1. Introduction

2. Materials and Methods

2.2. Soil Sampling

Soils were sampled in agricultural farms in the Manica province, namely two samples in the Manica district (Fields C1 and C2) and three samples in the Sussudenga district (Field C3, C4 and C5) (Figure 1). The areas of the five farms and their agricultural productions are:

C1 - 7 ha: corn, green beans, banana, lettuce, cucumber, strawberries and okra;

C2 - 2 ha; corn, tomatoes and beans;

C3 - 1.5 ha: corn and sesame;

C4 - 1 ha: corn and beans;

C5 - 1 ha: corn and beans.

Fertilizers and pesticides are used in these farms to improve the fertility of the soil and control pests.

Samples were collected in two campaigns (2021/2022 and 2022/2023), and in each of them samples were obtained before and after the rainy season, in the following periods (Table S1): 2021/2022 campaign - September and October 2021 (before the rainy season) and April 2022 (after the rainy season; 2022/2023 campaign - September 2022 (before the rainy season) and April 2023 (after the rainy season).

For each agricultural field, a sample was collected that was made up of a determined number of subsamples, which varied between 15 and 20. The collection of the subsamples for each field was done randomly and in a zigzag manner, in order to cover the entire area. The depth considered for the soil sampling was from 0 and down to 20 cm, and involved the use of a manual auger, two plastic buckets and some plastic bags. Table S1 shows the date of the samplings and the coordinates of all the sub-samples. After collecting the subsamples, they were mixed manually to homogenize, and about 1 kg of the mixture was kept in a plastic bag, placed inside a cooler to be transported and conserved in a freezer.

Figure 1. Mozambique map with its provinces, with the Manica province amplified and with its districts. Red arrows indicate the areas under soil analysis. With permission from reference [17].

Figure 1. Mozambique map with its provinces, with the Manica province amplified and with its districts. Red arrows indicate the areas under soil analysis. With permission from reference [17].

3. Results and Discussion

3.1. Agronomical Chemical Analysis of the Soils

Table 1 shows the results of the agronomical chemical analysis of the five soils sampled in the campaigns 2021/2022 and 2022/2023. The analysis of this table shows that all the soils are extremely to strongly acidic (pH ranging from 4.5 and 5.4, with an average of 5.0±0.3) and have no salinity problems (conductivity ranging from 4.2 to 11.8 mS/m). The difference in pH values determined in water and in KCl 1M was about 0.8, suggesting that these soils have negative charge and are cation exchangers [7]. All soils are characterized by a low amount of soil organic matter (SOM) ranging from 0.90% to 1.81%. The soils from the Sussudenga district (C3 to C5) have a very low CEC (average and standard deviation of 3.33±0.99 cmol_c/kg) while those from Manica district (C1 and C2) ranges from low to average CEC (C1, 7.30 and 13.11 cmol_c/kg) and very low to low CEC (C2, 3.59 to 9.74 cmol_c/kg). This analysis shows that these soils need liming, for pH correction, and incorporation of organic correctives to increase organic matter and improve CEC, in order to improve fertility. Also, the Sussudenga district soils have a phosphorous deficiency (values ranging from <20 to 38.5 mg/kg) while the Manica district soils usually have a high amount of phosphorous (with a concentration ranging from 106 to 174 mg/kg), with the exception of the sample C1 from the year 2022 (44.8 mg/kg). This results show that the samples of the Sussudenga district soils have marked fertility problems.

A study about the Mozambique soil fertility published in 2006 concluded that, in general, they can be classified as having low to moderate fertility [11]. Indeed, the median CEC was low, with an average of 5.0 cmol_c/kg, ranging from 0.4 to 14.5 cmol_c/kg, and 75% of the samples had less than 7.5 cmol_c/kg, which is considered the minimum adequate CEC [11]. The soils under analysis from Sussudenga have a particularly very low CEC value (3.33±0.99 cmol_c/kg). Mozambique soils have a median pH of 6.0±0.53, and ranged between 4.4 and 7.8, and a SOM ranging from 0.4% to 5.0%, with a median of 2.1% [11]. The Manica and Sussudenga soils under analysis fall within these pH and SOM intervals, but are close to the lower values, i.e. more acidic and poor in organic matter.

