Soil quality effects of accelerated erosion and management
systems in three eco-regions of Tanzania
F.B.S. Kaihura
a, I.K. Kullaya
b, M. Kilasara
c,
J.B. Aune
d, B.R. Singh
e, R. Lal
f,*aUkiriguru Agriculture Research Institute, PO Box 1433, Mwanze, Tanzania bAgricultural Research Institute Lyamungu, PO Box 3004, Moshi, Tanzania
cSokoine University of Agriculture, Soil Science Department, PO Box 3008, Morogoro, Tanzania dCentre for International Environment and Development Studies-Noragric, NLH, Aas, Norway
eDepartment of Soil Science, NLH, Aas, Norway
fSchool of Natural Resources, Ohio State University, 2021 Coffey Road, Columbus, OH 43210-1085, USA
Received 27 May 1999; received in revised form 8 July 1999; accepted 15 July 1999
Abstract
Soil erosion can adversely in¯uence soil quality, especially in tropical soils. Thus, a multi-location ®eld experiment was conducted on eight major agricultural soils with different degrees of erosion, in three eco-regions in Tanzania. The objective was to assess the impact of topsoil depth (TSD) and management on soil properties. Three eco-regions comprising of humid at Kilimanjaro, sub-humid at Tanga and sub-humid/semi-arid at Morogoro were selected. There were a total of eight locations within three eco-regions comprising two at Kilimanjaro (e.g., Kirima Boro and Xeno Helena), two at Tanga (Mlingano 1 and Mlingano 2) and four at Morogoro (Misu®ni 1, Misu®ni 2, Misu®ni 3, and Mindu). The soil management treatments consisted of farmyard manure (FYM), N and P fertilizer, tie-ridging and farmers' practice. Plant nutrient content was generally lowest on severely eroded and the highest on least eroded soil classes. Soil pH decreased with increasing severity of erosion on soils with higher content of Ca2
in the sub-surface. In general, there occurred a decline in soil organic carbon (SOC) and P with the decrease in TSD. The SOC content decreased on severely eroded soil class by 0.16%, 0.39% and 0.13% at Misu®ni 1, Mlingano 1 and Kirima Boro, respectively, compared to slightly or least eroded soil class. Corresponding decline in available P at these sites was 41%, 62% and 61%, respectively. Application of FYM signi®cantly increased soil pH at some sites. Soil content of SOC, N, P, K and Mg were signi®cantly increased by FYM application. Signi®cant effects of N and P fertilizers on SOC and P were observed at most sites. In comparison with farmer's practice, FYM application increased SOC by 0.55%, N by 0.03%, P by six-fold and K by two-fold. Nitrogen and phosphorus fertilizers had comparable effects for SOC and P only at some sites. The results indicate that FYM is a better soil input than N and P fertilizers in improving soil quality. The data show that SOC, N and P are most adversely affected with accelerated erosion and that FYM fertilizer applications have the potential to improve fertility of eroded soils.#1999 Elsevier Science B.V. All rights reserved.
Keywords:Soil degradation; Tropical soils; Maize; Cowpeas; Sub-Saharan Africa; Erosion and productivity
*Corresponding author. Tel.:1-614-292-2265; fax:1-614-292-7432
E-mail address: [email protected] (R. Lal)
1. Introduction
Soil erosion is a major threat to sustainable use of soil and water resources (Lal, 1998). The threat is more serious for the soils of the tropics that are highly susceptible to erosion and other degradative processes. Erosion in¯uences several soil properties, e.g., topsoil depth (TSD), soil organic carbon (SOC) content, nutrient status, soil texture and structure, available water holding capacity (AWC) and water transmis-sion characteristics that regulate soil quality and determine crop yield. Lal (1988) indicated that low levels of N, P, K, and low cation exchange capacity (CEC) are among the most important chemical and nutritional constraints accentuated by soil erosion. Soil erosion also decreases the AWC (Nizeyimana and Olson, 1988) and SOC content (Rhoton and Tyler, 1990) and increases soil bulk density (Frye et al., 1982).
