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TECHNICAL REPORTS
Plant and Environment Interactions

Soil Chemical Changes under Irrigated Mango Production in the Central São Francisco River Valley, Brazil

^a Dep. of Land Resource Science, Univ. of Guelph, Guelph, ON, Canada
^b Dep. of Soil Science, Univ. of Saskatchewan, Saskatoon, SK, Canada
^c Dep. of Agronomy, Federal Rural Univ. of Pernambuco, Recife, Brazil
^d Dep. of Nuclear Energy, Federal Univ. of Pernambuco, Recife, Brazil

^* Corresponding author (rheck{at}lrs.uoguelph.ca)

Received for publication April 15, 2002.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Irrigated areas in Brazil's Central São Francisco River Valley have experienced declines in productivity, which may be a reflection of changes in soil chemical properties due to management. This study was conducted to compare the chemical composition of soil solutions and cation exchange complexes in a five-year-old grove of irrigated mango (Mangifera indica L. cv. Tommy Atkins) with that of an adjacent clearing in the native caatinga vegetation. A detailed physiographic characterization of the area revealed a subsurface rock layer, which was more undulating than the current land surface, and identified the presence of a very saline and sodic (1045 µS cm^-1, sodium adsorption ratio [SAR] = 5.19) ground water table. While changes in concentrations of Ca, Mg, and K could be attributed to direct management inputs (fertilization and liming with dolomite), increases in Na suggested average annual capillary rise from the ground water table of 28 L m^-2. Accordingly, soil salinity levels appeared to be more dependent on surface elevation than the elevation of the rock layer or sediment thickness. The apparent influence of land surface curvature on water redistribution and the solution chemistry was more pronounced under irrigated mango production. In general, salinity levels had doubled in the mango grove and nearly tripled under the canopies, after only five years of irrigation. Though critical saline or sodic conditions were not encountered, the changes observed indicate a need for more adequate monitoring and management of water and salt inputs despite the excellent water quality of the São Francisco River.

Abbreviations: EC, electrical conductivity • SAR, sodium adsorption ratio

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
THE SãO FRANCISCO RIVER VALLEY is the main irrigation area in the semiarid region of northeastern Brazil. In 1988, the valley contained about 16 682 km² of irrigable land (Merlo, 1992). Large-scale irrigation near Petrolina, in Pernambuco state, began with the construction of the Sobradinho Dam in 1977. The government-coordinated Senador Nilo Coelho project, occupying 19 593 ha, remains the largest irrigation area in the region (Companhia de Desenvolvimento do Vale do São Francisco, 1976). Much of the area is composed of settler plots of about 6 ha, on which a large variety of fruit and vegetable crops are cultivated. Mango is one of the major tree fruit crops grown.

Even though the São Francisco River water is of very good quality for irrigation (classified as C1S1, according to Pereira and Cordeiro, 1987), several of the older irrigated areas along the São Francisco River have experienced declines in production with some being abandoned because of increased salinity. The soils in this region are developed on a post-Cretaceous sandy-to-clayey pediment covering pre-Cambrian magmatic and metamorphic rocks (Gomes, 1990). An estimated 46% of the soils in the Nilo Coelho Project area have restricted internal drainage (Companhia de Desenvolvimento do Vale do São Francisco, 1976), making them more susceptible to salination under irrigation. Increases in salinity have been noted for Oxisols (Pereira and Siqueira, 1979) and Vertisols (Pereira and Cordeiro, 1987) in the region, following the introduction of irrigation. According to Magalhães (1995), economic losses due to soil salination–sodification have been observed in about 20% of the irrigated areas coordinated by DNOCS (federal agency for drought mitigation) in northeastern Brazil.

Ultisols occupy approximately 62% of the Nilo Coelho Project area (Companhia de Desenvolvimento do Vale do São Francisco, 1976). These soils are characterized by a marked difference in texture from the A to B horizons, and are variable in color, thickness, texture, drainage, stoniness, and relief (Burgos et al., 1998). Their major limitations for irrigated production are low natural fertility and impeded drainage often due to a fragipan. About 45% are considered well suited for mango production; another 46% were moderately suited, but require artificial drainage. The local history of salination reveals the need for monitoring soil changes, and adapting management practices, before degradation advances to a point where reclamation or land abandonment becomes necessary.