Mozambique soils are poorer in the macronutrient phosphorous, and the following study cases demonstrates this problem:

(i) Cassava is the second most produced crop in Mozambique [9,10]. Cassava is produced mainly by small-scaled, resource-poor farmers, on nutrient-depleted soils [1]. Indeed, cassava can achieve reasonable yields in poor soils, where other crops would not thrive [18]. In Mozambique, about 75% of the economically active population is engaged in agriculture, and the majority in small farms with an average land of 1.78 ha [18]. A soil of Milha-14 in the coastal Dondo district (Sofala province, Mozambique) was analyzed with the following results [18]: pH = 4.9; P, 6 mg/kg; K, 149 mg/kg; Ca, 215 mg/kg; Mg, 60 mg/kg; Na, 16 mg/kg; and, SOM, 1.03%. The cassava tuber yield of this soil was 14.7±2.6 ton/ha. The fertilization of this soil with 60 kg/ha N and with 60 kg/ha P₂O₅ yielded 27.7 tons/ha [18].

(ii) Maize is the highest crop in Mozambique [9,10]. Besides the well-known nitrogen fertilization in maize production, the availability of phosphorous is a critical factor for crop productivity, especially under Africa acid soils conditions [19]. In a study on the Nacala corridor (Mozambique), it was suggested the fertilization with 32-74 kg P₂O₅ ha^-1 [19].

(iii) Soybean production is small, but it is growing in Mozambique, with a yield in the year 2020 of 1.67 t/ha [20]. Besides being used in human and animal nutrition, it is a legume crop that improves soil fertility [20]. The average soybean yield in the world was 67.8% higher than that of Mozambique [20]. Fertilization with 20 to 30 kg P ha^-1, potassium and starter nitrogen, and inoculants, improved soybean yields [20].

The soils under analysis have somewhat different textures because the Manica soils have higher percentage of clays, when compared with the Sussudenga soils, with higher percentages of sand: Manica C1 soil has a sandy clay loam/ clay loam texture; Manica C2 soil has a sandy-loam/sandy clay loam textures; C3, C4 and C5 Sussudenga soils have loamy sand/sandy clay loam/sandy loam texture.

The Manica and Sussudenga soils have a similar texture to other Mozambique soils, that fall in the loamy sand, sandy loam and sandy clay loam classes [11]. Typical mineral present in these soils are the kaolinite, illite, and the hydroxides, oxohydroxides, and oxides of Fe and Al [11].

The following observations can be drawn about the macro and micronutrients in the Manica and Sussudenga soil samples:

(i)

all the soils under analysis are deficient in boron, with an average concentration of the extractable boron lower than 0.2 mg/kg;

(ii)

soils C4 and C5 from the Sussudenga district have calcium, magnesium and potassium deficiencies;

(iii)

soil C3 from the Sussudenga district have calcium and zinc deficiencies;

(iv)

soil C4 from Sussudenga district have copper and zinc deficiencies;

(v)

soil C1 from Manica district have an excess of magnesium, manganese and iron;

(vi)

soils C2 from Manica district, and C3 from Sussudenga district, have excess of manganese and iron.

Table 1. Characteristics of the five soil samples in campaigns 2021/2022 (first row) and 2022/2023 (second row).

Table 1. Characteristics of the five soil samples in campaigns 2021/2022 (first row) and 2022/2023 (second row).

