A SOIL TEST FOR DETERMINING AVAILABLE COPPER IN ACIDIC SOILS OF VENEZUELA

 

Belkys Rodríguez and Ricardo Ramírez

 

 

Belkys Rodríguez. M.Sc., Universidad Central de Venezuela. Researcher, Instituto Nacional de Investigaciones Agrícolas, (INIA/CENIAP), Venezuela. Address: Apartado 4653, Maracay, Venezuela. e-mail: brodriguez@inia.gov.ve

Ricardo Ramírez. Ph.D., Purdue University, EEUU. Professor, UCV, Venezuela.

Resumen

La adopción de sistemas de producción más intensivos por los agricultores, en Venezuela, puede conducir a la aparición de deficiencias de cobre en el campo. Actualmente no se dispone en el país de un método de análisis calibrado para el diagnóstico de Cu disponible para las plantas. El objetivo de este trabajo fue evaluar cinco soluciones extractoras de Cu: DTPA, DTPA-HCl, EDTA, HCl y Mehlich 1, en experimentos de invernadero usando suelos ácidos y maíz como planta indicadora. La cantidad de Cu extraído con DTPA-HCl, DTPA y EDTA se correlacionó positivamente con la arcilla, materia orgánica, pH y capacidad de intercambio catiónico de los suelos. El poder de predicción de las soluciones extractoras se estimó por medio del coeficiente de determinación y se encontró que la capacidad para separar los suelos que responden al Cu de los que no responden fue de 69% para DTPA-HCL y DTPA, 56% para el HCl, 55% para EDTA y 51% para Mehlich 1. Los resultados sugieren que las soluciones extractoras DTPA-HCl y DTPA pueden ser utilizadas para la determinación de las deficiencias de Cu en suelos ácidos.

Summari

Copper deficiency can become a limiting factor in crop production in Venezuela due to the adoption of more intensive cropping systems by farmers. In spite of this, there is no calibrated test for proper soil Cu determination. The purpose of this study was to evaluate five chemical methods used to estimate plant available Cu in acidic soils. Five Cu extractants: DTPA, DTPA-HCl, EDTA, HCl, and Mehlich 1 were used in a greenhouse experiment using corn as a test crop. Cu extracted by DTPA, DTPA-HCl and EDTA was well correlated with soil clay, organic matter (OM), pH and cation exchange capacity (CEC). Determination coefficients were used to measure the prediction power of Cu extractants. DTPA-HCl and DTPA prediction capacity for separating responsive soils to Cu from non-responsive soils was 69%, followed by HCL with 56%, EDTA with 55% and Mehlich 1 with 51%. This suggests that DTPA-HCl and DTPA extractants could be a useful index for predicting Cu deficiency in acidic soils.

Resumo

A adoção de sistemas de produção mais intensivos pelos agricultores, na Venezuela, pode conduzir à aparição de deficiências de cobre no campo. Atualmente não se dispõe no país de um método de análise calibrado para o diagnóstico de Cu disponível para as plantas. O objetivo deste trabalho foi avaliar cinco soluções extratoras de Cu: DTPA, DTPA-HCl, EDTA, HCl e Mhelich 1, em experimentos em estufas usando solos ácidos, e milho como planta indicadora. A quantidade de Cu extraído com DTPA-HCl, DTPA e EDTA, correlacionou-se positivamente com a argila, matéria orgânica, pH e capacidade de intercâmbio catiônico dos solos. O poder de predição das soluções extratoras se estimou por meio do coeficiente de determinação e se encontrou que a capacidade para separar os solos que respondem ao Cu dos que não respondem foi de 69% para DTPA-HCL e DTPA, 56% para o HCl, 55% para EDTA e 51% para Mhelich 1. Os resultados sugerem que as soluções extratoras DTPA-HCl e DTPA podem ser utilizadas para a determinação das deficiências de Cu em solos ácidos.

KEYWORDS / Acid Soil / Copper / Venezuela /

Received: 07/21/2004. Modified: 05/05/2005. Accepted: 05/06/2005.

