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Soil differences between native brush and cultivated fields in the lower Rio Grande Valley of Texas.

ABSTRACT. -- Soil fertility and salinity were compared between native brush and cultivated fields of flood plain and upland areas. Organic matter and zinc were higher (P < 0.05) in native brush surface soils of both areas. Calcium and iron were also higher (P < 0.05) in native brush, but only in upland soils. No significant differences (P > 0.05) were observed between native brush and cultivated fields for the soil parameters of pH, nitrogen, copper, sodium, and salinity. Organic matter and potassium generally decreased with depth, whereas calcium and salinity correspondingly increased; changes with depth (to 150 centimeters) were not observed with other soil parameters. Key words: floodplain; old fields; organic matter; soil fertility; upland; Texas.


The U. S. Fish and Wildlife Service and Texas Parks and Wildlife Department are revegetating newly acquired cultivated fields in the lower Rio Grande Valley of southern Texas with native plants to improve habitat for wildlife. Plant growth may be reduced if cultivation has significantly lowered soil fertility. Much of the water used in irrigation is slightly saline and an accumulation of salts also might stunt growth (Everitt et al., 1977; Salisbury and Ross, 1985). Preliminary tests were conducted to identify differences in soil fertility and salinity between native plant (brush) communities and cultivated fields of floodplain and upland area.


Floodplain soils of the study area are deep, moderately well-drained, silty to clayey soils on bottomlands (Jacobs, 1981). Upland soils are deep, gently sloping, well-drained, loamy soils on ridges of middle to high stream terraces that are modified by wind (Turner, 1982.)

The lower Rio Grande Valley has a subtropical, semiarid climate, characterized by short, mild winters (< 10[degrees]C) and long, hot summers (32 to 40[degrees]C). Annual rainfall, mostly from irregularly distributed thundershowers, varies from 450 to 700 millimeters. Occasional tropical storms in late summer (September) produce heavy rains. Another rainy period occurs in late May and early June. Relative humidity is usually between 65 and 80 percent. Surface winds are common.


Surface soil samples (top 15 centimeters) were collected at 21 sites in native brush and 27 sites in cultivated fields. Twenty-four sites were located in the Rio Grande floodplain and another 25 in an upland area about 44 kilometers north of the river. Samples were taken from Matamoros silty clay, Rio Grande silt loam, Conmargo silt loam, Rio Grande silty clay loam, and Reynosa clay loam soils in the flood plain, and from Hargill fine sandy loam, Hidalgo fine sandy loam, Willacy fine sandy loam, and Yturria fine sandy loam soils in upland areas (Table 1). The five flood plain soils supported similar native vegetation as did the four upland soils. The cultivated fields in the flood plain had been irrigated. Additionally, soils were sampled to a depth of 150 centimeters at six sites in both brush and cultivated fields to examine differences associated with soil depth; samples were collected in relation to changes in soil texture, not at prior-specified depths. There was no visible water erosion at any of the sample sites (slopes less than two percent). Soils in brush were sampled where dense vegetation permitted space to take a soil core, such as under a tree or in interspaces among shrubs. Information was not available on length of cultivation or crops used. Soil analyses were done by the soil testing laboratory of Texas A & M University in College Station, Texas; Welch et al. (1980) described the analytical procedures used. Readily oxidizable organic matter was used for percent organic matter. Extraction of phosphorus, potassium, calcium, magnesium, and sodium was done with a solution of 0.025M H-EDTA in 1.4M ammonium acetate and 1.0M hydrochloric acid, pH 4.2 to 4.3. A DTPA extracting solution was used for zinc, iron, manganese, and copper (ph 7). Salinity was measured with a two to one extract of water and soil. Water-extractable nitrates were measured with a nitrate electrode (Onken and Sunderman, 1970). Ratings of high, medium, and low were also provided, and were based on comparisons with research plots at Texas Agricultural Experimental Stations. Differences between brush and cultivated fields of floodplain and upland sites were analyzed with t-tests.