Property	C1	C2	C3	C4	C5
Extractable K (K2O), mg/kg	157	251	157	40.1	49.3
	149	124	110	45.0	37.9
Extractable Mg, mg/kg	268	386	128	46.4	75.8
	622	102	121	47	40.1
Extractable Ca, mg/kg	916	1191	512	448	424
	1474	458	641	516	270
Extractable Fe, mg/kg	183	230	117	49.3	107
	88.9	170	81	50.9	74.3
Extractable Mn, mg/kg	263	307	180	45.6	22.6
	301	163	153	51.8	14.2
Extractable Zn, mg/kg	1.9	1.9	0.95	2.0	3.0
	1.4	1.6	0.86	1.3	2.7
Extractable Cu, mg/kg	3.5	3.6	2.0	0.45	0.60
	2.2	3.2	1.4	0.42	0.38
Extractable B, mg/kg	< 0.2	< 0.2	< 0.2	< 0.2	< 0.2
	< 0.2	< 0.2	< 0.2	< 0.2	< 0.2
Exchangeable Na, cmol(+)/kg	0.10	0.15	0.11	0.04	0.05
	0.17	0.04	0.07	0.05	0.04
Exchangeable K, cmol(+)/kg	0.33	0.44	0.39	0.14	0.16
	0.39	0.31	0.33	0.15	0.12
Exchangeable Ca, cmol(+)/kg	4.6	5.9	2.6	2.2	2.1
	7.4	2.3	3.2	2.6	1.3
Exchangeable Mg, cmol(+)/kg	2.2	3.2	1.0	0.38	0.62
	5.1	0.84	0.99	0.39	0.33
Exchangeable Al, cmol(+)/kg	< 0.025	< 0.025	< 0.025	< 0.025	< 0.025
	< 0.025	< 0.025	< 0.025	< 0.025	< 0.03
CEC, cmol(+)/kg	7.30	9.74	4.22	2.83	3.08
	13.11	3.59	4.67	3.26	1.91
pH(KCl) 1:5	5.2	5.4	4.8	5.2	4.6
	5.2	4.5	5.1	5.3	4.5
pH(H2O) 1:5	6.0	6.1	5.7	5.8	5.4
	6.2	5.4	6.0	5.9	5.2
Extractable P (P2O5), mg/kg	132	106	36.4	37.1	37.9
	44.8	174	<20	38.5	37.3
Organic Carbon (%)	0.63	0.77	1.0	0.60	0.78
	1.0	0.52	0.76	0.64	0.66
Organic Matter (%)	1.09	1.33	1.81	1.04	1.34
	1.77	0.90	1.31	1.10	1.14
Nitrogen Kjeldahl, g/kg	0.94	1.23	1.10	0.59	0.80
	1.24	0.78	0.68	0.66	0.68
Inorganic nitrogen, mg/kg	18.5	23.2	7.0	12.0	16.9
	19.8	14.5	4.7	14.3	20.1
Conductivity, mS/m	10.1	11.8	7.8	6.6	6.9
	6.3	5.7	4.2	4.9	5.3
Sand, Clay, Silt (USDA) (%)	62.7. 21.7. 15.6	55.2. 26.3. 18.5	67.9, 20.3, 11.8	77.4, 10.2, 12.4	79.4, 13.0, 7.6
	34.0. 32.3. 33.7	71.0. 17.0. 12.0	66.4, 19.2, 14.4	85.9, 9.7, 4.4	78.5, 10.5, 11.0
Texture (USDA)	sandy clay loam	sandy clay loam	sandy clay loam	sandy loam	sandy loam
	clay loam	sandy loam	sandy loam	loamy sand	sandy loam

These results show that corrections are required in the concentration of the soil macro/micro nutrients to achieve increased yields of Mozambique crops. However, before defining a correction scheme, soils must be analyzed to confirm its main deficiencies to allow a sustainable agro-environmental management of the food production.

3.2. ICP-MS Elemental Concentrations

Table 2 shows the concentrations of the elements present in the five mixtures of the soils sampled before and after the rainy season in the 2022/2023 campaign. The following elements were not detected: As; Sb; Be; Cd; Hg; Mo; Se; and, Sn.