 

Introduction

In Venezuela, soil testing is practiced in almost all laboratories; however, not enough calibrated soil tests are available for proper Cu diagnosis and analysis. Accurate prediction of Cu status in the soils permits the correction of crop Cu deficiencies before planting. Micronutrient deficiencies are likely to become a limiting factor in the maintenance of soil fertility levels as farming methods adopt more intensive cropping systems. These systems may include the planting of high-yielding varieties as well as using concentrated fertilizers (NPK). In the past, Arrieche and Ramírez (1991) reported Zn deficiencies in acidic soils with pH values between 4.5 and 5.6. De Quiñones (1991), working with sandy loam and sandy soil samples collected in the eastern part of Venezuela, found that 46% of the samples had low Cu levels. Casanova and Arvelo (1987) reported Cu and Zn deficiencies in calcareous soils. To date there are no local studies about the symptoms of Cu deficiency in annual crops.

Copper deficiency in sandy acidic soils can be expected due to continuous fertilization utilizing relatively high N and P rates. Nitrogen applications to soils can induce Cu deficiencies (Reuther and Labanauskas, 1966; Robson and Reuter, 1981). Prolonged phosphate fertilization of soils has also been reported as a cause of Cu deficiency in certain soils (Birghane, 1963).

Soil Cu occurs in organic compounds or as an exchangeable cation in soil colloids. Extractable Cu linked to organic matter has often been reported where organic matter content in the soil is rather low (Dolar and Keeney, 1971; Sillampaa, 1982). However, a negative correlation has been reported between organic carbon and extractable Cu where soil organic matter was above 6.0% (Sillampaa, 1982). Lindsay (1972) reported that Cu concentration in soil solution decreases as the soil pH increases. This behavior was explained on the basis of stronger Cu absorption at higher pH.

Copper build up in the soil was attributed to large Cu applications and to the pesticide applied routinely as foliar spray (Alva et al., 2000). Actually, in Venezuela farmers do not apply Cu fertilizers or Cu pesticides to annual crops; therefore, there is no contribution to Cu accumulation in the soils nor a potential watershed or runoff contamination. Acidity and low nutrient content in the soils are considered to be the main constraints for agriculture in the country (López de Rojas and Comerma, 1985).

A calibrated soil test is required in order to optimize recommendations for the application of Cu fertilizers, and different kinds of soil extractants have been suggested so as to estimate available Cu in the soils. The most common methods of Cu extraction from soils use diluted acids such as 0.1N HCl and organic chelating agents, namely ethylenediaminetetraacetic acid (EDTA) and diethlenetriaminepenta-acetic acid (DTPA). The purpose of this study was to evaluate five chemical methods used in estimating available Cu in plants growing in acidic soils and to establish critical Cu values to separate responsive from non-responsive soils. This correlation study has to be followed by field experiments to calibrate chemical test, so as to provide the basis for Cu fertilizer recommendations to farmers.

Materials and Methods

Fourteen surface soil samples (0-20cm) were collected in bulk from major agricultural areas located in acidic soils throughout Venezuela. All soil samples had a pH <6.5 and, therefore, qualified for the purpose of the experiment. The soils were air-dried and sieved through a 2mm screen. Oven dried (45ºC) soil samples were analyzed after they were ground to pass through a 1mm stainless steel screen. Organic matter (OM) was determined according to Walkley and Black (1934), pH was measured in water (1:1.25), P according to Olsen et al. (1954), cation exchange capacity (CEC) by ammonium acetate (Chapman, 1965) and texture as indicated by Boyoucos (Day, 1965).

Available Cu content in each soil sample was extracted following five methods:

1- Na-EDTA (Lindsay and Cox, 1985).

2- DTPA (Lindsay and Cox, 1985).

3- DTPA+HCl 0.005M DTPA in 0.1N HCl, soil:extractant ratio 1:5 and 20min agitation.

4- HCl 0,1N. (Brown and Rodríguez, 1983).

5- Mehlich 1 (CSTPA, 1980).

Extractant solutions were filtered in a Waltman Nº1 filter paper and the amount of Cu in each clear aliquot part was analyzed by means of a Perkin Elmer 3100 atomic absorption spectrometer.

