SOIL PH AFFECTS COPPER FRACTIONATION AND PHYTOTOXICITY

Authors: Alva, Ashok; HUANG, B - INST SOIL SCI, CHINA; PARAMASIVAM, S - UNIV OF FLORIDA

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2001
Publication Date: 4/1/2000
Citation: ALVA, A.K., HUANG, B., PARAMASIVAM, S. SOIL PH AFFECTS COPPER FRACTIONATION AND PHYTOTOXICITY. SOIL SCIENCE SOCIETY OF AMERICA JOURNAL, 64:955-962. 2000.

Interpretive Summary: In the early 1900s copper sulfate was routinely applied to citrus production as a control measure for citrus dieback. The repeated application of copper resulted in its accumulation in several soils. Since downward transport of copper in soil is extremely slow, most of the copper is accumulated in the top soil layer. Some citrus rootstock such as Swingle citrumelo (the most commonly used rootstock in the recent years) i quite sensitive to high content of copper in the soil. Therefore, replanting of an old grove site containing high accumulation of copper with trees on Swingle citrumelo rootstock can result in copper toxicity problems, particularly if the soil pH was low. This study showed that copper content of up to 400 mg/kg was not phytotoxic to Swingle citrumelo rootstock seedlings grown in a soil with pH 8.2. However, in soils with pH 5.7 and 6.2, significant growth reduction occurred when the copper levels exceeded 200 mg/kg. Copper in soils exist in different chemical forms. Readily soluble form of copper was most phytotoxic, therefore, managing the soils to decrease the proportion of this form of copper is desirable to minimize the copper phytotoxicity effects. Readily soluble form of copper content in the soil within 1.6 to 2.5 mg/kg concentrations decreased the top and root weight by 20%. Copper content in the root was a good index of copper phytotoxicity as compared to the copper content in the leaves. This study contributed to our understanding of the mechanism of copper phytotoxicity and use of appropriate index for its evaluation to develop management practices to minimize copper phytotoxicity in soils with high levels of accumulated copper.

Technical Abstract: Phytotoxicity of Copper (Cu) depends on the relative distribution of different chemical forms, which is a function of soil properties, such as soil pH and organic matter content. Sequential fractionation was conducted to partition the total Cu into exchangeable, sorbed, organically bound, precipitate, and residual forms. Three soils were sampled from existing citrus groves and 0 to 400 mg Cu kg -1 were added. The soils used were: Myakka fine sand (pH = 5.7), Candler fine sand (pH = 6.5), and Oldsmar fine sand (pH = 8.2). Phytotoxicity of added Cu was evaluated using citrus rootstock (Swingle citrumelo) seedlings grown for 330 d. In Cu-unamended soils, the major portion of the total Cu was in the organically bound form in the low pH soils. However, in the high pH soil, the precipitate form was the dominant form. As the rate of Cu increased, the concentration of the readily soluble Cu forms (exchangeable + sorbed forms) increased in the elow pH soils, i.e., from 0.8 to 89.5 mg kg-1 (8.4 to 25.3% of total Cu) in the Myakka soil, and from 2.2 to 70.3 mg kg-1 (3.1 to 20.3% of total Cu) in the Candler soil. In the high pH Oldsmar soil, however, the concentration of readily soluble Cu forms increased only from 1.1 to 5.3 mg kg-1. The citrus seedling growth was negatively correlated with Cu concentrations in the readily soluble forms and positively correlated with those of the precipitate form. A 20% decrease in the top and root weights occurred at 2.5 mg kg-1 of readily soluble Cu in the Candler soil (pH = 6.5). The critical concentration was lower (1.7 mg kg-1) for root growth on the Myakka soil (pH = 5.7). The critical Cu concentration in the plants varied from 60 to 68 and from 62 to 270 mg kg-1 in the leaves and roots, respectively.