4R Nutrient Stewardship is top of mind today. One of the program requirements is to maintain current soil samples­—none older than four years. Using current soil sample results allows us to make recommendations that account for changes in rotation practices, yields and nutrient applications from all sources. I doubt you would make financial decisions today from a balance sheet from 2005; the same applies to a soil report. Current information is always a better way to go.

A soil report measures the soil’s ability to supply the essential nutrients to meet the intended yield goals. The report is a reflection of how the fields have been managed in the past. If nutrients are low, that means more nutrients are being removed than what is being applied. Conversely, if nutrient levels are increasing, the application rates are higher than removal rates. This gives us the insight needed to adjust nutrient plans.

The most important value on the soil report is pH and BpH (buffer pH). The pH influences several soil conditions and reactions.

Soil pH influences:

1. Nutrient availability
2. Disease organism activity
3. Micro-organism activity
4. Potential crop damage by some herbicides

Cause of low pH:

1. Crops removal of base nutrients
2. Leaching losses – up to 350 lbs. of calcium per acre can be lost annually
3. Decomposition of organic materials
4. Nitrification of ammonium fertiliser takes 1.8 lbs. of calcium carbonate to neutralise acidity from 1 pound of ammonium fertiliser
5. Normal crop growth roots release hydrogen ions

Most crops grow well between 6.0-8.0 pH. The chart below indicates the optimum pH range for each essential nutrient.

Availability of nutrients relative to the pH

(Left) ideal pH in mineral soils is 6.8. (Right) ideal pH in muck soil is 5.5.

When soil pH is below 6.5 on a soil sample, we instruct the lab to perform a buffer pH test. The buffer pH is a test procedure that uses a buffered solution at a pH of 7.5. The buffered solution is mixed with your soil sample, and the pH is reread. The result is reported as the BpH. The purpose of the BpH is to determine the reserve acidity, which is used to determine the rate of limestone required to move the pH to a targeted level. Soil texture influences the pH, BpH and the resulting rate of limestone.

What is contributing to low pH readings is the hydrogen ion concentration in the soil. The higher the hydrogen content, the lower the pH. Sand and clay soil can have the same pH but entirely different BpH values. Clay soil will have a greater amount of hydrogen and will tend to have lower BpH and higher rates of limestone required. Conversely, sandy soil will have less total hydrogen, a higher BpH reading and a lower rate of limestone required to change the pH. For this reason, you cannot determine the correct rate of lime without a BpH value. You should never guess at rates of limestone. You can over lime soil, and it will really mess up the availability of some nutrients such as manganese.

Site-specific soil sampling is a good approach to determining the extent of not only nutrient variability but also pH distribution. This allows a targeted approach to lime applications and avoids over liming soil areas that do not need the lime.

There are two primary sources of lime regularly used in the area—Dolomitic and Calcitic lime. Dolomitic contains calcium and magnesium. Calcitic contains calcium and trace amounts of magnesium. When soil test magnesium is below 100 ppm, we choose dolomitic to have both magnesium and calcium sources to address the nutrient and pH issues together.

To measure the quality of limestone, we test for the Ag Index. The index measures the fineness and neutralising value. Our limestone recommendations assume an Ag Index of 75. We adjust the application rate based on different Ag index values of the limestone.

Application in the fall after harvest or mid-summer after wheat is the best time to apply lime, allowing time to react with the soils. A tillage pass to incorporate the lime thoroughly in the root zone offers the best result.

In conclusion, the practice of liming, when required, can improve crop production and optimise profitability. It all starts with a soil sample. We have records of past soil sampling and will be reaching out to inform you of the need for a current soil test. Otherwise, call your local AGRIS or Wanstead Co-operative to arrange for soil sampling.

If you have any questions or concerns with anything you are seeing in your fields, contact your local AGRIS Co-op Crop Sales Specialist.

Reference Source OMAFRA Pub 611.