Soil pH is simply the acidity or alkalinity of a soil and is measured along the pH scale from 0 to 14. Anything below a pH of 5 is strongly acidic and anything above a pH of 8 is strongly alkaline. The middle point of seven is neutral, making the soil neither acidic nor alkaline.
You can find out the pH of a given soil in a couple different ways. Most soil sample test reports come with a pH reading. If your lab doesn’t automatically supply a pH reading, you can request one. The other way is to do your own test. You can buy a pH soil tester from garden shop. These can range from simple probes that cost $20 to a more complex tester that costs up to $100.
IT CAN CHANGE
Soil pH depends on the parent and subsoil material that the soil is made from. Topsoil pH can change when you mix up the soil with tillage. There are also some proven methods for changing the pH up or down, as needed. Adding lime can make soil more alkaline, but it can take a very large amount of lime to create the desired effect. You could also add elemental sulphur to make the soil more acidic.
SOIL PH AND FERTILIZER UPTAKE
Nitrogen uptake is directly affected by soil pH. When a soil is near neutral, this is the best soil condition for the conversion of nitrogen and the plant is in a better position to use the nutrient more quickly. As a soil becomes more acidic, conversion of nitrogen fertilizer into a plant-available form is slowed down. This can actually be good for crops such as peas and other pulses that are able to uptake nitrogen better on their own. They will use the fertilizer more efficiently and have greater production as a result.
For crops that are more susceptible to chemical damage or for chemicals that have a history of being harder on crops, growers with low pH (acidic) soils may want to be more careful with their spray decisions.
Soil pH also influences the amount of available phosphorous. In a more alkaline soil, the phosphorous becomes more stable, binding more readily with other minerals such as calcium. In this case, phosphorous becomes less soluble and more difficult for crops to access and uptake. However, even though this phosphorus is tied up with the calcium, it can become available over time as the pool of soluble phosphorus goes down. So if all the phosphorus fertilizer is not accessed in one year, it may still be there to be used next year.
This may not be the case in more acidic soil. In soils with low pH, the bonding minerals create a more permanent situation. Crops will not have the same access, over time, to phosphorus reserves in the soil.
Potassium and micronutrient availability are also affected by the soil pH. With potassium, a slightly more alkaline soil can be more beneficial for potassium uptake. If you do add lime, this has shown to increase the availability of potassium reserves for the crop. As for micronutrients, it seems that the higher the soil pH, the more available they are to the crop. For manganese, copper and iron in particular, a slightly alkaline soil seems to be best for plant uptake.
When you know your soil pH, you can use the knowledge to predict how much of the fertilizer you apply will actually be available to the crop.
Nitrogen uptake is best in neutral to slightly acidic soils. Phosphorus and potassium availability, in the long term, is improved with alkaline soils.
SOIL PH INFLUENCESHERBICIDE PERFORMANCE
Producers put herbicide chemicals into or onto their soil multiple times every year. Soil pH influences the rate of uptake. In soils that are slightly acidic, herbicide uptake — by weeds AND crops — is much easier. That means weed kill is faster and more effective, but you’re also more likely to see crop injury. For crops that are more susceptible to chemical damage or for chemicals that have a history of being harder on crops, growers with low pH (acidic) soils may want to be more careful with their spray decisions.
It was also thought that soil pH may have an effect on the carryover of herbicides and the ability for that residual chemical to have an effect on the next year’s crop. But the research indicates that in a pH of around seven, some group 2 herbicides did carryover and some did not. This suggests that pH was not the major factor. It turns out that moisture and organic matter are much more important in determining herbicide carryover from year to year.
As you can see, pH is just another one of the many factors contributing to the productivity of the soil. Knowledge in this area is beneficial for understanding how your soil is working and how it affects your crops and the nutrients they are trying to access from the soil. Like most things in farming, every situation is different and each situation has different needs. However there are plenty of resources and previous research out there to help you understand how pH fits into the puzzle of more profitable crop production.
Jay Peterson farms near Frontier, Sask. He graduated from University of Saskatchewan