Submitted by: Cameron Ford, on behalf of Northeastern Ontario SCIA & NOFIA
The pH value of soil is an important indicator of overall soil health. Acidic soils are an issue in many regions across Ontario and can result in dramatically reduced yields for both crops and forages. Luckily, compared to other deficiencies or crop diseases, soil acidity can be reduced fairly simply through the application of ground agricultural limestone.
Soil acidity is measured on the pH scale, which is a count of the number of acidic hydrogen ions in a given solution. The pH scale ranges from 0, which is highly acidic, to 14, which is highly basic or alkaline. For reference, stomach acid has a pH value of 1, pure water has a pH value of 7, and bleach has a value of 13. In Ontario, agricultural soils can range somewhere between 4.0 to above 8.0 in pH.
The pH value of soil is largely determined by the composition of its parent material. Parent material is the underlying geologic material that makes up the foundation of the soil itself. For example, the parent material of much of northern Ontario is the Canadian Shield, which is igneous rock and is acidic in nature. Elsewhere, limestone is the parent material, resulting in more alkaline soils.
Awareness of soil pH is important because it plays a role in the availability of nutrients for crops. Different nutrients and compounds that plants need to grow become soluble at different pH levels. For example, manganese becomes increasingly available to plants in more acidic soils and can reach toxic levels for crops at soil pH values below 5. Additionally, microbial activity is reduced at both high and low pH levels, even further reducing the availability of nutrients.
While requirements for individual crops vary, most crops can take up their required nutrients at soil pH values of between 6 and 7.5. OMAFRA guidelines broadly recommend liming when test results show soil pH levels of 5.0 or 5.5, depending on the crop.
Table 1. Guidelines for lime application to Ontario Crops
Soil pH below
|Coarse and medium textured mineral soils (sand, sandy loams,
loams, and silt loams)
|Perennial legumes, oats, barley, wheat, triticale, beans, peas, canola, flax,
tomatoes, raspberries, strawberries, all other crops not listed below
|Corn, soybeans, rye, grass, hay, pasture, tabacco||5.6||6.0|
|Fine textured mineral soils (clays and clay loams)|
|Alfalfa, cole crops, rutabagas||6.1||6.5|
|Other perennial legumes, oat, barley, wheat, triticale, soybeans, beans, peas,
canola, flax, tomatoes, raspberries, all other crops not listed below
|Corn, rye, grass, hay, pasture||5.1||5.5|
|Organic Soils (peats/mucks)|
|All field crops, all vegetable crops||5.1||5.5|
Adapted from Table 3.1, OMAFRA Publication 611, Soil Fertility Handbook
Soil sample analyses often return two pH results. The first is the simple pH of the soil sample and is the value used in determining whether or not liming is necessary. If that soil pH value is less than 6.1, then a Buffer pH test will also be conducted. Buffer pH is measured by adding a lime solution to the sample and measuring the resulting change in pH. A significant difference between the pH and the Buffer pH values means that an application of lime will have a major change in the pH of that field.
Conversely, if there’s very little difference in the pH and Buffer pH scores, that means that the soil contains a high degree of ‘reserve acidity’, which refers to the ability of the particles of soil to hold hydrogen ions. The higher the reserve acidity stored in soil, the more lime it takes to make a difference in pH. Heavier soils generally have a higher reserve acidity, which is why it takes so much more lime in clay soils than in sandy soils to make the same changes in pH.
Lime neutralizes soil through a chemical reaction wherein oxygen molecules in the carbonate bind with hydrogen ions in the soil, eliminating the acid and resulting in carbon dioxide and water. However, in order for this reaction to occur the lime needs to dissolve. Lime dissolves very slowly, so it can take months after an application to begin to show neutralizing effects and years to completely dissolve and react. The best way to speed up the neutralization process is to maximize the surface area of the lime in contact with soil. The more finely ground the lime, the more surface area, and the faster it will dissolve. OMAFRA recommends using lime that has been graded through at least a #60 sieve (about .25 millimetres). Anything more finely ground than #60 might begin to neutralize the soil faster, but will ultimately not have a greater effect on neutralization.
