Crop fertility and nutrient uptake

Crop Nutrition

How Potassium Placement and Root Growth Affect Uptake

eKonomics News Team

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While conducting a tillage trial, researchers in Iowa examined the factors beyond rate that affect how nutrients move from the soil into plants. They took soil samples and applied nutrients based on recommended rates. In some of the plots, however, the corn plants still showed signs of potassium deficiency such as yellow leaves near the bottom of the plant and stunted growth.

According to John Kovar, soil scientist at the USDA National Laboratory for Agriculture and the Environment in Ames, this phenomenon could be a result of issues with nutrient uptake. In particular, it shows how potassium placement and how plant roots grow can affect nutrient uptake. If a plant is unable to properly absorb nutrients, despite adequate fertilizer application, growers will ultimately miss out on yields.

Potassium Placement

In Kovar’s study, researchers took soil samples six inches deep. These results showed adequate amounts of potassium overall.

Kovar then pulled more detailed soil samples from the research site that included no-till plots. He found potassium levels tested high in the top two inches of the soil profile but decreased as the soil profile got deeper; the lack of tillage did not mix the nutrients deeper in the soil profile.

To correct the subsurface potassium levels, Kovar applied higher potassium rates. After seven years, he started to see an increase in subsurface potassium levels.

“When soil tests show levels at the critical level, you have to add back potassium to build up the subsurface levels,” he noted.

Kovar is currently researching other methods to increase subsurface potassium levels, including subsurface potassium application and the use of liquid potassium. While more research is done, he advised farmers in a long-term no-till or reduced till system to monitor potassium at both the surface and subsurface levels to prevent deficiency issues.

The Root of the Matter

Plant roots play another key role in taking up potassium from the soil.

“When making potassium recommendation rates, we need to take into account that plant roots grow differently depending on the plant type,” said Kovar.

Some root growth factors that affect how plants take up nutrients include:

  • The amount of root hairs on the root system. Kovar said plants like corn, wheat and soybeans have numerous root hairs that assist them taking up nutrients like potassium. Other crops with fewer root hairs (such as onions and sugar beets) restrict the amount of nutrients the plant can absorb.
  • Uneven root growth in the soil profile. The roots do not grow uniformly in the soil. More roots may grow on one side of the plant or closer to the soil surface, but this might not be where the potassium is in the soil.
  • Soil compaction limiting root growth. In compacted soils, roots have more trouble growing due to increased bulk density and reduced pore size in the soil profile. Avoiding driving in the field when it is wet and reducing the overall weight of farm equipment are common ways farmers can prevent subsoil compaction.
  • Roots growing past potassium placement. Kovar explained roots could grow beyond the top six inches that are commonly sampled. Some of the corn hybrids marketed as drought tolerant might develop deeper roots. This can lead to plants mining subsurface nutrients such as potassium. “The best bet is not to mine the soil to begin with,” said Kovar.

While researchers like Kovar continue to work to improve potassium rates, farmers should continue to use tools such as soil sampling, nutrient removal rates and tissue sampling.

“I think farmers should keep testing the soil. If you don’t test the soil, you are just guessing,” he advised.

Kovar added farmers should calculate and replace nutrients that are removed from the field in the grain during harvest. Conducting plant tissue tests every few years will also allow them to monitor what nutrients the crops are taking up and be able to reapply accordingly before planting the next season’s crop.