The term “soil pH” is often heard in agricultural settings, especially when it comes to plant nutrition, but what exactly is it and why is it important to crop growth? Let’s find out! 

What is soil pH?

Soil pH refers to the degree of acidity or alkalinity of a type of soil, and it is measured based on the activity of H⁺ and OH^- ions in that soil. An equal amount of H⁺ ions and OH^- ions indicates a neutral solution, hence water which is made up of an equal part of these two ions, is neutral at a pH of 7. pH is measured on a scale of 0 to 14. Anything less than 7 is considered acidic, while anything greater than 7 is considered alkaline, or basic. 

Why is soil pH important?

Soil pH is also termed the “master variable” in measuring soil health, and for a good reason: it is an important indicator for the soil processes which support life. In the agricultural context, soil pH influences the nutrient absorption and soil fertility through controlling processes such as nutrient cycles, microbial activity and biodegradation of organic matter. With proper management of soil pH such as controlled use of soil amendments, fertilisers and irrigation, nutrient availability and uptake can be maximised, leading to better plant health and greater crop productivity.      

Why does soil pH affect nutrient availability?

Figure 1: Positive ion nutrients adsorbed to small clay particles are exchanged with H⁺ ions. (Hopkins and Hüner, 2008)

Nutrients exist as positive and negative ions in the soil, adsorbed or stuck to very small-sized soil particles. The nutrients adsorbed onto these particles can be exchanged with one another. For example, as seen in Figure 1, a Ca²⁺ ion can be exchanged for two H⁺ ions. Plants can utilise this process to obtain the nutrients that they need, which is how nutrient absorption occurs – plants secrete H⁺ and OH^- ions into the soil to exchange with the desired nutrient ions. Soil pH affects nutrient availability by controlling which nutrients are stuck to the soil particles and which nutrients are available for uptake. The nutrient ions available for uptake will change according to different soil pH ranges. This can also be the reason for nutrient deficiencies still persisting even though sufficient amounts of nutrients are applied - the soil pH causes certain nutrients to be unavailable for uptake. 

What is the optimal soil pH?

Generally speaking, an optimal soil pH for best overall nutrient uptake would be slightly acidic, at a range of 5.5 to 6.5. This range is influenced by the different optimal pH levels for absorption of each nutrient, and the type of crop grown. Each nutrient ion has its own range of pH where it is most available and easily absorbed by plants. The primary nutrients (eg. N, P, K) are usually best absorbed in more neutral conditions, while metallic trace elements (eg. Fe, Mn, Zn) are best absorbed in more acidic conditions. Furthermore, different crops have different nutrient requirements and preferred growth conditions – some crops grow better in more acidic conditions, others don’t. Hence, a pH range of 5.5 to 6.5 is a good overall average for optimal nutrient uptake without compromising the uptake of any nutrients, taking into account different types of crops. Figure 2 below shows the optimal pH ranges for different nutrient availability. 

Figure 2: Effect of soil pH on nutrient availability (Roques et. al., 2013)

What happens if soil pH is too high or too low?

Soil pH which is out of the 5.5 – 6.5 range leads to reduced nutrient uptake and crop growth. Usually, soil pH which is too low (acidic) is more of a problem than soil pH which is too high (alkaline). Nevertheless, soil pH which is out of the optimal range may lead to nutrient deficiencies and toxicities. 

Impacts of low soil pH:

When soil pH is too low, Aluminium and manganese will become more available in the soil, causing it to rise to toxic levels. This leads to plant root growth decreasing significantly, affecting the ability of plants to properly absorb nutrients. pH which is too low will also cause Calcium, Magnesium, Molybdenum, Boron, Nitrate-Nitrogen and Phosphorus deficiencies. Furthermore, low soil pH causes microbial activity to be reduced, as beneficial microbial populations cannot survive in these conditions. This leads to lower or no organic matter decomposition, causing nutrients to be trapped in undecomposed organic matter. 

Impacts of high soil pH:

Overly high pH can cause nutrient deficiencies with nutrients such as Phosphorus, Iron, Copper, Zinc and Boron. This can lead to trace element deficiency.

In other words, soil pH which is too low or too high will lead to poor root health, decreased nutrient availability, nutrient toxicities and deficiencies, and unhealthy root zones. Poor plant health subsequently leads to reduced quality and quantity of crop yields. 

Conclusion: 

Only by understanding the importance of soil pH and its impacts on plant health and nutrient uptake, can proper steps be taken to manage and keep it in optimal range for maximised crop yields. What are the factors that control soil pH and what kind of soil pH does Malaysia have? How can we fix non-optimal soil pH using AgroBridge’s product - AMICOTE? Stay tuned for our next article to find out!  

References

Havlin, J., Tisdale, S.L., Nelson, W.L. and Beaton, J.D., 2016. Soil Fertility and Fertilizers : An Introduction to Nutrient Management. 8th ed. Tamil Nadu, India: Pearson.

Mosley, L.M., Rengasamy, P. and Fitzpatrick, R., 2024. Soil pH: Techniques, Challenges and Insights from a Global Dataset. European Journal of Soil Science, [online] 75(6). https://doi.org/10.1111/ejss.70021.

Neina, D., 2019. The Role of Soil pH in Plant Nutrition and Soil Remediation. Applied and Environmental Soil Science, [online] 2019(5794869), pp.1–9. https://doi.org/10.1155/2019/5794869.

Rengel, Z. (2011). Soil pH, Soil Health and Climate Change. In: Singh, B., Cowie, A., Chan, K. (eds) Soil Health and Climate Change. Soil Biology, vol 29. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-20256-8_4

Roques, S., Kendall, S., Smith, K., Newell Price, P. and Berry, P., 2013. A Review of the non-NPKS Nutrient Requirements of UK Cereals Andoilseed Rape. [online] United Kingdom: Agriculture & Horticulture Development Board. Available at: <https://www.researchgate.net/publication/277669269_Review_of_the_non-NPKS_nutrient_requirements_of_UK_cereals_and_oilseed_rape> [Accessed 13 February 2025].

USDA Natural Resources Conservation Service, 2011. Soil Quality Indicators: Soil pH. Soil Quality Indicators. [online] Available at: <https://www.nrcs.usda.gov/sites/default/files/2022-10/soil_ph.pdf> [Accessed 13 February 2025].

William Garret Hopkins and HünerN.P.A., 2009. Introduction to Plant Physiology. 4th ed. Hoboken: John Wiley & Sons, Cop.