Sodium in the soil

Most of the sodium in soils is held in the exchangeable form on clay particles or is in solution. The amounts are less than those of potassium. Typical exchangeable Na levels (mg Na/l)

• Sand 15
• Light loam 18
• Medium loam 24
• Silt 30
• Clay 30
• Organic 50

Sodium is highly susceptible to leaching and available soil sodium can be lost over winter. Deeper rooted crops such as sugarbeet will be able to access sodium that has moved below the plough layer. It is not possible to build up reserves of sodium in the soil by repeated applications over years. High levels of exchangeable sodium can disperse clay particles resulting in a loss of soil structure. This is often seen where arable land has been flooded by seawater. Such effects are not significant where sodium is applied to land at rates generally associated with salt or fertilizer use. Such seawater inundations can be treated with Gypsum or Lime

British Sugar often test for sodium in their own soil analysis. The British Sugar interpretation scale is listed below.

The Role of Sodium in Plant Nutrition

Sodium is not generally regarded as an essential nutrient for all higher plants. However, sodium can substitute for potassium to a greater or lesser extent in different plant species (ref. 1). Some crops will show a yield response to sodium even in the presence of adequate amounts of potassium. For some species, including sugarbeet, red beet, fodder beet, mangolds and spinach, sodium is essential and can not be substituted completely by potassium (ref 2). The amounts of sodium removed by crops are highly variable but are usually around 95 kg Na2O/ha in sugarbeet (of which about 75 kg Na/2Oha is in the tops, the remainder in the roots). Other crops remove somewhat less sodium.

Sodium is essential for animals and herbivores depend on the content in plants to supply their needs. There is, therefore, a role for sodium in herbage species additional to that of plant nutrient.

Much of the agronomic research on sodium requirements of arable and horticultural crops was done before 1980. More recently, research on the role of sodium in grassland systems has been done at Bangor and Cambridge (refs 1, 12, 13). Members of the HDC can obtain a review of sodium use in horticulture ‘Review of the value of salt as a fertilizer and herbicide for vegetables’ by George Wadsworth (code FV51). For copies, tel HDC at 01732 848383 or visit the HDC web site www.hdc.org.uk