Experiments conducted on Ultisols in Nigeria showed that maize (Zea mays L.) yield reductions were 95%, 95% and 100% with 5, 10 and 20 cm re-moval of TSD, respectively (Mbagwu et al., 1984). For the same TSD removals on an Al®sol, yield reductions were 31%, 74% and 94%, respectively. The corre-sponding yield reductions on an Al®sol at Ilora in Nigeria were 73%, 83% and 94%, respectively. In all cases no fertilizer combination was effective in restor-ing maize yield when TSD was reduced by 10 or 20 cm (Mbagwu et al., 1984). Experiments relating effects of natural erosion on crop yield have indicated that the effects are even more severe than that of arti®cial topsoil removal. Lal (1981) observed that over a ®ve-year period, the grain yield of maize and cowpeas (Vigna unguiculata L. Walp.) decreased at the rate of 9 and 0.7 kg Mgÿ1
of soil loss, respectively. In another experiment, Lal (1985) observed that maize yield was reduced 16 times more due to topsoil loss from natural erosion compared to mechanical topsoil removal. In some cases, soil quality degraded by erosion can be improved by judicious use of inputs and improved soil management practices. Gajri et al. (1994) observed that application of farmyard manure (FYM) improved AWC and root growth in soils with unstable structure and low SOC content. Conventional or no-tillage plus mulching improved the soil hydro-thermal regime, resulting greater root growth, nutrient uptake and grain yields of maize and wheat (Triticum
aestivumL.) on a Typic Hapludalf in India (Acharya and Sharma, 1994).
In Tanzania, accelerated erosion has occurred since the pre-colonial period, but the severity and magnitude of damage had not been adequately assessed. Most of the work done so far concentrated on assessment of the amount of soil loss. Ahn (1977) reported that the Kondoa and Uluguru mountains in central and eastern Tanzania, respectively, were severely affected by ero-sion. Ngatunga et al. (1984) observed that soil loss was greatest on bare fallow soil compared to plowed, mulched or natural grass conditions. Soil loss ranged from 38 to 88 Mg haÿ1
on 10% to 22% slope under bare fallow conditions as compared to 0.08 to 0.10 Mg haÿ1
under natural grass cover for the same slopes. Recent studies conducted on eight major agri-cultural soils in Kilimanjaro, Tanga and Morogoro ecological regions of Tanzania indicated that decrease in TSD adversely affected a number of soil properties including SOC, CEC, pH, total soil N (TSN), available P, AWC, and water saturation percentage (Kaihura et al., 1996). It was also observed that maize grain yield was positively and signi®cantly correlated with TSD, SOC, TSN, CEC, and AWC. Maize yield declined at 38.5, 55 and 87.7 kg cmÿ1
decrease in TSD in Kili-manjaro, Tanga and Morogoro eco-regions, respec-tively.
Despite numerous world wide reports on the mag-nitude and extent of soil erosion and its adverse effects on soil quality and crop yields, little research has been done in sub-Saharan Africa on soil management tech-niques to restore productivity of eroded soils. There-fore, this study was conducted to evaluate the potential of selected soil management practices on improving soil physical and chemical qualities of eroded soils in three eco-regions of Tanzania.
2. Materials and methods
2.1. Identification of erosion classes
The experiments were conducted on eight major agricultural soils with different degrees of erosion in three ecological regions in Tanzania. Eco-region char-acteristics, as described by De Pauw (1984), and antecedent soil properties for each soil type are pre-sented in Table 1. Recognizing that TSD can be
in¯uenced by genetic factors and severity of past soil erosion, sites were carefully selected within the same parent and landscape unit so that variation in TSD was mainly due to soil erosion. With this assumption, the severity of erosion (e.g., as determined by the depth of the A horizon) was assessed in 1992 using morpho-logical characteristics of the top 50 cm depth and by determining the boundary between the topsoil and the sub-soil. Surface stoniness, color, soil consistence, texture and TSD were used to characterize severity of erosion. The TSD ranges considered to re¯ect differ-enterosionclasses were:severelyeroded(<15 cm TSD); moderately eroded (16±20 cm TSD); slightly eroded (21±25 cm TSD); and least eroded (> 25 cm TSD). Details on ecological zone characteristics and criteria for establishment of erosion classes are presented in Kilasara et al. (1995a). Cowpeas and maize growth and yields were determined on each erosion class for each soil during short rains (from November through December) of 1992 for cowpeas and long rains (from March through July) of 1993 for maize (Kilasara et al., 1995b).