The influence of irrigated grape (Vitis vinifera L.) production on chemical and physico–chemical characteristics of these Ultisols has been investigated (Heck et al., 2002). The present work examined how the management practices such as tree spacing, irrigation, and chemical inputs associated with irrigated mango production have altered the spatial variability of chemical properties of the solution and exchange complex of these soils. This was evaluated in the context of short-range physiographic characteristics, which can influence water redistribution and, therefore, soil development (Pennock et al., 1987; Pennock and de Jong, 1991). Understanding the factors influencing the variability of soil chemical composition and the mobility of salts is fundamental to developing suitable soil monitoring and management schemes. The objective is to document incipient changes that potentially lead to more severe salination and production constraints.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Study Site Description
The area selected for this study is located in Lot #1473 of Nucleus 10, Sector 3 of the Nilo Coelho Irrigation Project (9°18' S, 40°25' W) in the Municipality of Petrolina, Pernambuco state, Brazil (Fig. 1) . The climate is Köppen BSwh, very hot (mean annual temperature 26°C) and semiarid, with most of the annual precipitation (400 mm) falling during the winter season and an annual potential evapotranspiration of about 1400 mm (Jacomine et al., 1973; Companhia de Desenvolvimento do Vale do São Francisco, 1976). The soil is classified as allic, isohyperthermic Plinthustult (Soil Survey Staff, 1998) or Argisolo Amarelo Distrófico arênico (Empresa Brasileira De Pesquisa Agropecária, 1999). Selected physical characteristics of the soil are presented in Table 1 . At the time of sampling (April 1994), the area contained a 5-yr-old grove of irrigated mango (Mangifera indica L. cv. Tommy Atkins), spaced 5 x 7 m, adjacent to a recent clearing in the native hyperxerophyllic "caatinga" vegetation. The soil surrounding the mango trees was maintained free of other vegetation.

View larger version (43K): [in this window] [in a new window]   Fig. 1. Map of Pernambuco state in northeastern Brazil, showing the location of the Nilo Coelho study site beside the São Francisco River.

View this table: [in this window] [in a new window]   Table 1. Selected physical characteristics of the studied soil.

Sample Collection and Analysis
Before sampling, a 12 x 12 regular grid pattern was demarcated: 58% of the grid was located in the mango grove and 42% in the adjacent clearing. The internodal spacing was 10 m; therefore, the distances to mango tree trunks were variable: 6 nodes were within 1 m, 24 were between 1 and 2 m, 12 between 2 and 3 m, and 18 beyond 3 m. A theodolite was used to measure the surface elevation of each node, relative to a local datum (lowest point measured). The depth to the underlying rock layer was also determined through auger holes at each node point. Using the computer program TOPO (Pennock, 1993), the following primary landscape attributes were calculated for each 3 x 3 array of nodes: slope and its derivative (profile curvature), as well as the aspect and its derivative (plan curvature). Surface elevations were obtained for each node; the lowest point was taken as the local datum. Contour maps were generated by the "moving weighted least squares" gridding method of the software MacGRIDZO (Rockware, 1990).

Soil was collected at each of the central 100 nodes, from three depths: 0 to 0.3, 0.3 to 0.6, and 0.6 to 0.9 m, using a 9-cm-diameter cup auger. These depths correspond to Ap, BA, and Bt1 horizons (Table 1). Samples of the irrigation water and ground water were also obtained for analysis. Following gravimetric determination of field moisture, the soil samples were air-dried and crushed to pass a 2-mm sieve. Electrical conductivity of water extracts (1:1 soil to water [EC_1:1] and saturated paste [EC_e]) and the pH of 1:1 extracts (1 M KCl and water) were determined by direct potentiometry. The concentration of cations in saturated pastes extract and 1 M NH₄Cl solution (1:10 ratio) extracts were determined by atomic emission (Na and K) or absorption (Mg and Ca) spectroscopy. Sodium adsorption ratios (SARs) were calculated from cation concentrations in mmol L^-1 according to the equation:

[1]

