Determination of chemical forms of Cu and their relationship with pistachio seedlings responses and soil properties in some calcareous soils of Rafsanjan

Determination of different chemical forms of copper in soil is important in the evaluation of copper availability for plants, soil fertility management as well as plant nutrition. Availability of copper for plants depends on the relative distribution of different chemical forms of copper. Copper relative distribution in soil is a function of physical and chemical properties of soil such as the amount of organic matter, CaCO3 content, soil pH, clay content and CEC. Sequential extraction is one procedure to estimate the chemical forms of elements such as copper in soil that is considered as the perfect method for the description of elements behavior in soil. This study was done to determine different chemical forms of copper in some calcareous soils of Rafsanjan, relationship between these chemical forms and with physical and chemical characteristics of studied soils and also relation of different chemical forms of copper with plant responses (growth parameters and Cu concentration and uptake) of pistachio seedlings.

Eleven soil samples with different physical and chemical properties from different parts of Rafsanjan region before and after cultivation were fractionated by sequential extraction (Sposito et al.) to estimate the quantities of copper in exchangeable, sorbed, organic, carbonate and residual forms. The greenhouse experiment was carried out as a completely randomized design on 11 soil samples in three replications and each replication in three observations in pots containing 4 kg soil. After 8 months, pistachio seedlings were harvested and soils were air dried. Chemical forms of copper in soil samples were extracted by method of Sposito et al. and Cu concentration in plant tissues (leaf, stem and root) were determined by atomic absorption spectrometry.

Sequential extraction results indicated that exchangeable and sorbed forms of copper before and after planting were negligible, organic Cu 9.1 and 17.6 %, carbonate Cu 9.8 and 20.8 % and residual Cu 80 and 61.4 % respectively. After planting the amount of residual copper significantly decreased whereas organic and carbonate forms significantly increased. The highest correlation coefficient in soil samples, after cultivation was observed between carbonate Cu and stem Cu concentration (r=0.542**).

Organic and carbonate forms of copper could have an important role in pistachio seedlings nutrition, although their availabilities in soils are low. Generally, the pattern of copper distribution in soil samples before and after planting was as follows: Residual Cu > carbonate Cu> organic Cu> exchangeable Cu> adsorbed Cu