Soil acidity can limit nutrient availability and reduce crop yields, particularly in regions with highly weathered soils. This is a common challenge for South African crop farmers. Acidic soils require careful management through practices like liming to maintain productivity.
Soil acidity, measured by a pH level below the neutral mark of 7, is a key determinant of soil health and directly influences crop performance. Acidic soil conditions are prevalent in the eastern regions of South Africa. Also, soils tend to be naturally acidic in areas with higher rainfall, but human activities can accelerate this process.
Peter Mudau, a soil scientist with a master’s degree in soil chemistry, offers expert guidance on the causes, detection, and management of soil acidity. This empowers farmers to make informed decisions that enhance productivity and soil sustainability.
Why soil acidity matters
Soil acidity directly affects plant roots and nutrient availability in the soil. Mudau explains that acidic soils impair the ability of crops to absorb essential nutrients. This leads to reduced growth and increased vulnerability to diseases.
Recognising and addressing soil acidity is crucial for ensuring healthy and productive farming systems. “It is very vital for farmers to know about soil acidity because it affects crop performance by affecting plant roots and nutrient availability in the soil,” Mudau says.
Signs of acidic soils
Farmers may notice signs of soil acidity through stunted growth and fungal diseases in crops, Mudau shares.
“Soils prone to soil acidity are commonly sandy soils or soils which have gone through intensive leaching of nutrients such as albic soils.” Albic soils, found in the E horizon of a soil profile typically situated a few centimetres below the surface, are pale in colour. This pale colour signifies intensive leaching of nutrients and positive ions (cations), leading to acidic soil.
“Hence albic soils are related to natural acidity in soil properties,” he explains.
In addition, Mudau advises farmers to be vigilant for key indicators of soil acidity, which can significantly impact crop performance. He highlights two primary signs to watch for:
- Stunted crop growth: Poor growth may indicate that acidic soil restricts root development and limits nutrient uptake.
- Increased susceptibility to fungal diseases: Acidic conditions often create an environment where crops are more vulnerable to fungal infections, further hampering yield potential.
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Detecting soil acidity
Accurately identifying soil acidity is a crucial first step in managing soil health. Soil testing provides reliable results and enables farmers to make informed decisions about corrective measures.
“Farmers can determine soil acidity or pH by asking agriculturalists to collect soil samples and send them to soil laboratories for chemical analysis. The methods to be used at the lab involve techniques such as electrodes or colourimetric testing determine soil pH,” says Mudau.
For a quicker and more convenient alternative, however, farmers can use portable field instruments, such as soil testing kits available from retailers. Mudau explains that these on-site tools allow farmers to monitor soil pH levels without needing to send samples to a lab, making them an accessible solution for regular checks. To determine exact soil acidity, Mudau explains that acid saturation and exchangeable acidity need to be tested for.
Addressing soil acidity
The primary method for managing soil acidity is the application of lime, which helps neutralise acidic soils. “Lime commonly responds late or does not correct acidity at once. It needs time to be worked into the soil and react, so it is advisable to be applied earlier before planting,” Mudau says.
Lime is available in two main types:
- Calcitic lime is rich in calcium.
- Dolomitic lime contains both calcium and magnesium.
Farmers should select the appropriate lime based on their soil’s calcium and magnesium content.
Monitoring soil acidity
“Soil acidity should always be monitored before crop establishment, but [the frequency of testing] depends on the crop type. Some crops can be tested every four seasons, like grains, for example,” he says.
Mudau advises testing vegetables every two seasons. Also, check crops showing declining performance or increased susceptibility to diseases immediately for soil acidity.
Causes of soil acidity
Mudau identifies several key factors that contribute to acidity in soil, each of which plays a significant role in altering soil pH and affecting crop health:
- Excessive fertiliser use: “Overuse of fertilisers, particularly urea and sulphur-based varieties, can increase soil acidity by introducing excess nitrogen and sulphur into the soil, disrupting the delicate nutrient balance.”
- Water quality and nutrient leaching: Mudau explains that excessive irrigation, especially with poor-quality water, can cause nutrient leaching, where essential soil nutrients wash away, leaving the soil more acidic and less fertile over time.
- Contributing crops: Crops like legumes, which are known to mine soil nutrients or release organic acids into the root zone, can exacerbate acidity. While beneficial in certain contexts, these crops contribute to a gradual build-up of acidity in the soil.
“Managing soil acidity is an ongoing process that requires proactive measures, regular monitoring, and timely interventions,” says Mudau.
For more tailored advice on mitigating acidity and improving soil health, Mudau encourages farmers to consult agricultural experts and soil scientists who can offer guidance and develop effective, site-specific soil management strategies.
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