Statistical Analysis
Statistical analysis of differences was conducted using the nonparametric tests Wilcoxon signed-rank (for comparison of values between horizons), Mann–Whitney U (for comparison of values in groupings of values based on subarea, distance from mango trees, and surface curvature), and Kruskal–Wallis (for comparison of groupings of values based on distance to the collector canals and quartiles of other properties) of the software Statview SE + Graphics (Abacus Concepts, 1988). The Spearman rank correlation test was used to obtain correlation coefficients (r) between groups of data. Unless specifically indicated, the level of statistical probability was less than 5%. Less rigorous probability levels (up to 20%) have been used in landscape-based studies (Pennock et al., 1994; Corre et al., 1996; Beckie et al., 1997).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Local Physiographical Characterization
The land surface of the study area slopes generally southward, toward the river, at approximately 1% (Fig. 2) . According to the primary landscape attributes calculated with TOPO, most landscape elements exhibited a southward aspect at the 10-m scale. Potential convergence of surface water flow was predicted in some elements: 32 of the 60 points examined in the mango plantation exhibited a concave plan-curvature and 34 concave profile-curvature; in the clearing, 17 of the 37 points analyzed had concave plan-curvature and 21 concave profile-curvature. The underlying rock layer also drains toward the river (Fig. 3) ; the slopes are, however, about twice as large and convergence of water flow is expected.

View larger version (42K): [in this window] [in a new window]   Fig. 2. Surface elevation map of the Nilo Coelho study site. The lowest relative elevation measured was taken as the local datum. Arrows indicate slope direction calculated from the digital elevation model by the program TOPO (Pennock, 1993).

View larger version (27K): [in this window] [in a new window]   Fig. 3. Elevation map of the subsurface rock layer at the Nilo Coelho study site. The local datum is the same as in Fig. 2.

The dolomitic lime commonly applied to soils in Pernambuco state contains about 18% Ca and 12% Mg (Cavalcanti, 1998), the superphosphate contains 18% Ca, and the potassium chloride contains 48% K. Thus, the initial liming added 1.1 mol m^-2 of Ca and 1.2 mol m^-2 of Mg. Subsequent localized amendments (under canopy) have also contributed 11.2 mol Ca and 12.3 mol of Mg (both from lime), 33.7 mol of Ca (from superphosphate), and 30.7 mol of K (from potassium chloride). The urea applied in the same area has contributed an acid equivalent of 3.4 kg of CaCO₃ (Cavalcanti, 1998), which corresponds to about 3.1 kg of dolomite. This is about 50% more than that applied as dolomitic lime (which, according to the Ca assay, contains a maximum of 82% dolomite) under the canopies.

Soil Moisture
The absolute moisture content at sampling time was smaller near the surface for both subareas (Table 3) , attributable to the lack of recent rainfall or irrigation as well as the textural gradient through the profile (Table 1; Santos, unpublished data, 2002). Moisture levels in the Ap horizon did not reflect surface elevation within either subarea, but significantly more moisture was present in the B horizons for the lowest quartile of points in both subareas, suggesting lateral redistribution of water inputs. No effect of surface curvature on moisture content was found in the clearing; under the mango trees, however, subsurface horizons contained more moisture in landscape elements with concave than convex curvature, indicating an enhancement of lateral redistribution under irrigation. Moisture levels in the Bt1 horizon were also notably higher for points where the rock layer was the deepest (compare Fig. 3 and 4) , reflecting the region of subsurface water coalescence. No relationship was observed between the moisture content of any horizon and sediment thickness.

View larger version (29K): [in this window] [in a new window]   Fig. 4. Ratio of actual moisture to field capacity of the Bt1 horizon at the Nilo Coelho study site at time of sampling. Values above 100% indicated the water potential was above 0.3 MPa.

Ammonium Chloride–Extractable Basic Cations
For the most part, the concentrations of basic cations in the NH₄Cl extracts were significantly larger in the mango grove than in the clearing (Table 4) . Based on the bulk densities available for each horizon (Table 1) and the average extractable cation concentrations to a 0.9-m depth (Table 4), the soil under the mango trees contained 0.8 mol m^-2 of Na, 4.1 mol m^-2 of K, 32.4 mol m^-2 of Ca, and 20.6 mol m^-2 of Mg more than in the clearing. Of these changes, only the increase in Na concentration exceeded known production inputs. Assuming that the balance of the Na increase came from the saline ground water (Table 2), this corresponds to about 140 L m^-2 of capillary rise since the introduction of mango. On an annual basis (28 L m^-2 or 28 mm), this is approximately half of that estimated for heavier-textured soils under irrigated grape in the same region (Heck et al., 2002).