All 14 soils were fertilized with Cu in the amounts (mg·kg^-1 of soil) of 0, 5, 10, and 15 by using reagent grade copper sulfate (CuSO4.7H20). All pots used for sowing were added a general fertilizer that contained 200mg·kg^-1 N as ammonium sulfate, 80mg·kg^-1 P as monoammonium phosphate, 90mg·kg^-1 K as potassium sulfate, 20mg·kg-1 Mg as magnesium sulfate, and 15mg·kg^-1 Zn as zinc sulfate. All fertilizers were well mixed with 3kg of soil and the mixture was potted in 4 liter plastic containers in a greenhouse using in a randomized design with four replications.

Four corn seeds of the PB-8 hybrid were sown in each pot, and ten days later plants were thinned down to 2 per pot. De-ionized water was used to water the pots so as to maintain soil moisture at 30-80% available water. Thirty five days after sowing the plants were harvested by cutting them 1cm above the soil surface. Plants were washed with de-ionized water, dried at 70°C for 48h in a mechanical convection oven, then weighed and ground up in a small mill and passed through a 1mm screen. The plant material was processed in a mixture of sulfuric acid and hydrogen peroxide (Parkinson and Allen, 1995) and the digest was assayed for its Cu content via atomic absorption spectroscopy.

Critical values for soil Cu were calculated according to the linear discontinuous model (Cate and Nelson, 1971; Nelson and Anderson, 1978).

Results and Discussion

The soils used in the experiment were acidic with pH between 4.6 and 6.5 (Table I). They were low in OM which was over 1.5% in only 5 out of the 14 soils sampled. CEC for 12 soils was between 1.3 and 15.9cmol·kg^-1 and only in 2 soils CEC reached 30.5 and 31.5cmol·kg^-1. Clay content varied from 3.4 to 47.4%, and only one soil sample was over 21.4%.

The Cu content in air-dried soil samples with the 5 extracting solutions are reported in Table II. The data show a large variation in the amount of Cu extracted from the soil by each extracting solution.

Based on the mean values obtained for all 14 soils sampled, the DTPA-HCl extractant removed the most Cu from the soil, varying from 0.13 to 5.06mg·kg^-1 with a mean of 1.33mg·kg^-1. EDTA extracted larger amounts of Cu from the soils than did DTPA alone and Mehlich 1, and Cu removed by HCl was close to that removed by EDTA. Mehlich 1 extracted smaller amounts of Cu than did the other extractants. Cu removed from the soil by Mehlich 1 ranged from 0.08 to 1.29mg·kg^-1 with a mean of 0.44mg·kg^-1. Average Cu removal, in order of extractants used, was DTPA-HCl > DTPA > Na-EDTA > HCl > Mehlich 1.

Despite the differences in the amounts of Cu removed from the soils, significant and positive correlations ranging from 0.522 to 0.987 were found among the varying extracted Cu quantities by all five extractants (Table III). Correlation coefficients above 0.90 corresponded to DTPA-HCl:DTPA; DTPA-HCl:EDTA and Mehlich 1:HCl. No correlation was found between extracted Cu by Mehlich 1 and EDTA.

Soil Cu removed by DTPA-HCl, EDTA and DTPA was closely related to soil clay content, pH, OM and CEC. Correlation coefficients of these four soil characteristics and Cu removed from the soil by these three extractants were positive and statistically significant, except for DTPA:OM (Table III). The higher correlations coefficients, of 0.721 to 0.826, corresponded to organic extractants with soil clay and pH. These relationships indicate that the amount of Cu removed by organic extractants increases as the soil clay content and soil pH rise within the limits for tested soils. A significant effect of pH on available Cu was reported by Li and Mahler (1992). However, the present findings do not agree with those reported by Olumuyiwa (1973) working in tropical soils in Nigeria. Sedberry and Bligh (1988) reported a lack of correlation between soil Cu and organic matter.

There was no relationship between Cu removed from the soil by Mehlich 1 or HCl with soil OM, clay content, pH and CEC. Calculated correlation coefficients for these inorganic extractants and soil characteristics were not statistically significant.

