Understand pH in Order to Understand Nutrient Availability

No other soil characteristic is more important in determining the chemical environment of plants and soil microbes than pH. The degree of soil acidity, expressed as soil pH, greatly influences the root uptake availability of many critical nutrients.


Effects of Soil pH

Strongly acidic soil presents a host of problems. These include toxicities of aluminum, manganese, and hydrogen, as well as deficiencies of calcium, magnesium, molybdenum, and phosphorus. Since it is difficult to separate one problem from another, the situation is sometimes simply referred to as the acid soil ‘headache’.


Causes of Low Soil pH

One cause of low soil pH is fertilization with ammonium forms of nitrogen. Excessive H+ ions are generated during the bacterial conversion of ammonium nitrogen to nitrate nitrogen. Moving pH levels away from the optimum range impact the availability of nutrients in your soil and cause interactions with other ions that impede their uptake.

Acid rain is another cause of acidic soil. While acid rain has been significantly reduced in North America, it can still be a factor in other parts of the world.


Optimal Soil pH for Plant Growth

Optimal pH conditions for plant growth varies considerably. For example, certain legume crops, such as alfalfa and sweet clover, grow best in near neutral or alkaline soils. Corn and soybeans prefer a pH range between 6.0 – 6.5 depending on your soil and region.

How to Measure Soil pH

The most accurate method of determining soil pH is with a pH electrode. In this method, a pH-sensitive probe is inserted into a soil:water (1:1 ratio) slurry. The hydrogen ion (H+) activity is measured and reported as pH. ‘360 Yield Center’ has added pH measurement as part of its 360 SOILSCAN™ in-field soil testing system. Growers can get an instant measurement of nitrate nitrogen and pH.

Raising Soil pH with Lime

If your soil is not at the optimal pH, liming is a common way to decrease soil acidity and raise the pH. Since lime must be added in large enough quantity to chemically react with a large volume of soil, the choice of liming material is often based on what is locally available to avoid excessive transportation costs. But all lime is not equal.

There are two things to consider in determining the quality of the lime and its ability to shift pH – CCE and the particle size. Calcium Carbonate Equivalent (CCE) provides a numeric value for the effectiveness of various lime materials. The CCE gives a producer a comparison of the quarry’s limestone compared to the neutralizing ability of pure calcium carbonate. A stone with a CCE of 100% is equal to the neutralizing effect of pure calcium carbonate. Dolomitic limestone contains magnesium carbonate and can have a CCE greater than 100%. Particle size is important in determining how quickly the limestone becomes effective. Particles smaller than 60 mesh (powder like) are effective immediately, while particles over 60 mesh (bb size) need a year or more to become effective. Effective Calcium Carbonate Equivalent or ECCE combines the stone’s chemistry and particle size to indicate effectiveness compared with pure calcium carbonate with all particles smaller than 60 mesh.

Of course, while 60 mesh limestone may be more effective more quickly, it is more difficult to spread. The best value lime for long term pH management is often a high CCE lime with a combination of particle sizes that improve the spread and extent the calcium carbonate release.

The quantity of lime required to change your soils pH is determined by both the soil water pH and the buffer pH which is a measure of the reserve acidity held in your soil. A soil’s buffering capacity is largely a function of clay and organic matter content and the buffer pH is essentially a measure of your soil’s ability to resist changes in pH. Buffer pH is a different measurement than water pH.

Which ag lime and how much to apply can be determined by consulting your agronomist and supplier.