2.2. Soil management treatments
Soil management treatments were imposed on dif-ferent TSD classes in 1994. In order to alleviate erosion-induced constraints of low soil productivity (soil crusting, soil moisture stress and low soil ferti-lity) that in¯uence crop yield in three eco-regions, four soil management treatments imposed included:
1. Farmers' practice (FP) or control treatment involving shallow tillage in Tanga and Morogoro,
and hand hoeing to 10±15 cm depth and planting in Kilimanjaro.
2. Tie-ridging (TR) or a ridge-furrow system at 1 m interval with cross-ties for runoff control and water conservation.
3. Tie-ridging plus 20 Mg haÿ1
FYM on air dry bases (TRFYM).
4. Tie-ridging plus N and P fertilizers at recom-mended rates, i.e., 100 kg N haÿ1
13 kg P haÿ1 at Kirima Boro and Xeno Helena; 50 kg N18 kg P at Mlingano and 80 kg N36 kg P haÿ1
at Morogoro.
The recommended rate of FYM applications for most soils in Tanzania is 20 Mg haÿ1
, which is equiva-lent to application of 59 kg N and 16 kg P haÿ1
in Tanga and Morogoro, and 15 kg N and 2.5 kg P haÿ1 in Kilimanjaro. The FYM was applied and mixed with the soil prior to ridging. Urea and triple superpho-sphate were used as sources of N and P, respectively, at all sites. Phosphorus was applied at planting and N was applied in split applications comprising 1/3 at planting and 2/3 at knee height stage. Ridges were made 30 cm high, and with cross-ties of 20 cm height at 1 m interval using hand hoes. Maize was planted as test crop with variety `Kilima' for Kilimanjaro and `TMV-1' for Morogoro and Tanga eco-regions, respectively. Each erosion class constituted a block in which treatments were randomly distributed and replicated three times for Mlingano and Morogoro sites, while erosion class plots and treatments were randomly distributed in a 2 ha ®eld at Kilimanjaro. The TSD classes were not replicated as there were not enough plots in the same TSD class. Gross plot size was 4.5 m4.5 m and harvested area was 9 m2. Each
Table 1
Eco-regions, soil types and selected topsoil properties for each soil type at the beginning of the experiment in 1992
Eco-region Soil type Location pH SOC N Av-P
Taxonomy FAO (H2O) (g kg
ÿ1) (g kgÿ1) (mg kgÿ1)
Kilimanjaro (humid) Umbric Hapludalfs Humic Nitisols Kirima Boro 7.0 24 2.1 38.0
Umbric Hapludalfs Humic Nitisols Xeno Helena 6.0 20 1.4 7.6
Tanga (sub-humid) Tropeptic Haplustox Rhodic Ferralsols Mlingano 1 6.6 27 2.2 4.0
Typic Rhodustalfs Haplic Lixisol Mlingano 2 6.5 23 1.9 4.0
Morogoro Lithic Eutrochrepts Eutric Cambisols Misufini 1 6.5 12 1.1 <1.0
(sub-humid/ Typic Eutrochrepts Chromic Cambisols Misufini 2 6.6 11 1.2 1.0
semi-arid) Typic Rhodustalfs Chromic Luvisols Misufini 3 6.3 10 1.2 1.0
plot comprised six rows with 75 cm between and 30 cm within-row spacing. All plots were prepared using a hand hoe, and the same procedure was repeated for the second maize crop in 1995.
2.3. Soil sampling and analysis
Soil core samples were obtained from 0 to 10 cm depth in triplicate from each treatment to determine bulk density (b), and measure soil water retention at 0.01 and 1.5 MPa suctions (Cassel and Nielsen, 1986). The AWC was calculated as a difference between water content at 0.01 MPa and 1.5 MPa suctions. Composite topsoil samples from each treatment were obtained, air dried, ground, and sieved to pass through a 2 mm sieve. Samples were analyzed for pH in water and KCl solution (McLean, 1982), SOC (Nelson and Sommers, 1982), TSN (Bremner and Mulvany, 1982), available P (Watanabe and Olsen, 1965), CEC and exchangeable bases (Thomas, 1982). Soil samples were obtained after crop harvest.