The dry matter production (DM) of corn plants grown in greenhouse conditions are presented in Table IV. Corn DM responded well to Cu fertilization in all 14 soils. Average DM increments due to Cu fertilization were 15.7, 23.11 and 8.73% for 5, 10 and 15mg·kg^-1 of applied Cu, respectively. These results demonstrate that the soils used in the experiment are Cu deficient and that a crop response to Cu fertilization can be expected.

Corn DM varied widely among the soils with and without Cu fertilization (Table IV). Where Cu was not applied to the soil, DM ranged from 2.26 to 5.50g per plant. The higher yield of 5.00g per plant in Lairen soil corresponds to a higher soil OM, clay content, and CEC. A second higher DM yield of 4.43g per plant in San José soil corresponds to a higher soil OM and clay content (Table I).

Where 5mg·kg^-1 Cu was applied to the soil, significantly higher DM yields occurred in Lairen and Orurita soils (Table IV), while increasing Cu fertilization to 10mg·kg^-1 led to higher yields in seven soils. A significant yield DM production was only recorded in one soil upon Cu fertilization of 15mg·kg^-1.

The coeficients of determination (R2) for the regression data relating DM to extracted Cu and soil properties are reported in Figure 1. When soil tests alone were used as DM predictors, 45.0% of the variation in DM production also accounted for variations in DTPA-HCl and DTPA extracted Cu, 40.7% for EDTA, 16.2% for HCl and 8.6% for Mehlich 1. The inclusion of OM, pH and CEC in the regression improved the predictive power for each soil test. Soil OM had a greater effect on DM predictive ability in each soil test, more than any other soil property, and was followed by pH. When OM and pH were included in the regression, its predictive ability improved to 67.6% for DTPA and DTPA-HCl, 64.9% for EDTA, and 61.7% for HCl.

To relate DM yields to the amount of Cu removed by the five extractants, percent relative yields (PRY) were calculated as the ratio between the yield for the unfertilized control and the maximum yield corresponding to a given Cu fertilizer treatment multiplied by 100 (Cate and Nelson, 1971). The relationship between extracted Cu and relative yield was established by a quadratic function (Figure 1) for all five extractant solutions.

Only 2 out of the 14 PRY values were over 90%, which means that these soils did not respond to Cu fertilization because their Cu content was good enough for plant growth. Only a few acidic soils in the sampled areas are well provided with available Cu to support good crop growth. The soil sampling procedure was random; therefore we can assume that the soil population used represents, at a macro scale, the acidic soils of the flat lands.

Critical values (CV) for the soil Cu tests were determined for each of the extractants used to remove Cu from the soil. This was performed by separating responsive soils from non-responsive soils to Cu fertilization, as described by Nelson and Anderson (1978). These critical values for available Cu in tested soils may help differentiate soils that will respond to Cu fertilization from non-responsive soils to Cu fertilization.

Percentage relative yield values ranged from 0.42 to 99.0; however, most of the the PRY values were below 70%, which indicates a high response rate in plants to Cu application to the soil.

Correlation coefficients for PRY, extracted Cu for the five extractants, and calculated critical values for available Cu in the soil are also included in Figure 1. PRY was plotted against extractable soil Cu, following the Cate and Nelson (1971) method. Both DPTA and DTPA-HCl proved to be the best for identifying critical Cu available in the soil, since both extractants displayed the highest correlation coefficients for removed Cu and PRY, 0.829 for DTPA and 0.828 for DTPA-HCl. Correlation coefficients for the other three extractants were lower.

Taking into account the calculated coefficients of determination (R2) the predictive capacity of DTPA-HCl and DTPA extractants was 69% for separating Cu responsive soils from non-responsive soils, while it was lower for the other extractants (Figure 1). Critical values (CV) for predicting plant response to Cu fertilization in mg·kg^-1 ranged from 0.90 to 1.5 in the different extractants.

Available Cu in 12 out of 14 soils was below the calculated CV for Cu removed by DTPA-HCl and DTPA. According to these results, crops growing in these soils may respond well to Cu fertilization. Only in San José and Lairen soils available Cu was over the CV limits; therefore, no response to Cu fertilization was expected in these two soils.

The data indicate that DTPA-HCl and DTPA extractants could be useful as an index for predicting Cu deficiency in acidic soils.

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