Sodium occurs only in a compound form in soils, predominantly as salts.
Sodium does adsorb onto clay minerals but the bond is weaker than that of potassium ions and therefore sodium has a higher propensity to be leached Therefore in areas of high rainfall such as trpicall or sub-troical climates, soils are generally depleted in sodium which is washed down into deeper soil layers
In contrast, in arid or semi-arid areas an accumulation of Na in the top soil frequently occurs because the rate of evaporation exceeds the replacement of water from the soil. This often results in a deterioration of the soil structure which has a negative effect on the water and air balance of the soil. In addition, the pH becomes more alkaline with an increasing Na content.
Sodium in the plant
Even though sodium plays a less important role in plant nutrition than potassium or magnesium, the positive effects of sodium fertilisation on yield and quality of sodium loving crops (e.g. Chenopodiaceae) are clearly evident. Sugarbeet as the most important crop species in this group is well known example which has a relatively high sodium requirement. Sodium supports the synthesis of glucose and its conversion to fructose which is stored in the beet.
Sodium controls osmotic pressure in plant cells and results in a more efficient use of water.
Na ions can often substitute K ions in some metabolic and osmoregulatory functions and therefore the two nutrients are interchangeable to varying degrees according to plant group
Sodium is important for some C4 plants (e.g.. amaranth) for CO2 uptake
Sodium in animals
In animal nutrition, a sufficient supply of sodium is an important factor in maintaining the productivity of the animals. Sodium deficiency results in loss of appetite, decreased milk production, weight loss and has implications for health and fertility of the animals.
Approximately 2g Na kg-1 DM in the basic ration is necessary to satisfy the daily requirement for Na in dairy cattle. Field studies have demonstrated that in many regions the average sodium content of grassland is just 0.1-1g Na kg-1 DM and therefore clearly below the required amount.
Sodium containing fertilisers such as Magnesia-Kainit have been successfully used for over a century for greatly increasing the sodium content of grassland and can guarantee meeting the Na demands of the animals.
Studies and trials have demonstrated that besides an improved Na supply from the nutritional point of view, the increased sodium also has a major effect on palatability of forage which can increase dry matter intakes by around 10%.
This nutrient is contained in the following fertilisers