2.4. Statistical analysis
The results obtained were analyzed according to the ANOVA procedure usingMSTATpackage (Nissen et al., 1994).
3. Results
3.1. Antecedent soil properties
The data on antecedent soil properties are presented in Table 1. According to soil fertility rating (Landon, 1991), soil pH was medium ranging from 5.8 to 7.0 being highest at Kirima Boro in Kilimanjaro eco-region and lowest at Mindu in Morogoro eco-eco-region. Soils of the Kilimanjaro eco-region (e.g., Kirima Boro
and Xeno Helena) are of volcanic origin developed in a humid climate. Therefore, these soils are character-ized by higher SOC content than those of the semi-arid regions. The SOC content, depending on the climate and the parent material, ranged from a low of about 10 g kgÿ1
in the Morogoro eco-region to 20±25 g kgÿ1
in the Tanga and Kilimanjaro eco-regions. The TSN was in the low range of <2 g kgÿ1
at all sites except for Kirima Boro and Mlingano 1 with 2.1 and 2.2 g kgÿ1
, respectively. Available phosphorus (Av-P) was very low for all sites in Tanga and Morogoro and high at Kirima Boro site. The data suggest that soil fertility is high, medium and low for soils in Kili-manjaro, Tanga and Morogoro eco-regions, respec-tively. In particular, P levels are low and very limiting to crop yields for Tanga and Morogoro soils.
3.2. Soil properties as affected by topsoil depth
Antecedent soil properties, prior to imposition of the soil management treatments, are shown in Table 1. Soil chemical quality was in the order Morogoro < Tanga < Kilimanjaro eco-regions. This ecological gra-dient in soil quality is especially true for Av-P, TSN and SOC content.
3.2.1. Misufini 1 Ð Morogoro eco-region
Soil pH ranged from 6.6 on severely eroded class to 6.4 on moderately eroded class (Table 2). The Av-P consistently declined with the increasing severity of erosion. The severely eroded class was also low in SOC and K
contents by 1.6 g kgÿ1
and 0.38 cmol kgÿ1
, respectively, compared to moderately eroded class. The TSN was low and remained almost constant for all erosion classes, and there was no clear trend with regard to erosion for exchangeable bases and soil bulk density.
Table 2
Effect of soil depth class on soil properties at Misufini 1 eco-regions
Depth class
<15 6.57 9.8 1.6 7.01 0.59 11.65 2.22 21.75 1.37
16±20 6.44 11.4 1.6 8.30 0.97 11.11 1.98 21.47 1.41
21±25 6.49 9.9 1.5 9.91 0.78 10.10 2.42 20.75 1.39
a<15: severely eroded; 16±20: moderately eroded; 21±25: slightly eroded.
3.2.2. Kirima Boro and Xeno Helena Ð Kilimanjaro eco-region
At Kirima Boro, soil pH was higher in severely and moderately eroded soils compared to the less eroded classes (Table 3). The SOC, TSN, Av-P, K
and Ca2 contents were highest in the least and slightly eroded classes and their concentrations declined with dec-reasing TSD. Thebdecreased with decreasing sever-ity of erosion. There was no consistent trend in Mg2 content with regard to the severity of erosion. A similar trend was observed at Xeno Helena for most nutrients tested. Unlike at Kirima Boro, Mg2
content decreased with erosion severity, andbwas not affected.
3.2.3. Mlingano 1 and 2 sites Ð Tanga eco-region Soil pH was not much affected by erosion severity at Mlingano 1 but was lowest on severely eroded class at Mlingano 2 (Table 4). Average nutrient content was higher in least and slightly eroded classes compared to severely and moderately eroded classes. At Mlingano 2, the severely eroded class contained the lowest content of all soil nutrients. The severely eroded class was lower by 3.2 g kgÿ1
in SOC, 0.4 g kgÿ1 in TSN and 2.42 mg kgÿ1
for Av-P compared to the slightly eroded class. Exchangeable bases decreased consis-tently with increasing severity of erosion. Thebwas the highest in the severely eroded class.
