | 2 | 0.0102* | 0.0016* | ns | 0.0002* | 0.0366* | ns | ns |
Hombolo-Fertility management | 2 | 0.0103* | 0.0223* | ns | 0.0005* | 0.0009* | ns | ns |
Ihumwa-Fertility management | 2 | ns | ns | 0.0034* | ns | 0.0164* | 0.0004* | 0.0120* |
df = degrees of freedom; * = significant at alpha = 0.05; ns = not significant at alpha = 0.05.
In Hombolo, the fertility management practices included application of FYM + Urea, FYM alone, or FYM + foliar fertilizers. The results showed that among these practices FYM + urea resulted in higher soil total N (2.8%), available P (23.3 mg/kg) and Zn (4.5 mg/kg) than other management practices. However, FYM + urea had significantly the lowest available Cu (1.2 mg/kg) in soil (
In Ihumwa site, the common management practices consisted of the use of urea alone, FYM + urea, FYM alone or Urea + CAN. The results showed that soil K was significant (p < 0.05) lowest (0.16 cmolc/kg) in soils managed by FYM + urea compared to soil K in other management practices (ranged from 0.50 cmolc/kg in FYM alone to 0.69cmolc/kg in Urea+CAN) (
Soil fertility management practices slightly differed among the three sites studied, but the use of FYM and FYM in combination with inorganic N fertilizer were common practices. The soil fertility management effects on soil nutrient status differed among sites due to differences in inherent soil properties among these sites. Reference [
In Hombolo, the high soil total N, P, and Zn suggest that the FYM is the major source of N, P and Zn in the soil, because these soils are naturally very low in N, P and Zn [
Ihumwa soil is deficient in soil K (
Higher Cu concentration in soil but lower Cu concentration in vegetables due to FYM alone may be due to supply of Cu from FYM, and organic matter interference with Cu uptake. It was established that Cu uptake is reduced by SOM because Cu is tightly bound by OM [
The analysis of variance results showed that none of the micronutrients (Cu, Fe, Zn and Mn) concentrations in vegetables were significantly affected by soil fertility management practices. Soil fertility management practices resulted in significant differences in P, K and Mg concentrations in vegetables from Kilombero, N concentration in vegetables from Hombolo, and K and Ca in vegetables from Ihumwa (
Source of Variation | df | Vegetable N | Vegetable P | Vegetable K | Vegetable Mg | Vegetable Ca |
---|---|---|---|---|---|---|
p value | ||||||
Kiombero-Fertility management | 2 | ns | 0.0193* | 0.0040* | 0.0001* | ns |
Hombolo-Fertility management | 2 | 0.0080* | ns | ns | ns | ns |
Ihumwa-Fertility management | 2 | ns | ns | 0.0011* | ns | 0.0086* |
df = degrees of freedom; * = significant at alpha = 0.05; ns = not significant at alpha = 0.05.
The results showed that management practices directly influenced the absorption of macronutrients and hence nutrient concentrations. The SFMP effect differed among sites due to differences in soil physical and chemical properties. The higher concentrations of P, K, and Mg in vegetables that received compost + rice husks in Kilombero are due to higher concentration of these nutrients in rice husks than under the other fertility management practices. Higher N in Hombolo vegetables due to FYM + Urea suggests better supply of N when organic and inorganic N sources are combined in these sandy loam soils with low SOM [
To determine the relationship between soil properties and nutrient concentration in vegetables, the soil properties were pulled across all fertility management practices to obtain sufficient observations for correlation and regression analysis. Due to differences in agro-ecological conditions and soil types among studied sites, the relationship between soil properties and plant nutrients were investigated separately by site. This approach was expected to reduce wide variations, and allow understanding of soil-plant processes that can be explored to enhance micronutrients quality of vegetables and understanding the how micronutrient concentration in soils affect mineral quality of vegetables.
