Soil Water Studies in Oxisols and Ultisols of Puerto Rico: I
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Soil Water Studies in Oxisols and Ultisols of Puerto Rico: I. Water Movement 1 James M. Wolf and Mattheu; Drosdoff2 ABSTRACT Soil water experiments were conducted to determin e t he water su pplyin g characteristics of two clayey Ultisols, a clayey Ox isol, and a sandy Ox iso l. Water infiltration into all soils was very rapid, reaching 9 cm/hr after 1 hr of continuous floo ding. T he strong structural stability of the clay soils perm itted infiltration rates in excess of that for the sandy soil. Lateral water move ment, downslope, was a significant fa ctor in observed high rates of water infiltration and may part ially account for downslope move ment of nitrates. Values of soil water tension after 2 to 3 days of free drainage did not exceed 20 to 80 em of water (roughly Yso to Yi 2 bar). Field capacity was established to be ';1 5 bar for the sandy Oxisol and Ylo bar for the clayey Oxisol and Ultisols. Two avenues of soil water movement were postulated: Capillary pores (between soil particles) and non- capill ary pores (bet ween soil aggre gates) . Because of water movement in non-ca pillary pores, fl ow charac teristics of the cl ay soils resembled t hat of the sand. In terms of soil wa ter rele ase characteristics, the clays and t he sand were sim ilar. INTRODUCTION In vast areas of the Humid T ropics soils are strongly leached and very acidic. So il acid ity is co m monly assoc iated with AI toxicity, and these factors create a hostile soil environment which chemicall y limits t he 1 Submitted to Editoria l Board April 25. 1975. 2 Formerl y Lecturer, Department of Agricultural Engineeri ng, now with CIDIAT, Merida , Venezuela . and Professor of Soil Sci ence, Department of Agronomy. respectiv ely, Cornell University, It haca, N .Y., in cooperation with the Agri cultural Experiment Stat ion of the Uni versity of Peurto Rico and with the support of the U.S . Age ncy for Internat ional Development under Research Contract csd 2490. T he authors are indebted to several people for assistance d uring the course of the work. Dr. Ri chard Fox was singularly helpful in provid ing logistic support. Gaston Amedee assisted greatly in both the laboratory and field . Drs. Levine and Bo uldin willi ngly gave t heir t ime in reviewing this materia l. Many other peopl e at Rio Piedras. Coro zal, and the other sites gave assistance when needed. S pec ia l thanks are due Dr. Miguel A. Lugo -L6pez fo r his assistance in preparing the manuscript for publication. Lastl y, the sen io r author acknow ledges the cont in ued support by the Agricultural Engineering Department , Cornell Uni versity. wh ic h enabled hi m to undertake this work. 37.5 376 JOURNAL OF AGRICULTl' RE OF li:"-Jl\' ERSITY OF PUERTO RICO depth of rooting of crops such as corn and sorghum. At present, liming soil to a depth of more than the surface foot is not practical. Crops which are sensitive to soil acidity are effectively limited to t he depth of the soil profile to which lime may be incorporated. T his severely lim its t he amount of soil water and nutrients wh ich a crop can ut ilize. In the case of water, rooting limitations increase the likelihood of yield decreases when rainfall distribution is anything short of ideal. In order to assess the potential for successfu l cropping, informat ion regard ing climatic factors must be su perimposed upon those soil physical proper ties which affect t he water supplying characteristics of the soil s. T he climatic information required should include rainfall d istribution and poten tial evapotranspiration . The purpose of the soi l water experiments conducted in P uerto R ico d uring the summer of 1971 was to determine the water su pplying characteristics of fo ur soils on which fertility experimentation was in progress or planned. A previo us s urvey of moisture relationships of over 100 Puerto Rico soils has been made by Lugo-Lopez (l) . The soil water information obta ined from t he experiments herein reported should be usefu l in characterizing the soil s fo r purposes of su bsequent experimentation by fu t ure workers in the a rea. An evaluat ion of the soil p hysical characteristics together with climatic data may be useful to predict the ir combined effect upon crop y ield and perm it conclusions relative to the likelihood of successful cropping. T his is t he first of a seri es of four papers on soil water studies conducted on Oxisols and Utisols of Puerto R