View this table: [in this window] [in a new window]   Table 4. Average composition of 1 M NH4Cl-extractable cations according to subarea and distance to mango trees.

No relationship between cation concentrations and elevation was observed in the clearing, but under the mango trees Na levels were significantly smaller in the Ap and larger in the Bt1 horizon for points at higher elevations. This is consistent with the behavior described previously for moisture; whereas good internal drainage at higher points favors leaching of the Ap horizon, lateral redistribution decreases the volume of water moving through the profile at higher points and increases it at lower points in the landscape. Under the mango trees, levels of Na in the Ap horizon, as well as Ca and Mg in the Bt1, were also significantly higher in areas where the rock layer was lowest, reflecting the coalescence of subsurface moisture and associated capillary rise. Moreover, where the sediment was thinnest under the mango trees, there was significantly more Na in the Bt1 horizon. Cation concentrations in most horizons did not reflect surface curvature; however, lower K and Ca levels were found in the Bt1 horizons within elements with concave profile curvature.

Salinity
As indicated by the EC of 1:1 extracts (Table 5) , the Ap horizon contained significantly more soluble salts than the underlying B horizons in both subareas; such a profile is typical of a moisture regime dominated by capillary rise or of the surface application of salts. Salinity levels of all horizons were also significantly higher under the mango trees than in the clearing (Table 5), probably reflecting production inputs. Though the average EC_e of the Ap horizon was less than 2 dS m^-1 (Table 6) , and still considered nonsaline, two sampling points (corresponding to 5% of the subarea) under the mango trees had exceeded this criteria.

The predominant lack of difference in salinity of the Ap horizon, as a function of distance to the mango trees, was reflected by the EC of both water extracts and the concentration of soluble cations (Table 5 and 6). In the Bt1 horizon salinity levels were higher closer to the trees (Table 5), which is consistent with the trend observed with the NH₄Cl-extractable cations (Table 4). The saturated paste extracts did not exhibit such regular compositional gradients (Table 6), but significant differences in cation concentrations were observed with distance from the trees. Though the lateral gradients in the Ap horizon were not as clear as for the Bt1, the EC values of 1:1 extracts from the two horizons were more strongly correlated under the mango trees (r = 0.45**) than in the clearing (r = 0.31*, where * indicates significance at the 0.05 probability level), suggesting a greater linkage of solute dynamics in the two horizons.

No significant effect of surface elevation, rock layer elevation, or sediment thickness was observed for EC_1:1 in either subarea. The composition of saturated paste extracts also did not reflect elevation under the mango trees, but in the clearing larger EC_e and Ca concentrations occurred in the Ap horizon at lower points. In contrast, while K concentrations of both horizons were larger where the rock layer was higher under the mango trees, no such differences were observed in the clearing. Both subareas did, however, demonstrate significantly higher Na concentrations where the sediment was thickest. Though profile curvature did not exert an effect on gross salinity levels (EC_1:1 and EC_e) in either subarea, higher K and lower bivalent cation concentrations occurred in the Bt1 horizon of concave elements in the clearing. Landscape elements under the mango trees with concave plan curvature did demonstrate significantly lower salinity levels in A and B horizons as well as lower K and bivalent cations in the Ap.