In Kilombero soils with soil Zn concentration range of 3.9 to 40.6 mg/kg, soil Zn was positively correlated with K and Mg concentrations in vegetables (
Vegetables’ nutrient concentrations | |||||
---|---|---|---|---|---|
Parameter | n | Mean | SD | Min | Max |
K (%) | 16 | 4.74 | 1.04 | 2.96 | 6.41 |
Mg (%) | 16 | 0.14 | 0.08 | 0.05 | 0.29 |
Cu (mg/kg) | 16 | 0.08 | 0.01 | 0.07 | 0.08 |
Soil properties | |||||
pH (H2O) | 16 | 6.41 | 0.39 | 5.72 | 6.98 |
EC (dS/m) | 16 | 0.11 | 0.06 | 0.02 | 0.30 |
CEC (cmolc/kg) | 16 | 19.41 | 5.34 | 12.40 | 33.60 |
P (mg/kg) | 16 | 94.42 | 40.02 | 44.80 | 145.6 |
Zn (mg/kg) | 16 | 14.70 | 11.64 | 3.90 | 40.6 |
Cu (mg/kg) | 16 | 2.73 | 0.60 | 2.00 | 4.10 |
Pearson correlation coefficients (r) | |||||
Soil Zn | Soil Cu | Soil P | Soil pH | ||
Vegetable K | 16 | 0.538* | 0.0.451 | 0.528* | -0.143 |
Vegetable Mg | 16 | 0.703* | 0.553* | 0.434 | -0.325 |
Vegetable Cu | 16 | 0.401 | 0.212 | 0.536* | -0.645* |
n = number of observation; SD-standard deviation; Min = minimum; Max = maximum; *Significant at alpha = 0.05.
due to positive effect of P on root growth, which increases root interception and hence absorption of K and Cu by the vegetables [
Nutrient concentrations in vegetables grown in Hombolo had significant and positive correlation with Soil Fe, CEC and pH. Concentration of K in vegetables was significantly and positively correlated with soil Fe concentration (r = 0.447; p < 0.05) (within the soil Fe range of 3.29 to 42.50 mg/kg) but negatively correlated with soil pH (r = −0.686; p < 0.05) (within the soil pH range of 6.41 to 8.73) (
These results shows that in soils of Hombolo the soil Fe influenced availability of K in vegetables, while soil pH influenced availability of K, P, and Zn in vegetables. The negative influence of pH on P and Zn concentrations in the vegeta-
Vegetables’ nutrient concentrations | |||||
---|---|---|---|---|---|
Parameter | n | Mean | SD | Min | Max |
K (%) | 22 | 4.143 | 1.54 | 0.38 | 6.53 |
Zn (mg/kg) | 22 | 28.23 | 14.18 | 1.77 | 57.63 |
Fe (mg/kg) | 22 | 316.71 | 267.37 | 17.26 | 1235.00 |
Cu (mg/kg) | 22 | 8.29 | 4.93 | 1.87 | 21.66 |
Soil properties | |||||
pH (H2O) | 22 | 8.02 | 0.60 | 6.41 | 8.73 |
EC (dS/m) | 22 | 0.25 | 0.13 | 0.01 | 0.41 |
CEC (cmolc/kg) | 22 | 15.73 | 3.63 | 10.80 | 24.20 |
P (mg/kg) | 22 | 15.19 | 9.93 | 3.22 | 31.71 |
Zn (mg/kg) | 22 | 2.63 | 1.75 | 0.45 | 7.36 |
Fe (mg/kg) | 22 | 12.66 | 8.93 | 3.29 | 42.50 |
Cu (mg/kg) | 22 | 1.25 | 0.28 | 0.83 | 1.98 |
Pearson correlation coefficients (r) | |||||
Soil Zn | Soil Fe | Soil CEC | Soil pH | ||
Vegetable K | 22 | −0.216 | 0.447* | −0.162 | −0.686* |
Vegetable P | 22 | 0.207 | 0.407 | −0.472* | −0.496* |
Vegetable Zn | 22 | 0.026 | 0.417 | −0.501* | −0.571* |
Vegetable Fe | 22 | −0.439* | −0.172 | 0.126 | −0.055 |
n = number of observation; SD-standard deviation; Min = minimum; Max = maximum; *Significant at alpha = 0.05.
