Copper is an essential element that is needed by natural healthy growing plants. Some agricultural soils does, however, contain large amounts of anthropogenic copper as a result of soils receiving copper based pesticides such as copper sulphate or copper oxychloride (Komàrek et.al,  2008).  

The presence of high concentrations of copper has a strong environmental impact; in fact, copper is among the most toxic of the heavy metals for living organisms such as annelids, algae and soil microbes. As a rule, its toxicity on these organisms exceeds that of Cd, Cr, Ni, Zn, Pb, Co, Mn and Sn, and is only surpassed by that of Hg (Ross and Kaye,  1994). 

High copper concentrations in soils can have adverse effects on soil biota and plants. This is especially true for acidic soils, tilled soils and soils affected by intensive erosion with a lower sorption capacity (Komàrek et.al,  2008).  The problem regarding rooting of plants and the effect on earthworm in copper contaminated soil is widely documented (Calisi et.al,  2009).  Warning and critical legislative limits, valid in the EU, set copper concentrations in soils to 50 and 140 mg / kg, respectively (Council Directive 86/278/EC, 1986). Czech regulations set the limit for coarse textured agricultural soils (<20% clay) as 60 mg Cu / kg and for agricultural soils with high clay contents (>20% clay) as 100 mg Cu / kg (Ministry of the Environment of the Czech Republic, regulation number 13/1994). The Australian and New Zealand guidelines for the assessment of contaminated sites recommend that if the total copper concentrations in soil exceeding 60 mg/kg require environmental investigation (Pietrzaka and McPhail,  2004) is required. Fernández-Calviño et.al  (2008) conducted a study on vineyard soil degradation caused by copper accumulation and availability. These authors reported that the total copper concentration in soils ranged from 25 to 272 mg kg^-1. The number of years during which each soil was cultivated with vines was found to affect both the total concentration of copper and its distribution. Copper bound to organic matter and to amorphous aluminium and iron colloids were greater in old vineyards than in young vineyards. The prevalence of copper bound to the organic fraction reduces the adverse environmental effects of this metal on soil organisms and plants (Fernández-Calviño et.al,  2008).   

The pH of the soil is one of the physico-chemical variables most markedly affecting the mobility of metals in general and copper in particular. Agricultural soils can undergo substantial pH changes under the action of fertilizers or calcareous amendments used to correct pH in acid soils. Although copper is more readily soluble in acid soils, its solubility also depends on the type and content of colloids present. In addition, the concentrations of other cations that could compete with copper in adsorption and/or absorption processes also must be taken into account. Some management practices such as the addition of organic matter or liming should be implemented in order to delay or avoid soil degradation because of the addition of copper.  The total acid soluble concentration of copper soil may not be significant in terms of environmental safety and availability to plants, but will indicate the extent of a potential problem.  In this work, we determined copper from a limited number of avocado, mango and litchi orchards in order to evaluate whether the use of copper presents a thread to the sub-tropical fruit industry in South Africa.  

Material and methods 

A soil drill was used to collect samples from the different orchards. The samples were collected at depths of 0-10 cm, 10-20 cm and 20-30 cm at each location and respectively marked as 1, 2 and 3. All samples were oven dried prior to analysis.  Samples were prepared by digestion of approximately 20 g dried material with 20 ml 50 % nitric acid.  Copper concentrations were determined using an ICP-OES and reported as mg/Kg Cu in dried soil.   

Results and discussion