Agricultural limestone is graded with an Agricultural Index rating, which is a quantified description of the efficacy of a given source of lime in neutralizing acidity. Lime from different limestone sources will contain different amounts of calcium carbonate and will therefore vary in its ability to actually change pH.
An Agricultural Index score is calculated by multiplying a lime’s neutralizing quality by its fineness rating and dividing by 100. The higher the Agricultural index, the more effective a given amount of lime will be at neutralizing acids. The average agricultural index score in Ontario is 75, though it can range between ~30 to above 100.
The Agricultural Index of a chosen lime source, soil pH and Buffer pH are combined in an equation to determine the total amount of lime required to adjust the pH of a field.
Table 2. Ground limestone requirements to correct soil acidity based on soil pH and soil buffer pH, assuming lime Agricultural Index of 75.
|Target soil pH=6.5 (if pH below 6.1)||Target soil pH=6.0 (if pH below 5.6)||Target soil pH=5.5 (if pH below 5.1)|
Adapted from Table 3.2, OMAFRA Publication 611, Soil Fertility Handbook
As an aside, Muskoka SCIA conducted a research project between 2005 and 2008 testing different rates of lime application to see if the recommended amount was actually the most effective. While they tried using 100%, 150% and 200% of recommended lime on plots, they found that the recommended (100% level worked best for maintaining soil pH, and it also resulted in increased yield in hay bales per hectare compared the other two higher levels.
There are two different kinds of lime: calcific and dolomitic. Calcific lime is ground from limestone primarily composed of calcium carbonate. Dolomitic lime comes from limestone that is a combination of calcium carbonate and magnesium carbonate. Both types have comparable effects on pH value, but dolomitic lime adds magnesium to the soil. If soil tests show a deficiency in magnesium (i.e. less than 100 parts per million) use dolomitic lime, otherwise use whichever is more economical.
Shortly after spreading the lime, it is important to till the field to ensure that the lime is incorporated into the soil and the resulting pH change will be relatively evenly spread down to the depth of the plow.
No-till fields may experience issues with pH stratification. The application of nitrogen heavy fertilizers could result in the acidification of the top couple of inches with the rest of soil having an otherwise acceptable pH. In this situation, rather than tilling the soil, lime can be applied directly onto the surface of soil and left there. Since this method of application is much less effective than tilling the lime into the soil, more frequent applications are required and it may take years to bring that top layer of soil back to a sufficiently neutral pH value. A trial of this type is being conducted at the Temiskaming Community Pastures near Earlton, Ontario. Lime was applied to the surface of the pastures in 2017, and since then the top layer of soil has showed increasing pH levels as the lime dissolves.
On the other side of the scale, having alkaline soils is generally not a cause for concern as most crops are still able to metabolize the nutrients they need in mildly basic soils. That said, if crops are showing signs of deficiencies due to insolubility of nutrients at high pH values, the best way to counter that is to directly provide the crops with the deficient nutrient, rather than attempt to reduce the pH. This is because in most scenarios it would take so much sulphur to make a real difference in soil pH that it wouldn’t be worth trying.
There are also some alternatives to lime for increasing soil pH. One option is wood ash, the effectiveness of which was studied at Lakehead University between 2004 and 2008. It was found that lime and wood ash had comparable effects on pH, but since wood ash has a lower Agricultural Index than lime, some two or three times more ash was required. Wood ash has the added benefit that it contributes many micro- and macronutrients to soils. In the Lakehead study the ash plots showed higher alfalfa yields than the lime plots. In addition, there is some concern about wood ash depositing detrimental heavy metals in soil, but after 4 years the wood ash plots showed comparable heavy metal levels to the lime and control plots. If wood ash is locally and cheaply available, it could be tried as a viable alternative to lime.
Liming is an important tool in the overall soil health toolbox. Vigilance in conducting soil testing and applying lime when necessary is an easy way to lead to increased yields.