The reference values for agriculture soils accordingly to the Portuguese Environmental Agency [21] are included in Table 2. The comparison of the results with the reference values show that soil samples C1 and C2 from the Manica district have severe contamination problems for the following elements: C1, Cr (1400 mg/kg), Co (80 mg/kg), Ni (680 mg/kg) and V (86 mg/kg); C2, Cr (280 mg/kg) and Ni (78 mg/kg). The soils from the Sussudenga district show no contamination with the measured chemical elements.

The presence of the elements Cr, Co, V and Ni in the agricultural soils is of geogenic origin [97]. In a study of the top soils from the Beira city (Mozambique) the concentration of these elements ranged [22]: Cr, 11.0 to 3930 mg/kg (with an average of 89 mg/kg); Co, below the detection limit to 56.0 mg/kg (with an average of 3.00 mg/kg); and Ni, 1 to 120 mg/kg (with an average of 7.00 mg/kg); and, V, 2.00 to 87.0 mg/kg (with an average of 17.0 mg/kg). Soil pollution with an anthopogenic origin, namely of Cu, Pb and Zn, were not detected. Moreover, taking into consideration that the Manica district area under investigation has illegal artisanal gold mining [23-25], no Hg contamination was detected in the studied soils.

Comparing the contamination of the C1 and C2 Manica soils with other agricultural soils al over the world we can conclude that these are considered outliers, due to the relative high concentration levels of pollutants. For example, it compares with Iranian agricultural soils that have an average (minimum/maximum) concentration of Cr, Co, Ni and V, respectively, 101 (5.67/633), 27.9 (6.80/519), 68.0 (2.79/770) and 101 (20.3/1202) mg/kg [26]. In a review from Indian agricultural soils values for metals Co, Cr Ni and V usually are lower that the values found in this work [27]. Also, the analysis of the agricultural soils from the Shanghai region has an average Cr value of 41.00 mg/kg [28] and show that although this region is highly industrialize, the heavy metal levels in agricultural soils are within the safe ranges according to the Chinese environmental regulations.

Due to the absolute and relative abnormal concentrations of some elements in samples C1 and C2, these two soils were subject to a detailed chemical analysis of organic pollutants, and the following were detected: C1 - p-isopropyltoluene (0.06 mg/kg), ethyl chlorpyrifos (0.03 mg/kg), diethylhexyl phthalate (0.3 mg/kg) and total petroleum hydrocarbons (C30-C35) (7.6 mg/kg); C2 - diethylhexyl phthalate (0.4 mg/kg). The presence of these organic pollutants suggests that, besides the geogenic origin of the pollutants in these two soils, there is an unknown anthropogenic contribution to that pollution that will be the subject of further research.

Table 2. ICP-MS results (in mg/kg) of the analysis of the five soil samples collected in the 2022/2023 campaign.

Table 2. ICP-MS results (in mg/kg) of the analysis of the five soil samples collected in the 2022/2023 campaign.

Element	C1	C2	C3	C4	C5	Reference Value1
As	-	-	-	-	-
Sb	-	-	-	-	-
Ba	67	32	51	19	17	210
Be	-	-	-	-	-
Cd	-	-	-	-	-
Cr	1400	280	34	-	4.1	67
Co	80	17	7	-	-	19
Cu	32	13	9.1	-	-	62
Hg	-	-	-	-	-
Pb	8.8	6.4	13	4.3	5.1	45
Mo	-	-	-	-	-
Ni	680	78	11	-	-	37
Se	-	-	-	-	-
Sn	-	-	-	-	-
V	86	36	30	3.0	5.1	86
Zn	30	17	15	-	13	290

¹ Reference values for agriculture soils according to the Portuguese Environmental Agency [21].

3.3. XRF Elemental Concentrations

Table 3 shows the concentrations of the elements present in the soils under analysis for the year 2022 and 2023. This table only shows the elements that were detected by XRF.