3.3. Soil chemical properties as affected by soil management
3.3.1. Misufini 1 Ð Morogoro eco-region
The application of FYM signi®cantly increased Av-P, TSN, K
, and Mg2
contents at Misu®ni 1 (Table 5). The effects of N and P fertilizer application
were not signi®cant on soil nutrient contents. Theb also signi®cantly decreased with FYM application (Table 5).
3.3.2. Kirima Boro and Xeno Helena Ð Kilimanjaro eco-region
Application of FYM signi®cantly increased pH by 0.34 units while N and P fertilizer application slightly decreased soil pH (Table 6). Increase in Mg2
and K
contents were affected by FYM applications only at Kirima Boro. At Xeno Helena, a signi®cant increase in SOC and Av-P was observed with both treatments. The increase in TSN and Av-P was affected by FYM application only. There were no signi®cant effects on
bdue to soil management at Xeno Helena. Applica-tion of P fertilizer was a better source of P for both sites than was FYM application.
3.3.3. Mlingano 1 and 2 in Tanga eco-region At Mlingano 2, FYM application signi®cantly increased pH but applications of N and P fertilizers slightly reduced pH (Table 7). Both treatments sig-ni®cantly increased soil contents of Av-P, K
and CEC at Mlingano 1 compared to the FP. Increase in SOC, Ca2
and Mg2
occurred due to FYM application only. At Mlingano 2, only Av-P content was signi®-cantly increased by both FYM and fertilizer treat-ments. The SOC, TSN, K
, Ca2
and Mg2 were increased by FYM application only at Mlingano 2. There were no signi®cant effects of any treatments on CEC of the soil.
3.4. Soil physical properties
Effects of TSD and management for two sites on soil physical properties are shown in Table 8.
Table 3
Effect of soil depth on soil properties at Kirima Boro and Xeno Helena eco-region sites, where ND is not determined
Site Depth
Kirima Boro <15 6.13 24.0 1.6 29.75 1.71 11.29 0.57 1.18
16±20 6.16 25.1 1.6 32.17 1.96 11.02 0.53 1.21
21±25 5.96 25.3 1.7 41.67 1.56 12.48 0.56 1.24
>25 5.89 24.9 1.8 47.75 1.81 13.05 0.57 1.24
Xeno Helena <15 5.87 21.9 2.0 33.08 ND 6.79 0.54 1.09
16±20 5.96 22.0 2.3 34.08 ND 7.73 0.57 1.08
Table 4
Effect of soil depth on soil properties at Mlingano 1 and Mlingano 2 eco-region sites where ND is not determined
Site Depth
class (cm) pH (H2O)
SOC (g kgÿ1)
N (g kgÿ1)
Av-P (mg kgÿ1)
K
(cmol kgÿ1)
Ca2
(cmol kgÿ1)
Mg2
(cmol kgÿ1)
CEC (Mg mÿ3)
b
(Mg mÿ3)
Mlingano 1 <15 6.05 18.9 1.9 2.42 1.36 7.09 2.56 16.10 ND
16±20 6.05 20.0 2.0 1.99 1.20 6.83 2.56 15.14 ND
21±25 5.99 22.8 2.2 3.37 1.43 8.95 2.85 18.58 ND
>25 5.95 21.5 2.0 3.93 1.30 7.70 3.64 19.32 ND
Mlingano 2 <15 6.30 22.1 1.7 5.63 1.00 7.72 3.93 17.33 1.16
16±20 6.46 24.5 2.1 5.76 1.25 8.86 4.27 18.35 1.10
21±25 6.32 25.3 2.1 8.05 1.30 8.17 4.07 17.75 1.14
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3.4.1. Effect of topsoil depth
There were minor effects of TSD on b and the moisture retention capacity of the soils of Kirima Boro. These effects at Misifuni 1 were inconsistent among TSDs. The middle depth of 15±20 cm showed slightly higher values of b and moisture content at saturation and 1.5 MPa suction.
