State producers face challenges managing soil pH

In many instances on the farm, the word “fixed” is used to describe the known and understood factors impacting the grower’s business and profitability.

However, when associated with soil pH and nutrient uptake, “fixed” describes one of the most unknown and challenging issues: fertilizer tie-up. When key nutrients like phosphate are unavailable because they are fixed in the soil, the efficiency of a fertilizer application greatly decreases.

Key Points

Soil pH is one indicator of the level of nutrient tie-up.

Soils in Minnesota vary greatly from one side of the state to the other.

Follow a three-step process to ensure crops are getting adequate phosphate.


Nutrient tie-up in the soil can be an added cost to the grower with little benefit, since these critical nutrients are tied to other compounds and cannot be used by the current crop.

Phosphate ties up

One indicator of soil tie-up is pH, which is the measure of hydrogen ion concentration. More hydrogen ions makes the environment more acidic (a pH of 0-7) while fewer hydrogen ions creates a basic environment (a pH of 7-14). Under low pH conditions, (below 6.0), phosphate is tied up in the soil with aluminum and iron. Under high pH conditions (greater than 7.3), phosphate is tied up with calcium and magnesium.

“Phosphate fertilizer is crop available, but since it reacts with other compounds in the soil, it becomes fixed in a form unavailable to the plant very quickly,” says Scott Inman, Novozymes Crop Production agronomy group leader. “Phosphate is least available in cool soils. However, as soil temperatures increase so does the microbial activity in the soil that can cause phosphate to become unavailable to the plant when it is most active and requiring phosphate.”

Growers in Minnesota face greater challenges than other states in the Corn Belt when it comes to soil pH. First and foremost, the soils of Minnesota vary greatly from one side of the state to the other. As many as 14 soil suborders exist in the state, which can translate to variability in soil pH. The pH of soils in Minnesota ranges from less than 5.0 (where soils are sandy with a low organic matter content) to 8.1 or higher in the western and northwestern part of the state. This variability in soil pH reinforces the importance of soil testing for optimal yields.

“The same compound that makes up our bones — calcium phosphate — is what is responsible for holding phosphate in the soil,” Inman says. “We see that bone can be broken down with acids like citric acid in fruit. Biological technologies are available to do the same in soils, making the phosphate more available to the plant.”

Test and know

Since climate and weather conditions that affect soil temperature are uncontrollable, testing soil pH is a critical step in determining how available nutrients are to the plant. Follow this three-step process to ensure your crops are receiving the phosphate they need to thrive when they need it, regardless of soil pH:

Send in a soil sample for testing. With the extreme variability seen in Minnesota soils and their pH, soil testing is critical for the success of your farm and can be a first step to identify phosphate availability.

While soil pH does not change annually, taking a soil test frequently will help to identify possible shifts in pH and nutrient content, particularly if lime has been applied or cropping systems have applied large quantities of fertilizer.

Keep in mind that while a soil test provides an indication of the level of soil phosphate that is available for plant use, it does not reveal the total amount of phosphate within that soil unless soil sampling depth and soil analysis parameters are increased.

Identify solutions for phosphate tie-up. In alkaline soils, phosphate is especially tied up with calcium and unavailable to the plant. In these soils, biological solutions have been shown to break down the bonds between phosphate and calcium, making the key nutrient available to the crop. A commercially available fungus, such as Penicillium bilaii, breaks down the phosphate tie-up.

Provide phosphate at the right place at the right time. A starter application of phosphate is very beneficial, especially in alkaline soils. Providing phosphate to the plant during cool growing seasons can be beneficial for emergence and seedling vigor.

Source: Novozymes Crop Production

Soil suborders of Minnesota


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Source: Minnesota Agricultural Experiment Station in cooperation with the USDA Forest Service and Soil Conservation Service, Soil and Landscape Analysis Laboratory

This article published in the January, 2011 edition of THE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.