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.
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.