Few differences in surface soil characteristics were observed between native brush communities and cultivated fields (Table 2). Organic matter and zinc were higher (P < 0.05) in native brush of floodplain and upland areas. Calcium and iron were also higher (P < 0.05) in native brush, but only in upland soils. Manganese was higher (P < 0.05) in cultivated fields of the floodplain.

Differences between native brush and cultivated fields diminished with soil depth. Organic matter decreased with soil depth (from 3.5 to 0.38 percent), except in cultivated fields in upland sites. In these sites the organic matter increased between 12 and 24 centimeters (0.7 to 1.3 percent) and then decreased with depth. Potassium decreased with depth in the flood plain (747 to 346 milligrams per liter).

Calcium increased with depth in upland areas (923 to more than 3521 milligrams per liter) and was high throughout the profile in the flood plain (more than 3521 milligrams per liter). Salinity (all water-soluble salts) generally increased with depth (408 to 1202 milligrams per liter).

A comparison of soil surface characteristics between floodplain and upland sites (Table 2) showed a lower, more neutral pH in the sandier, upland soils. Nitrogen, phosphorus, magnesium, zinc, calcium, copper, sodium, and salinity were all much lower in the upland sites. Organic matter was lower in upland brush than in floodplain brush.


Higher organic matter in native brush soils would be expected given the annual leaf drop of the plants and additions of organic matter by root growth. Tilling soils may lower organic matter content by accelerating decomposition and incorporating subsoil low in organic matter. Tillage may have caused changes in physical properties of soils (that is, structure) that would return more slowly to their natural state under native conditions than would soil fertility. Higher organic matter in floodplain brush soils, compared to upland brush communities, would be the result of greater, or more lush growth of the brush in the floodplain (more water).

Lower amounts of zinc and iron (upland only) in cultivated fields may have been a result of depletion through crop harvest, reduced nutrient cycling compared to native brush, and greater leaching with irrigation. Higher calcium concentration in upland brush samples than field may have been due to calcareous soils (Hidalgo) that were present only in brush samples. We do not have an explanation for the higher amounts of manganese found in floodplain fields.

Low amounts of phosphorus, magnesium, zinc, and calcium in comparison to Texas Agricultural Experimental Station research plots were characteristic of both native brush and cultivated fields in the upland area. High levels of these elements in the floodplain were not the result of irrigation because they were also high under native brush, but instead were due to less weathering of the younger floodplain soils.

The soils of the lower Rio Grande Valley were typically low in nitrogen and organic matter, and high in phosphorus, potassium, calcium, magnesium, zinc, iron, copper, and manganese. Organic matter decomposes rapidly and does not appear to influence soil pH; it may be lost through wind or water erosion in some areas. Sodium and salinity did not appear to be a problem; all sites sampled were moderately well-drained to well-drained. Areas of salt accumulation or elevated water tables could occur if areas with poor drainage were irrigated.

Soils are naturally low in nitrogen despite the presence of a large number of leguminous species. Presumably, most of the nitrogen is in the plants and litter layer, and little in mineral soil. The native plants are probably adapted to this condition. Fertilization, nevertheless, may aid establishment but probably also will enhance growth of competing vegetation if broadcast over the entire site.
TABLE 1. Sampling in relation to soil type and taxonomic class (Jacobs,
1981; Turner, 1982).

No. of plots Soil
brush Field Soil name Family or higher taxonomic class

0 1 Conmargo Fine-silty, mixed (calcareous), hyperthermic
 Typic Ustifluvents
3 0 Matamoros Fine, mixed (calcareous), hyperthermic Vertic
1 0 Reynosa Fine-silty, mixed, hyperthermic Fluventic
6 13 Rio Grande Coarse-silty, mixed(calcareous), hyperthermic
 Typic Ustifluvents

4 7 Hargill Fine-loamy, mixed, hyperthemic Udic
2 0 Hidalgo Fine-loamy, mixed, hyperthermic Typic
1 4 Willacy Fine-loamy, mixed, hyperthermic Udic
4 2 Yturria Coarse-loamy, mixed, hyperthermic Pachic

TABLE 2. Summary of analyses of the top 15 centimeters of soil (mean and
standard deviation in parenthesis; milligrams per liter except pH and
organic matter).