bles suggests that the high soil pH in Hombolo soils, with a mean pH of 8.08, rendered P and Zn unavailable due to P fixation and precipitation of Zn with hydroxyl ions at those high pH levels [
The concentration of Zn in soils was significantly and negatively correlated with vegetable Zn (r = −0.704) and Fe (r = −0.587) concentrations but positively correlated with Mn (r = 0.543) concentration in vegetables grown in Ihumwa soils (
Vegetables nutrient concentration | |||||
---|---|---|---|---|---|
Parameter | N | Mean | SD | Min | Max |
K (%) | 29 | 2.07 | 0.88 | 0.55 | 4.23 |
Zn (mg/kg) | 29 | 96.02 | 30.05 | 29.11 | 144.13 |
Fe (mg/kg) | 29 | 1037.00 | 517.22 | 0.56 | 1881.00 |
Cu (mg/kg) | 29 | 30.11 | 36.40 | 8.83 | 175.54 |
Mn (mg/kg) | 29 | 195.42 | 134.17 | 100.00 | 748.53 |
Soil properties | |||||
pH (H2O) | 29 | 7.58 | 0.71 | 5.70 | 8.80 |
EC (dS/m) | 29 | 0.23 | 0.16 | 0.07 | 0.90 |
CEC (cmolc/kg) | 29 | 14.59 | 2.59 | 9.60 | 20.00 |
P (mg/kg) | 29 | 12.02 | 9.06 | 2.31 | 31.95 |
Zn (mg/kg) | 29 | 1.80 | 0.85 | 0.36 | 3.43 |
Fe (mg/kg) | 29 | 6.61 | 2.14 | 3.29 | 12.48 |
Cu (mg/kg) | 29 | 0.99 | 0.25 | 0.61 | 1.48 |
Mn (mg/kg) | 29 | 8.08 | 3.60 | 1.59 | 15.04 |
Pearson correlation coefficients (r) | |||||
Soil Zn | Soil Fe | Soil CEC | Soil pH | ||
Vegetable Zn | 29 | −0.704* | −0.093 | −0.411* | −0.018 |
Vegetable Fe | 29 | −0.587* | −0.587* | −0.600* | −0.277 |
Vegetable Mn | 29 | 0.543* | 0.213 | 0.195 | −0.239 |
n = number of observation; SD-standard deviation; Min = minimum; Max = maximum; *Significant at alpha = 0.05.
were significantly negatively correlated (r = −0.587). Soil CEC (9.6 to 20 cmolc/kg) was significantly and negatively correlated with vegetable Zn (r = −0.411) and vegetable Fe (r = −0.600) (
The results show that for Ihumwa, micronutrient concentrations in vegetables were influenced by soil Zn, soil Fe and CEC. The negative correlation of vegetable Zn and soil Zn concentration was unexpected, but further alludes to complex phenomena and possibly nonlinear relationship between soil constituents and micronutrient availability in soils [
The chemical properties and plant nutrient concentrations in the soils studied are diverse, with medium soil pH in humid alluvial soils of Kilombero to high soil pH in the semi-arid soils of Dodoma. Soil P was deficient in some vegetable growing soils of Dodoma, but sufficient in Kilomero soils. Among the micronutrients, Zn and Fe were sufficient in most of the soils but were deficient in one vegetable growing soil of Dodoma, while Cu was deficient in Kilombero soils. All vegetables from all sites had mineral micronutrient (Zn, Cu, Fe and Mn) concentrations at sufficient levels and within MAC for human health for most sites, except the vegetables from Kilombero that had low Cu, and vegetables from two sites in Hombolo that had low Zn. Soil fertility management affected concentrations of macronutrients (N, P, K, Ca and Mg) but not micronutrients in the vegetables. However, SFMP affected Zn, Cu, Fe and Mn, and the effect differed among soils. The relationships between soil chemical properties and vegetable mineral concentrations were direct; some were complex and some differed among sites due to differences in soil properties across SFMP. Soil fertility management for vegetables influenced vegetable mineral quality for human health, and differed among soils and agro-climatic zones.
This work was supported by the Innovative Agricultural Research Initiative (iAGRI), a Feed the Future Project of USAID. The field and laboratory assistance from Geoffrey Malimwengu, Farid Magoma and Mr. Waziri are highly appreciated. The technical assistance in laboratory analysis by Mr. Mohamed Hamis and Mr. Salum Marangi is gratefully acknowledged.
Amuri, N.A., Mhoro, L., Mwasyika, T. and Semu, E. (2017) Potential of Soil Fertility Manage- ment to Improve Essential Mineral Nu- trient Concentrations in Vegetables in Do- doma and Kilombero, Tanzania. Journal of Agricultural Chemistry and Environment, 6, 105-132. https://doi.org/10.4236/jacen.2017.62007