Soil pH is a key factor in farmland as it controls availability of nutrients, microbial activity and crop productivity. Before delving into what causes soils to become acid or alkaline and the steps to take to treat and correct soil pH, we must first establish what is considered an optimal pH for crop production.
The only sure way to determine the pH content of soil is to do a soil test. Soil pH measures hydrogen (H) ions in soil and typically ranges between 5.5 and 8.0. For most prairie crops, the ideal soil pH range for optimal growth and development is of 6.5 to 6.8.
Read more about soil testing in our post Soil Testing: What Should You Look For?
Soils with pH ranging from 5.6 to 6.0 are considered moderately acid, while strongly acid and very strongly acidic soils have pH ranging from 5.1-5.5 to <5.0, respectively. Crops have difficulty establishing and show a decline in productivity and yield in soils with a pH below 6.0.
When soil pH is less than 5.5, the soil is very acidic and likely has high levels of aluminum and/or iron, alongside lower content of calcium and magnesium. Low pH reduces availability of nutrients such as phosphorus and molybdenum, affects nitrogen fixation and causes crop toxicity with elements such as aluminum or manganese that become more soluble at a lower pH. Soil liming may be necessary when the pH is less than 6.0. Additionally, root growth can increase by 40% when pH rises from 5.5 to 7.2.
What Causes Soils to become Acid?
Acidic soils tend to be high in iron and aluminum oxides, as they are the slowest minerals to weather in soil. Aluminum in these increasingly acidic soils is solubilized and will combine with water to release additional hydrogen ions contributing to further acidity. Soils in areas with large amounts of rainfall tend to be acidic because the water leaches basic cations such as calcium, magnesium, potassium and sodium out of the soil profile, leaving room for acidic cations such as hydrogen and aluminum.
The parent substrate from which the soil developed can be a source of acidity. For instance, soils developed from land with high organic matter containing conifers or with high contents of iron or aluminum tend to be acidic. Acidity can also be increased by several additional factors, including the nitrification of ammonium fertilizers, which yields hydrogen ions.
Consequences of Soil Acidity
Low pH caused by soil acidity increases the solubility of zinc, manganese, iron and aluminum, which can lead to rapid accumulation and toxicity. As well, the high levels of aluminum and iron in acid soil cause tie-up, reducing phosphorus availability; molybdenum becomes less available to plants, which affects nitrogen and phosphorus uptake and utilization, restricts root growth and reduces nodulation in pulse crops; microbial activity, the decomposition of organic matter and mineralization are negatively affected; and, under acid conditions, calcium, magnesium, and potassium deficiencies become more pronounced.
Improving Acidic Soils
A common and economical way to raise the pH of the soil is by liming. The amount of lime material required depends on the pH of the untreated soil and the desired pH to grow the crop, the amount of soluble and exchangeable acidity, the crop’s tolerance to acidity/alkalinity, the amount of organic matter in the soil, and the type of clay present in the soil. Lime is most effective at neutralizing acidity when it is incorporated or tilled into the soil to the full depth of the root zone.
OMEX has developed several products that complement liming and can help mitigate the effects of aluminum toxicity:
- Pulse Primer, Pulse Pak and Primer Soybeans – Applied to seeds prior to sowing, these calcium-based seed primers protect the emerging radicle from the toxic effect of aluminum, encourage nodulation, and provide a sufficient amount of molybdenum unavailable in acidic soils.
- TPA – This in-furrow liquid Starter improves efficiency of phosphorus, diminished due to aluminum or iron tie-up. TPA is formulated with a patented Thermo Poly Aspartate molecule with a high CEC that breaks the bonds between aluminum/iron and phosphorus, rendering the latter more available to the plants.
- Sequestri-Cal – Is a calcium-based product designed for tank-mixing with in-furrow applied starter and is able to improve pH around the root zone of the seedlings.
Alkaline soil is used to describe a soil with a high pH level (greater than 7.3).
When soil pH is greater than 8.0, availability of nutrients like phosphorus or micronutrients (i.e. Zn, Cu, Mn, Fe, …) can be reduced and soil pH greater than 8.3 can indicate high sodium or sodic soil problems and often drainage issues. Elemental sulfur may be required on these soils especially if they are high in magnesium.
What Causes Soils to Become Alkaline?
Soils become alkaline for a variety of reasons. They may be located on very dry land and/or in areas with little rainfall. The alkalinity might be caused by the nature of the soil itself, or as a result of receiving water that contains highly alkaline substances (i.e. calcium or magnesium carbonate). Land that has never been broken tends to be pH neutral but once cultivated, it tends to become alkaline because of calcium and other salts pulled from the soil’s lower horizons.
Consequences of Soil Alkalinity
Generally speaking, alkaline soil contains a high calcium carbonate content, which prevents plants from absorbing most of the other essential nutrients. Even if adequate nutrients are present in the soil, plants cannot benefit from them. For instance, soybeans grown on high alkaline soils in the Prairies tend to develop iron chlorosis, which can be easily corrected through the use of iron, supplied in-furrow or by foliar application.
Improving Alkaline Soils
To manage alkaline soil, growers can try the following methods for lowering soil pH: incorporate organic matter into the soil – the acidic reaction produced by decomposition will help reduce pH levels and the process will help improve drainage and aeration; use humic acid (i.e. OrganoHume) regularly to stimulate biological activity involved in mineralization and decomposition of organic matter, which in turn can generate organic acids; incorporate sulfur or sulfur-coated urea into the fertility program.
As well, if you are struggling with alkaline soils and have hard water, you could be exacerbating the problem when you spray or irrigate. Consider softening your spray water with pHix or SopHtner95. Formulated with naturally occurring organic acids, SopHtner95 is safer to use in-crop than other common pH reducers, such as ammonium sulfate, and is considered an environmentally sound management tool for lowering alkaline soil conditions to reduce crop-stress.
Talk to Your OMEX Rep
If you struggle with acidic or alkaline soils, your OMEX representative can guide you in the selection of the right products for your crop’s needs. Your rep can provide you with a nutrient management strategy that incorporates our wide range of seed-applied Primers, in-furrow Starters, in-crop foliars and other specialty products to help you meet your yield goals, under a wide range of conditions.