 Floodplain (a)
Measured Native brush Field (c)
parameter (n = 10) (n = 14)

pH 8.2(0.1) 8.2(0.2)
O.M. (d) (%) 3.5(0.6) (*e) 1.4(0.7)
Nitrogen 9.1(5.5) 9.7(5.1)
Phosporus 50(21) 68(33)
Potassium 747(101) 598(254)
Calcium >3,521 (g) >3,521
Magnesium >426 (g) >426
Zinc 1.47(0.44)* 0.67(0.43)
Iron 11.4(5.5) 8.8(3.8)
Manganese 2.5(0.9) 5.1(3.3)*
Copper 1.03(0.40) 0.69(0.22)
Sodium 278(58) 254(90)
Salinity 408(84) 528(234)

 Upland (b)
Measured Native brush Field (c)
parameter (n = 11) (n-13)

pH 6.7(1.0) 7.1(0.5)
O.M. (d) (%) 2.0(0.9)* 0.6(0.4)
Nitrogen 1 (f) 1.3(1.1)
Phosporus 6(7) 3(4)
Potassium 377(155) 318(109)
Calcium 923(568)* 481(213)
Magnesium 212(116) 159(63)
Zinc 0.33(0.24)* 0.13(0.09)
Iron 13.8(7.4)* 7.1(4.5)
Manganese 5.6(3.2) 6.4(2.8)
Copper 0.52(0.28) 0.32(0.18)
Sodium 185(32) 164(111)
Salinity 220(163) 112(48)

(a) Soil textures: silt loam, silty clay loam, silty clay, clay, clay
loam, loamy fine sand.
(b) Soil texture: fine sandy loam.
(c) Field = cultivated field.
(d) Organic matter.
(e*) = significantly higher (P < 0.05).
(f) One sample had 23 parts per million, the remaining nine had one.
(g) Upper limit measured in laboratory.


The authors thank R. Schumacher of the USFWS for his support of the project, and B. Pinkerton of Clemson University, R. Wiedenfeld of Texas Agricultural Experiment Station, Weslaco, O. Van Auken of The University of Texas at San Antonio, J. Everitt of the U. S. Department of Agriculture, T. Fulbright of Texas A & I University, R. Lonard of Pan American University, and K. Manci for their suggestions.


Everitt, J. H., A. H. Gerbernamm, and J. A. Cuellar. 1977. Distinguishing saline from non-saline rangelands with skylab Photogrammetric Engineering and Remote Sensing, 43:1041-1047.

Jacobs, J. L. 1981. Soil survey of Hidalgo County, Texas. U. S. Dept. of Agric., Soil Conserv. Serv., Harlingen, Texas, 171 pp + maps.

Onken, A. B., and H. D. Sunderman. 1970. Use of nitrate electrode for determination of nitrates in soils. Soil and Plant Analysis, 1:155-161.

Salisbury, F. B., and C. W. Ross. 1985. Plant physiology. Wadsworth Publ. Company, Inc., Belmont, California, 3rd ed., 540 pp.

Turner, A. J. 1982. Soil survey of Willacy County, Texas. U. S. Dept. Agric., Soil Conserv. Serv., Harlingen, Texas, 137 pp. + maps.

Welch, C. D., C. Gray, D. Pennington, and M. Young. 1980. Soil testing procedures. Texas Agric. Exten. Serv., Texas A & M Univ., College Station, 29 pp.


Rio Grande Valley National Wildlife Refuge, Alamo, Texas 78516, and Soil Conservation Service, Alice, Texas (deceased)

Current address of author: U. S. Forest Service, 1170 4th Avenue South, Park Falls, Wisconsin 54552.
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Author:Vora, Robin S.; Jacobs, Jerry L.
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:May 1, 1990
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