Acidity
Companhia de Desenvolvimento do Vale do São Francisco (1976) reports that these soils are naturally very acid with pH in KCl of 4.0 for A, 3.9 for BA, and 3.7 for Bt1 horizons. Measurements of pH in KCl solutions showed similar results in the clearing, including increased acidity with depth, but significantly higher values under the mango trees (Table 5). The pH (pH in water - pH in KCl solution), which can be interpreted to reflect exchangeable acidity, was also significantly lower under the mango trees, especially in the Ap horizons. Acidity levels in the Ap and BA horizons were relatively uniform with proximity to the mango trees, but values in the Bt1 suggest less efficient neutralization at distance. These tendencies in acidity can be attributed to preferential application of lime under the mango tree canopies. No relation was found between land surface curvature and acidity (at p = 0.1); in the clearing, however, the B horizons had lower pHs where the elevation was the lowest and the sediment the thickest. Overall, the coefficient of variation of exchangeable acidity (pH) in the Ap horizon was greater in the mango grove, but the correlation between pH in the surface and Bt1 horizons was considerably stronger (r = 0.45** vs. -0.10 in the clearing).

Sodicity
Despite the similarity of Na to K ratios with depth found in NH₄Cl extracts from the clearing (Table 4), soluble Na to K ratios were significantly lower near the surface (Table 6). In contrast, both extracts from under the mango trees exhibited increasing ratios with depth, as well as significantly higher values for the Bt1 horizon and lower values in overlying horizons compared with the clearing. This behavior can be attributed to the enhanced leaching to depth of Na in response to the application of K-bearing fertilizer and irrigation. The higher ratios observed closer to the mango trees (<1 m), especially in the Bt1 horizon (Tables 4 and 6), were probably due to transport of Na during water extraction by the plant roots. The solution ratio of Na to K was highly variable and not correlated between the Ap and Bt1 horizons in either subarea.

The SAR of the saturation extracts was generally low throughout the study area (Table 6); no sodic (SAR > 13) points were encountered. Though no significant difference was observed between horizons in the clearing, SAR values under the mango trees were significantly smaller in the Ap than the Bt1. Ratios in the Ap horizon were also significantly lower in the mango grove than in the clearing. Aside from the stimulated mobility of Na, mentioned above, these changes in SAR can be attributed to addition of Ca and Mg in the dolomitic lime. As with the changes in acidity and Na to K ratios, SAR values of the Bt1 horizon reflected the application of lime under the canopy. Like the total salinity levels, SAR values were more variable under the mango trees, but the correlation between Ap and Bt1 horizons was stronger than in the clearing (r = 0.41** vs. r = 0.22 at p = 0.09).

Surface elevation influenced sodium ratios in both subareas: higher SAR for both horizons at higher points in the clearing, as well as smaller Na to K ratios (both extracts) and SAR for the Ap horizon under the mango trees. These results are consistent with the significant influences observed for individual cations in the two extracts. No influence of rock layer elevation or sediment thickness was observed for either Na to K ratio or SAR in the clearing. Under the mango trees, however, larger Na to K ratios were observed in the Ap and Bt1 horizons, where the rock layer was lower, and in the Bt1 where the sediment was thickest. Surface curvature did not exert any influence on Na to K ratio and SAR in the clearing (p = 0.9), but significantly higher ratios (both extracts) were observed for the Ap horizons in elements with concave profile curvature, as well as for SAR in the Ap with concave plan curvature. This is consistent with the preferential losses of K and bivalent cations noted above.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
While the introduction of irrigated mango production onto these Plinthustults has led to an increase in salinity and a slight decrease in acidity, neither saline nor sodic conditions have been reached. Of particular importance, however, were the elevated levels of salinity and sodicity in the B horizons closer to the mango trees, attributable to localized applications of fertilizers, capillary rise from the ground water table, and salt exclusion in the mango root zone. To ensure long-term sustainability of mango production on these soils, there is a need for the implementation of regular monitoring of salinity, as well as the adjustment of fertilization and irrigation processes to minimize further degradation. Particular attention must be given to the local variations in surface topography, which apparently has a stronger influence on salt redistribution than either the elevation or depth to rock layer.

	ACKNOWLEDGMENTS

 
Funding for this work was provided by the Canadian International Development Agency (CIDA, Project no. 01842 S45070).

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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JEQ 2003 32: 1167-1172. [Full Text]  

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Related articles in JEQ Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Heck, R. J. Articles by Santos, M. C. Search for Related Content PubMed PubMed Citation Articles by Heck, R. J. Articles by Santos, M. C. Agricola Articles by Heck, R. J. Articles by Santos, M. C. Related Collections Capillary Fringe Processes Field-Scale Studies Soil Salinity Other Crops Irrigation