The analysis of Table 3 confirms the results obtained by ICP-MS, showing that the soil sample C1 from the Manica district are severely contaminated with V, Cr, Co and Ni and that the contamination is observed both in the samples collected before and after the rainy season. Also, sample C2 is also contaminated with V, Cr and Ni. Also, comparing the results obtained before and after the rainy season, it shows that the elemental concentration remains in the same order of magnitude, and it demonstrates that the rain that washed the soil in the summer months had no effect in the attenuation of the contamination. A probable cause for this observation is the geogenic origin of the most concentrated elements, and whose minerals are not soluble in water.

The comparison of the ICP-MS and XRF concentration estimations shows that the results obtained by XRF are usually higher than those obtained by ICP-MS for the elements Ba and Cr, which were the elements with the highest concentrations in the soils under analysis. For the others, the XRF estimates were in the same order of magnitude of ICP-MS – the plot of the two sets of results, resulted in linear plot with a slope 1.1 and an intercept of 7. This result supports that the use of portable XRF equipment can be use for screening on-site straightforward estimation of the concentration of chemical elements in the soils, allowing the identification of potential contaminations that are above the regulated threshold values.

Table 3. XRF results (mg/kg) of the analysis of the five soil samples in the campaign 2022/2023, before (first row) and after (second row) the rainy season (averages and standard deviation of three independent measurements).

Table 3. XRF results (mg/kg) of the analysis of the five soil samples in the campaign 2022/2023, before (first row) and after (second row) the rainy season (averages and standard deviation of three independent measurements).

Element	C1	C2	C3	C4	C5	Reference Value1
K	5591(101)	14013(696)	15344(2062)	40165(3925)	22545(1113)
	6841(161)	13877(620)	15772(524)	23067(3136)	22609(511)
Ca	7186(1327)	6432(478)	2170(167)	3703(659)	4360(258)
	8335(1046)	6585(113)	2450(1131)	3057(403)	3034(146)
Ti	3833(72)	5814(849)	5243(779)	2242(690)	2073(65)
	3798(159)	5284(228)	4840(308)	1353(29)	2018(217)
V	132(20)	97(27)	-	-	-	86
	112(10)	24(42)	-	-	-
Cr	2675(308)	803(67)	60(13)	-	-	67
	2543(119)	700(8)	52(54)	-	-
Mn	1429(238)	783(174)	690(111)	318(34)	167(18)
	1423(159)	799(23)	686(115)	268(60)	126(5)
Fe	83114(6083)	29774(2118)	22247(2373)	4412(530)	5109(95)
	75161(2541)	28393(960)	25610(3308)	4048(553)	4871(141)
Co	44(38)	11(18)	-	-	-	19
	49(46)	-	11(20)	-	-
Ni	823(70)	154(13)	23(6)	4(8)	4(7)	37
	684(42)	158(2)	25(5)	-	-
Cu	26(4)	14(1)	-	-	-	62
	25(6)	17(3)	4(8)	-	-
Zn	39(3)	19(2)	16(1)	-	4(6)	290
	36(5)	22(3)	20(6)	-	-
Rb	47(8)	53(8)	91(4)	202(30)	101(2)
	45(1)	53(2)	84(3)	106(20)	98(5)
Sr	48(9)	59(5)	49(2)	91(14)	88(3)
	55(4)	63(4)	38(5)	49(8)	96(2)
Zr	164(24)	261(52)	393(53)	169(34)	196(65)
	199(66)	294(23)	292(18)	158(2)	165(8)
Ba	-	256(3)	-	414(71)	299(25)	210
	-	256(22)	-	294(19)	293(30)
Ta	29(4)	-	-	-	-
	10(18)	7(13)	12(11)	-	-
Pb	3(6)	7(6)	22(3)	30(10)	18(2)	45
	3(6)	5(5)	19(2)	17(2)	19(2)

¹ Reference values for agriculture soils according to the Portuguese Environmental Agency [21].

4. Conclusions

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