3.4.2. Effect of management practices
Application of FYM decreasedbfor both Misu®ni and Kirima Boro soils (Table 8). Fertilizer application, although increased the crop yield, did not show any effect on b. The moisture retention at 1.5 MPa was signi®cantly higher with FYM application as com-pared to FP. No such effect was observed in moisture retention at saturation.
3.5. Maize yield
Effects of soil management practices on maize grain yield for eight sites are shown in Table 9. Application of FYM and fertilizer increased maize grain yield by 1732 and 1353 kg haÿ1
, respectively, for all sites. The relative increase in maize grain yield by the application of FYM in comparison with the control treatment was most pronounced for the Kili-manjaro eco-region where the increase in yield was 2830 kg haÿ1
for the Kirima Boro site and 2346 kg haÿ1
for the Xeno Helena site. Because of the favorable soil moisture regime, nutrient use ef®-ciency may have been higher for the Kilimanjaro compared with other eco-regions. Application of FYM increased yield by 915 to 1851 kg haÿ1
for other two eco-regions. The higher nutrient use ef®ciency for
Table 5
Effect of soil management on soil chemical properties at Misufini 1 eco-region where NS is non-significant
Treatment pH
FP 6.28 0.94 0.15 2.68 0.46 10.79 2.04 20.62 1.46
TR 6.51 0.94 0.15 4.40 0.48 11.24 2.13 21.23 1.38
TRFYM 6.63 1.33 0.18 19.84 1.48 10.87 2.53 22.37 1.32
TRNP 6.59 0.94 0.14 6.70 0.71 10.91 2.13 21.07 1.39
LSD (5%) NS NS 0.001c 4.35c 0.31c NS 0.17b NS 0.06a
aSignificant at 5% level of probability.
bSignificant at 1% level of probability.
cSignificant at 0.1% level of probability.
Table 6
Effect of soil management on soil properties at Kirima Boro and Xeno Helena where ND is not determined and NA is not applicable
Site Treatment pH
Kirima Boro FP 5.93 2.07 0.16 27.83 1.52 11.96 0.53 1.22
TR 6.12 2.24 0.16 30.25 1.53 12.21 0.50 1.21
TRFYM 6.27 2.99 0.18 44.33 2.41 11.91 0.72 1.18
TRNP 5.81 2.63 0.18 48.92 1.58 11.77 0.48 1.26
LSD (5%) 0.23b 0.20c 0.03a 5.18c 0.43c NS 0.04c 0.03b
Xeno Helena FP 5.62 1.86 0.20 23.22 ND 6.16 0.38 1.11
TR 5.76 1.97 0.18 31.56 ND 7.42 0.42 1.08
TRFYM 6.19 2.63 0.26 40.44 ND 8.59 1.00 1.08
TRNP 5.83 2.31 0.20 44.89 ND 6.99 0.48 1.10
LSD (5%) 0.23c 0.23c 0.05b 4.15c NA NS 0.16c NS
aSignificant at 5% level of probability.
Table 7
Effect of soil management on soil chemical properties at Mlingano 1 and Mlingano 2
Site Treatment pH
(H2O)
SOC (g kgÿ1)
N (g kgÿ1)
Av-P (mg kgÿ1)
K
(cmol kgÿ1)
Ca2
(cmol kgÿ1)
Mg2
(cmol kgÿ1)
CEC b
(Mg mÿ3)
Mlin-gano 1 FP 5.88 2.01 0.20 1.07 0.96 7.32 2.64 16.67 ND
TR 6.00 2.07 0.20 1.48 1.08 7.62 2.93 16.93 ND
TRFYM 6.22 2.32 0.22 6.45 1.54 8.09 3.31 17.61 ND
TRNP 5.95 1.93 0.19 2.71 1.73 7.54 2.73 17.95 ND
LSD (5%) NS 0.03c NS 1.17c 0.31b 0.28b 0.33a 0.75a ND
Mlin-gano 2 FP 6.26 2.27 0.18 0.86 0.93 7.19 3.52 15.91 1.11
TR 6.24 2.40 0.19 2.69 1.09 7.58 3.83 17.13 1.13
TRFYM 6.74 2.62 0.22 14.80 1.93 10.55 5.18 20.65 1.15
TRNP 6.19 2.30 0.19 7.56 0.78 7.67 3.82 17.55 1.14
LSD (5%) 0.07c 0.24a 0.03a 4.05c 0.24c 1.40b 0.84a NS NS
aSignificant at 5% level of probability.
bSignificant at 1% level of probability.
cSignificant at 0.1% level of probability.
