By Ruth Vaughan, CropNuts Technical Director
In Africa, and especially in Kenya, high soil sodium is one of the major causes of yield losses, quality reduction and crop failure in irrigated agriculture. So today, in our first newsletter, we are going to talk about sodium in the soil.
Soils High With Sodium
Soils with a high percentage of sodium are called sodic soils. Sodic soils negatively impact plant growth for several reasons:-
- Specific ion toxicity to sodium sensitive plants.
- Nutrient deficiencies or imbalances in the plants.
- High soil pH – resulting in lock up of phosphates, iron and other micronutrients.
- Dispersion of clay and silt particles in the soil – collapsing the soil structure and blocking soil pores.
Where does soil sodium come from? In low rainfall areas with high evaporation, like many parts of Kenya, sodium and other salts build up in the soil surface over time. This effect is rapidly enhanced due to over grazing, which removes the plant cover and compacts the soil, reducing water infiltration and causing a salt build up.
In coastal areas sodium comes from sea spray and sea salt in the rain. Irrigating the soil with water containing high sodium is a major cause of high soil sodium in Kenya. It is important to identify the root cause of the sodium build up so you can deal with it. If you are irrigating, the FIRST thing to test is the water quality, with an irrigation water analysis.
Effect Of High Sodium In Soil
High sodium levels compete with calcium, potassium and magnesium for uptake by plant roots. Some plants take sodium up with a high affinity (sodium sensitive plants). Sodium sensitive plants include potatoes, beans, woody plants, vines and stone fruits. Sodium toxicity can be seen as necrosis of leaf tips and plant yellowing like these tomato plants in Thika, and onions in Naivasha.
Long before you see the classic toxicity symptoms in the plant leaves – you will notice the plants struggling with high pest and disease pressure. Spidermite infestations are very often associated with high soil sodium. Death of plants from Fusarium is very common in sodic soils. Sometimes the sodium uptake affinity is bred out of plants creating sodium tolerant varieties. To avoid sodium toxicity plant sodium tolerant plant species or sodium tolerant varieties. Don’t struggle with sodium sensitive plants in high sodium soils – you will never win.
Correcting Soils High With Sodium
Soil nutrient imbalances, high pH soil and soil structure can all be improved by getting a complete soil analysis. The soil correction recommendations will help reduce the sodium in the soil, improve the soil structure and address nutrient imbalances. Ideally this should be done before you plant, but you can still do this in long term crops like roses, while the plants are still growing.
The effect of sodium on soil structure depends on the amount of clay or silt in the soil. The cation exchange capacity (CEC) gives an indication of soil type. Heavy soils with a high CEC generally have a high clay content. Fine silty soils, like those in Naivasha, are also highly affected by sodum. The sodium attaches to the soil particles making them repel each other. This disperses the soil and breaks down the soil crumb structure. These small particles move in the soil water and block the soil pores that are so important for root growth, gaseous exchange (oxygen in and carbon dioxide out) and water movement.
Measuring Soil Sodium Levels
Sodium levels are reported as exchangeable sodium percentage (ESP) in your complete soil analysis. At an ESP of <5% the soil structure starts to break down, and water infiltration reduces. As water infiltration reduces, the sodium quickly builds up, creating a rapid upward spiral in sodium levels and a fast collapse of soil properties and plant health. At an ESP of 15%, pretty much everything goes wrong and the soil is very difficult and very expensive to rehabilitate. The earlier you deal with sodium the better.
That’s all for this week, in the next GROW HOW we will discuss how to manage high sodium. In the meantime, if you suspect you have a soil sodium problem – send in a soil and water sample for analysis at CropNuts, or have a good look at your existing soil analysis reports.
Till next time,
Ruth Vaughan is the Technical Director at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). Ruth is also a contributing author to Kenya’s leading horticulture magazines such as the HortFresh Journal, HortiNews and Floricu