Farmers frequently ask “What does soil pH really mean?” and “What effect does pH have on availability of nutrients in the soil?” Both are excellent questions! The effects of pH are complex and vary with different nutrients. However, some broad generalizations are useful to keep in mind when understanding soil pH and making nutrient management decisions.
What is soil pH?
Soil pH comes from a French term meaning the “Power of Hydrogen.” It is a measure of hydrogen ion (H+) and hydroxyl ion (OH-) concentration in soil. It refers to the acidity (low pH) or alkalinity (high pH) of soil and is measured in pH units.
The pH scale goes from zero to 14. A pH of seven is neutral. As the amount of H+ ions in the soil increases, the soil pH decreases, and the soil becomes more acidic. As the amount of OH- ions in the soil increases, the soil pH increases, and the soil becomes more alkaline. From pH seven to zero, the soil is increasingly more acidic, and from pH seven to 14, the soil is increasingly more alkaline or basic.
Using a strict definition, pH is the negative log of H+ ion activity in a solution. This means the pH values are reported on a negative log scale. So, a one-unit change in soil pH value signifies a 10-fold change in the actual activity or concentration of H+, and the H+ activity increases as the pH level decreases.
To put this into perspective, a soil pH six has 10 times more H+ions than a soil pH seven, and a soil pH five has 100 times more H+ ions than a soil pH seven.
Soil testing labs usually determine soil pH in a 2:1 distilled water-to-soil mixture or in a weak solution of calcium chloride, and results are expressed as pH. The pH test using calcium chloride is considered by some as more accurate as it reflects what the plant root experiences in the soil. In a soil test report, pH is often reported with a descriptive modifier (see the table). Soil pH can easily vary by half a unit over a growing season depending on environmental conditions.
Soil acidity comes from H+ in soil but also from aluminum (Al3+) ions in the soil solution and held on soil colloids. Soil Al3+ is important in acid soils because below a pH of six, Al3+ reacts with water (H2O) to form aluminum hydroxide compounds that release H+ ions into soil solution. Various processes can contribute to the development of acid soils including rainfall, fertilizer use and natural weathering of soil minerals.
In Western Canada, nitrogen (N) and sulphur (S) tend to be the most acidifying fertilizers. For example, anhydrous ammonia (NH3), urea [CO(NH2)2] and other ammonium (NH4+) fertilizers react in the soil in a process called nitrification to form nitrate (NO3-), and in the process release H+ ions. So, as we continue to use significant amounts of N fertilizer, pH of many of our agricultural soils will gradually become more acidic.
Higher pH soils or alkaline soils have a high saturation of base cations potassium, calcium, magnesium and sodium (K+, Ca2+, Mg2+ and Na+). (I will discuss this in more detail in my next article).
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.
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.
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.
Sulphate sulphur (S042-), the plant available form of S, is relatively unaffected by soil pH.
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.
Soil pH can play a role in soil nutrient and fertilizer availability. Should you be concerned on your farm? Be more aware than concerned. Keep the pH factor in mind when planning nutrient management programs.
Over the past 20 years, declining soil pH seems to be occurring more noticeably in continuously cropped, direct-seeded land than conventionally tilled land. A soil with an optimum pH now may very gradually become more acidic over the next 20 years depending on fertilizer use and land management. What is most important is to keep historical records of soil pH of your fields. Soils tend to acidify very gradually over time when higher rates of N and S based fertilizers are used.