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the Kilimanjaro eco-regions was also evident by the higher increase in yield due to application of fertilizer for this eco-region. In comparison with the control treatment, increase in yield was 2931 kg haÿ1
for the Kirima Boro site and 2686 kg haÿ1
for the Xeno Helena site. Increase in maize grain yield with ferti-lizer application over the control treatment was 581 to 1187 kg haÿ1
for other eco-regions. Therefore, soils of the Kilimanjaro eco-region respond more to use of best management practices than those of other eco-regions.
4. Discussion
4.1. Effect of topsoil depth on soil properties
Soil pH increased with severity of erosion at Mis-u®ni 1, Kirima Boro and Mlingano 1 sites. At MisMis-u®ni
1, accelerated erosion was also associated with increase in exchangeable Ca2
and Mg2
contents. Erosion appears to expose the sub-surface material containing bases that also increased soil pH at this site. Increase in soil pH with severity of erosion was also reported by Cihacek and Swan (1994), where soil erosion exposed the CaCO3 rich material that increased soil pH. At the other two sites leaching or illuviation may have concentrated bases in sub-soil at Kirima Boro and Mlingano 1 such that erosion exposed sub-soil bases and increased pH. The decrease in soil pH at Mlingano 2 on eroded soils may be due to exposure of acidic sub-soil, and decline in soil fertility as is evidenced by decreases in other nutrients on the severely eroded class. A similar decline in soil pH with erosion was reported by Lal (1981) on Al®sols in Nigeria. Similarly, Manu et al. (1996), observed decline in soil pH as erosion exposed the acidic, Al-rich and P de®cient sub-soil for an
Table 8
Physical properties of soils at selected sites in three eco-regions of Tanzania as affected by TSD and management practices where SE is standard error
Depth Misufini 1 (Morogoro) Kirima Boro (Kilimanjaro)
b(Mg m
ÿ3) Moisture content (%)
b(Mg m
ÿ3) Moisture content (%)
Saturation 1.5 MPa Saturation 1.5 MPa
<15 1.37 35.6 9.0 1.18 33.8 19.0
15±20 1.41 37.6 9.9 1.21 33.2 18.9
21±25 1.39 35.8 10.2 1.24 33.4 19.0
FP 1.40 36.9 9.4 1.22 32.8 18.6
TR 1.40 36.5 9.4 1.21 33.5 19.0
TRFYM 1.40 36.9 10.5 1.18 32.5 19.3
TRNP 1.40 35.0 9.3 1.26 32.9 19.2
SE 0.02a 1.9a 0.2a 0.01 0.46 0.18
aSignificant at 5% level of probability.
Table 9
Effect of soil management on maize grain yield (kg haÿ1) for eight locations in Tanzania
Misufini 1 Misufini 2 Misufini 3 Mindu Kirima Boro Xeno Helena Mlingano 1 Mlingano 2
Control 3613 4361 3947 3103 1974 3499 1993 2411
Ridges 3925 4024 4241 3584 2170 2954 1915 2933
RidgesFYM 5454 5317 5759 4840 4804 5448 3384 3361
RidgesNP 4736 4942 4605 3766 4905 6185 2993 3598
SE 180 271 130 206 80 170 171 248
Aridisol in Niger. The observed decline in pH with erosion at Xeno Helena and Mlingano 2 may be associated with similar processes. The impact of ero-sion on soil pH is likely to be in¯uenced by the land use, and speci®c properties and processes associated with each soil type.
