Soil pH is a measure of the acidity or alkalinity in soils. In the pH scale, pH 7.0 is neutral. Below 7.0 is acidic and above 7.0 is basic or alkaline. Soil pH affects nutrients available for plant growth. In highly acidic soil, aluminum and manganese can become more available and more toxic to plant while calcium, phosphorus, and magnesium are less available to the plant. In highly alkaline soil, phosphorus and most micronutrients become less available.

When designing or planting new garden or landscape, it is helpful to check the soil pH as different plants thrive in different soil pH ranges. pH determination can give indication whether soil is suitable for the plants to be grown or it needs to be adjusted to produce optimum plant growth.

What is soil pH?

Soil pH generally refers to the degree of soil acidity or alkalinity. Chemically, it is defined as the log10 hydrogen ions (H+) in the soil solution. The pH scale ranges from 0 to 14; a pH of 7 is considered neutral. If pH values are greater than 7, the solution is considered basic or alkaline; if they are below 7, the solution is acidic. It is important to recognize that because the pH scale is in logarithmic units, a change of just a few pH units can induce significant changes in the chemical environment and sensitive biological processes.

For example, a soil with pH 5 is 10 or 100 times more acidic than a soil with pH 6 or 7, respectively. Sources of H+ ions in soil solution include carbonic acid produced when carbon dioxide (CO2) from decomposing organic matter, root respiration, and the soil atmosphere is dissolved in the soil water. Other sources of H+ ions are root release, reaction of aluminum ions (Al+3) with water, nitrification of ammonium from fertilizers and organic matter mineralization, reaction of sulfur compounds, rainwater, and acid rain. Certain soils are more resistant to a drop or rise in pH (buffering capacity). Therefore, the lime requirement, which is the quantity of limestone (CaCO3) required to raise the pH of an acid soil to a desired pH, must be determined specifically for each field before amending the soil.

What Impacts Soil pH?

Effect of pH level on nutrient availability

Plants take up N in the ammonium (NH4+) or nitrate (N03-) form. Most annual crops tend to take up most N as nitrate. At a soil pH range of six to eight, the microbial conversion of NH4+ to N03- (nitrification process) is rapid. In acidic soils (pH less than six), microbial activity is reduced, the nitrification process is slower and plants with the ability to take up NH4+ such as canola, may have a slight advantage.

Soil pH can play a role in N volatilization losses. Ammonium in the soil solution exists in equilibrium with ammonia gas (NH3). The equilibrium is strongly pH dependent. The difference between NH3 and NH4+ is one H+ ion. For example, if NH4+ were applied to a soil at pH seven, the equilibrium condition would be 99 per cent NH4+ and one per cent NH3. At pH eight, approximately 10 per cent would exist as NH3 gas. This means that a fertilizer like urea (46-0-0) is generally subject to slightly higher volatilization losses at a higher soil pH. But it does not mean that losses at pH seven will be one per cent or less. The equilibrium is dynamic. When one molecule of NH3 escapes from the soil, a molecule of NH4+ converts to NH3 to maintain the equilibrium.

The pH effect is only part of the full story of volatilization. Other factors such as soil moisture, temperature, texture and cation exchange capacity can also affect volatilization. The important point to remember is that under conditions of low soil moisture or poor urea incorporation, volatilization loss can be considerable even at pH values as low as 5.5.

Legume crop roots live in association with Rhizobium bacteria which in turn provide N for crop growth. Soil pH is a critically important factor in N fixation by legume crops. The survival and activity of Rhizobium bacteria declines as soil acidity increases. This is the concern when attempting to grow alfalfa on soils with a pH less than six. It is a serious concern for pulse crops when soil pH is less than 5.5.

Phosphorus

Soil phosphate (PO4) is quite pH dependent. Plants take up soluble PO4 from the soil solution, but this soluble PO4 pool tends to be extremely low, often less than one lb./ac.

The limited solubility of soil P relates to its tendency to form a wide range of stable P compounds in soil. Under alkaline soil conditions, P fertilizers such as mono-ammonium phosphate (11-55-0) generally form more stable (less soluble) minerals through reactions with calcium (Ca). Some agronomists will recommend higher rates of fertilizer P for crops growing on slight to moderately alkaline soils.

However, contrary to popular belief, the P in these Ca-P compounds will still contribute to soluble soil PO4 for crop P uptake. As plants remove PO4 from the soil solution, more Ca-P compounds will dissolve, and soil solution P levels are replenished for crop uptake. Alberta field research trials have clearly shown that over 90 per cent of the fertilizer P tied up after application will still become available to crops in subsequent years. Therefore, I normally do not recommend higher rates of P fertilizer just because soil pH is slightly to moderately alkaline.

The fate of added P fertilizer in acidic soils is different. When phosphate fertilizer reacts with aluminum (Al) and iron (Fe),which are more soluble in acid soils, formation of Al-P and Fe-P compounds will occur. The P tie-up with Al or Fe is much more permanent than with Ca-P compounds. I am far more concerned with P tie-up in acid soils than alkaline soils.

Potassium

Generally, soil potassium (K) is relatively unaffected by soil pH. Some agronomists become concerned when soil pH is increased by liming and feel soil K availability is reduced and that additional K fertilizer is needed after liming. However, this is not the case. Liming increases K availability, through the displacement of exchangeable K by Ca in the lime.

Sulphur

Sulphate sulphur (S042-), the plant available form of S, is relatively unaffected by soil pH.

Micronutrients

The availability of the metal micronutrients manganese (Mn), iron (Fe), copper (Cu),and zinc (Zn) tend to be very slightly decreased as soil pH increases up to pH eight but for the majority of soils and crops this decrease is not a concern until soil pH is greater than eight. Boron (B) availability is reduced when soil pH increases above 7.5 but is really only a concern if soil test B is very low (less than 0.4) and boron sensitive crops such as alfalfa or canola are grown. The mechanisms responsible for reducing nutrient availability differ for each nutrient, but can include formation of low solubility compounds, greater retention by soil colloids (clays and organic matter) and conversion of soluble forms to ions that plants cannot absorb.

Molybdenum (Mo) behaves opposite to the other micronutrients. Plant availability is lower under acid conditions.