ico. Forthcoming pa pers will deal with water retent io n and availability, capillary conductivity, and t he comparison of several possible tech n iques for obta ining this information fo r t hese types of soils . MATERIALS AND METHODS SOILS Humatas Series Location: Corozal S u bstation Classification: Typic Tropohumults, clayey, kaoli nitic, isohypert her- mic Texture: Clay loam to clay Slopes: Top of hi ll ; 0 to 14 '7r Vegetation : C ul t ivat ed; corn and grass Catalina Series Location : Margaro's farm near Barranquitas Classification: Typic Haplorthox, cl ayey, oxidic, isohypert herm ic WATER MOVEMENT IN OXISOLS A:'-lD ULTISOJ.S 377 Texture: Clay Slopes: 3 to 8% Vegetatio n: Cultiv ated ; starchy food crops Torres Series Locatio n: Dr. Hilda Soltero's fa rm near Cid ra Classification: Orthoxic Palehumul ts, clayey, kaolinitic, iso hypert her- mic Texture: Clay Slopes: 1 to 3% Vegetation: Cultivated; pasture Bayamon Series Location: Fundador farm east of Manat! Classifi cation : Typic Haplorthox, clayey, ox idic, iso hypertherm ic Texture: Loamy sand Slopes : 0 to 2% Vegetation: Cultivated ; pinea pples To characterize t he so il s in the instrumented areas two p its were dug to a depth of 150 em. Location of the pits close to t he instrumented are as fac ilitated st udy of lateral water movement. Soil samples were taken at 15-cm intervals to a depth of 1:1 0 em. Undisturbed soil core sampl es, two per depth per pit, were taken at the 30-, 60-, 90-, and 120-cm depths. T hese samples, 7.62 em in diameter by 7.62 em lon g, were obtain ed by driving aluminum core rin gs into t he ground with a modified Uhl and sampler. Cores were removed from the ground wi t h pick and shovel. T hey were trimmed wi t h a kn ife, placed in pin t-size ice cream contain ers, and stored in a refrige rator. Loss of water while in the refrigerator resulted in so me soi l shrinkage from the walls of t he rings on some samples high in clay. Cores were later sealed in plastic bags to reduce wa ter loss. Upon saturation, t he soils expanded to fill t he rings. Undisturbed core samples were used to determin e soil bulk density. Bulk densities were calculated fro m the dry weight of soil in the undisturbed cores after making soil water release measurements. Cores, still in the metal rings. were placed in T em pe Cell s. T hey were initially we tted with 5 x 10 • M CaCl 2 at sli ght tensions and finall y saturated with the solution at slight positive pressure. To ensure saturation of t he entire core sample. the wetting process took more t han 24 hr. Pressures between 20 em of water and .l ' atmosphere were appli ed and the eff1uent volume was collected beneath each cell at each pressure step. The time required to run all pressure steps was approx imately 1 month. 378 JOl'RNAL OF AGRIC'UL TlfRE OF CNI\"ERSITY OF PUERTO R!C'O S ince there were 80 samples to be run, a second method was employed to speed the determinations. A single piece of cheesecloth was placed at the bottom of each core and attached to the metal ring using a rubber band. These cores were wetted from the bottom in low washing dishes using the same solution of calcium chloride. Six samples were then placed in a pressure plate extractor and pressure was applied at similar steps. After cores reached equilibrium with the pressure plate, they were removed and weighed. Before the cores were replaced on the pressure plate, the plate was wetted to ensure good contact between core and plate. 1 After t he pressure step at 1 atmosphere, cores were weighed, then oven dried and reweighed. Soil water contents were calculated as percent by weight and then converted t o volumetric water contents which were graphed versus tension. Conversion to volumetric water content was made through the relationship: Pu = p ·Pw where: Pu Volumetric water content Pw Water content on a dry weight basis p Soil bulk density Water content and tension determinations for 1 1, 2 1, 1, 5, and 15 atmospheres were also made on a pressure plate apparatus using disturbed soil samples and standard techniques. Tensiometers have been used widely as a tool in agronomic and soil water investigations (2,3,4,5,7) . The tensiometers used in this study were easily and cheaply constructed. Information on tensiometer construction and calibration is given by Richards (5) . The tensiometer cups at the 7 .5-cm depth were placed in a horizontal position (L-shaped) while those at greater depths were ertical. Ten siometer readings were recorded as soil water tension values at the ap propriate depths.