Soil nutrient content declined with decrease in TSD with lowest concentrations on severely eroded classes and highest concentrations on slightly and least eroded classes at all sites. Consistent decrease was observed for SOC, TSN, Av-P, and K
contents at all sites, and decrease in only Ca2
, Mg2
and CEC for sites in Tanga. A very sharp and consistent decline was observed for SOC and Av-P suggesting that these nutrients are drastically in¯uenced by soil erosion. Since these soils have inherently low to medium contents of the major plant nutrients (Table 1), erosion effects on productivity are very severe. Pimentel et al. (1995) observed that soil erosion causes loss of basic plant nutrients such as N, P, K
and Ca2
and that water erosion selectively removes the ®ne organic particles leaving large particles and stones on the surface. Lal (1988) pointed out that progressive soil erosion increases the magnitude of soil-related con-straints to production. The concon-straints can be physical, chemical or biological. Among important physical constraints are reduced TSD and loss of AWC. Soil chemical constraints and nutritional disorders related to erosion include low CEC, de®ciency of major plant nutrients (N, P, K) and trace elements (Zn, S), nutrient toxicity (Al, Mn) and high soil acidity (Lal, 1981,1998). On the other hand, biological constraints include low microbial biomass carbon and reduced activity of soil macrofauna (Lal, 1991). A sharp decrease in plant nutrients in the severely eroded class at Mlingano 2 indicates that most nutrients are con-centrated in the surface soil, which makes quality of these soils highly sensitive to accelerated erosion. The
b increased with increasing severity of erosion at Mlingano 2 and decreased at Misu®ni 1. The increase inbwith erosion may be due to decrease in aggrega-tion of soil particles because of decline in SOC content that also reduces the microbial activities in the eroded soils. Rhoton and Tyler (1990) reported increase inb with erosion and associated this with decrease in TSD to fragipan and decrease in SOC content. The results for Misu®ni 1, are contrary to observations made by Rhoton and Tyler (1990).
4.2. Effect of soil management on soil properties
Application of FYM signi®cantly increased soil pH at both sites in Kilimanjaro and Mlingano 2 in Tanga but no effects at other sites. Applications of N and P fertilizers had no effects on soil pH. The increase in soil pH following FYM application needs further investigation. Soil contents of SOC, TSN, Av-P, K
and Mg2
at Kirima Boro site were signi®cantly increased by FYM application. In contrast, application of FYM decreased b for the Kirima Boro site. Applications of N and P fertilizers were only effective for increasing SOC and Av-P at most sites. Increase in soil chemical quality with FYM application can be explained by its potential to release CO2;
NH 4; NO
ÿ 3; PO
ÿ3
4 and undecomposed humic pro-ducts to the soil through mineralization (Stevenson, 1994). In this process FYM also contributes to CEC, which increased signi®cantly at some sites. Meelu (1981) reported that application of 12 Mg haÿ1 FYM produced a residual effect equivalent to 30 kg of N and 13 kg of P to the succeeding crop. The FYM applied in this study was equivalent to an average of 37 kg N and 9 kg P across sites. This amount was much lower compared to an average of 77 kg N haÿ1 and 22 kg P added through inorganic fertilizers. John-son (1986) attributed some effects of FYM to improvements in AWC and availability of N in ways that cannot be mimicked by application of N fertili-zers. These observations support the ®ndings of this study that FYM is overall more effective in restoring productivity than fertilizers. The bwas also signi®-cantly reduced by FYM application. Decrease in b may be due to simple physical effect of mixing organic matter in the mineral fraction or formation of stable aggregates, which in turn improve water retention, permits gas exchange and improve permeability. Simi-lar effects of FYM on soil quality were reported by Hussain et al. (1988), Tegene, (1992) and Herrick and Lal (1995).
5. Conclusions
The data support the following conclusions:
Severely eroded phases contained the least amount of plant nutrients, the lowest soil pH, and SOC content.
Application of FYM increased soil pH, SOC, and plant available nutrients, and enhanced soil quality.
Application of N and P fertilizers also increase SOC content for some sites.
Maize grain yield was significantly improved by application of FYM and fertilizer for all sites.
Adverse effects of severe erosion on soil quality and crop yield can be mitigated through application of FYM and judicious use of chemical fertilizers.
Acknowledgements
The authors thank the Research Council of Norway for ®nancial support to carry out this research in Tanzania.
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