9.1 Soil Application

Soil-applied copper (fertilizer copper) is preferred in many regions and has the advantage that treatments do not need repeating very often, except with smaller banded applications or on organic soils where the copper is strongly sorbed. The copper can also be put on together with a fertilizer dressing and incorporated into the topsoil by cultivations before the crop is sown. The initial outlay on copper for soil treatment can be considerable and some farmers may prefer to apply smaller amounts of copper annually as foliar sprays. However, an important benefit of soil application is that it ensures that the crop has an adequate supply of copper throughout its period of growth.

The rates of copper applied to soils to correct deficiencies vary widely (0.7-23 kg/ha of Cu. ie 300 foId) and can be due to:

Differences in the placement of the copper in relation to the plant roots, less being required if the metal is in the zone around the roots;

Sorption of copper by soil constituents, especially organic matter;

Release of copper ions by the copper fertilizer compound and differences in the copper requirements of crops. [2]

Copper sulphate is the most commonly used source of copper for soil application because it is widely available (can be made from recycled scrap) and cheaper than other compounds. Its sulphate content also makes a useful contribution but is not adequate to satisfy crop requirements for sulphur. Finely ground slags, containing copper and other metals in various forms are suitable for slightly acid to acid soils, which bring about the gradual solution of the copper. Such slags are popular on cultivated sandy podzols especially where maize is grown, as in Les Landes de Gascogne, and the slag's appreciable zinc content is also of value since maize has a high zinc requirement.

Opinions vary on the maximum application rate of copper which it is safe to apply to soils. These vary from a total of 20-40 kg/ha Cu altogether to a maximum of 44 kg/ha in any one application. However, it has been demonstrated that a cumulative application of 280 kg/ha copper, as copper sulphate, over 17 years was not harmful to crops. [3] Soil or plant tests should be carried out at reqular intervals after soil treatment to determine whether another application is required. The frequency with which repeat applications need to be made to soils varies widely. In Australia, some soils still had adequate available copper contents 35 years after the first soil treatment, but these soils had low organic matter contents. In general, soil applications of copper need to be repeated at intervals between 5 and 18 years, depending on the soil, crop and management. The higher the organic matter content the more frequently soil applications are needed.