The case for soil sampling

September 3, 2024 | Joe McClure

The Iowa farmer is dependent on a lot of factors, but can only control a few.

Nutrient availability and the application of fertilizer are two of the best management practices a farmer can use to impact and drive profitability on each acre.

Soil pH is a foundational factor of crop production and can be adjusted in as little as one year, making it a good investment even on most rented acres, according to Antonio Mallarino, professor emeritus and former professor of agronomy at Iowa State University.

Soil pH is a measure of the acidity of the soil, and optimal soil pH allows for optimal crop growth, ensuring the ideal availability of key nutrients necessary for soybean and corn production.

Impacts on yield

Research from Mallarino and others at Iowa State University shows that crop yield response to liming is consistent when soil pH is below 6.0, but there are higher and more consistent yield responses to lime products when there is a low pH subsoil, as in eastern and southern Iowa.

When pH is below 6.0 in Iowa, phosphorus, potassium, calcium and magnesium availability is restricted and many micronutrients increase to toxic levels. As field pH decreases to 5.5, the problems with nutrients are exasperated and nitrogen availability and plant health are negatively affected.

Crop root growth is negatively affected by low pH, leading to decreased soil aggregation, which affects water infiltration. Soybean nodulation is also reduced in acidic soils and the majority of nitrogen fertilizers, including swine manure, acidify the soil, adding to the need to maintain soil pH.

Questions farmers should ask themselves:

• Has it been more than four years since my fields were sampled?
• Do I have areas in the field with a soil pH below 6.0?
• Am I spending money on extra fertilizers or biologicals before I soil sample and manage my soil pH?

If the answer is yes to any of these questions, Iowa Soybean Association (ISA) recommends that farmers build a soil sampling plan independently or with a trusted advisor to understand the soil conditions and the opportunities to drive yield.

According to Craig Struve, CEO at SoilView, LLC, soil pH and soil buffer pH continue to be two of the most important attributes taken from a soil sample report. Nearly every field in Iowa has areas where the soil pH can easily be adjusted with variable rate lime application, according to Struve. Results from SoilView show 48% of the samples have a buffer pH that indicates ag lime needs should be closely monitored and adjusted.

Soil test data summarized by The Fertilizer Institute from their participating member labs indicate that nearly 30% of the soil samples analyzed for pH from Iowa are at, or below, 6.0. Soil sampling increased between 2001-2020, according to The Fertilizer Institute. However, the number of samples analyzed for Iowa in 2020 also indicates there are many acres not being sampled periodically even when the assumption is that the average field is sampled every four years at a three-acre grid size.

“Many Iowa soils have varying pH levels, and when soil sampling, care needs to be taken to ensure that the sample size is small enough to identify the soil pH spatially across the field,” Struve says. “A grid size of 2.5 acres or smaller is the most practical and cost-effective way to identify the variability in soil pH within a field.”

Soil sampling and proper soil pH management are crucial to maximizing profitability and increasing availability of key nutrients to produce soybeans and corn. This is especially important prior to investing in additional fertilizers or biologicals focused on increasing the availability of nutrients. ISA recommends that every farmer make a soil sampling plan this fall or find a partner who can assist with the plan to drive productivity and profitability of each field.

For a list of resources, contact Joe McClure at jmcclure@iasoybeans.com

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