Managing Soil pH in Cotton Can Save Costs - Here's Why

Newsletter - February 2019

What impacts soil pH

U.S. cotton production occurs on a wide range of soil types and different pH levels. Nutrient availability is directly impacted by soil pH. Knowing soil pH is the starting point, but that level may change over time depending nitrogen (N) source applied. Knowing how this change, or preventing it, is critical to optimum fertility management in your cotton fields.

When it comes to changes in soil pH, it can be influenced by both natural and management causes. An example of natural acidification includes cation removal through plant uptake or heavy leaching. When soil breaks down organic matter or ammonium to release nitrate-N, acid is also produced. Management practices can likewise influence soil pH, of which N source has the greatest effect. When plants utilize ammonium as an N-source or nitrification of ammonium occurs, it increases acidity. Soil pH is also reduced by any fertilizer containing elemental sulfur. Lastly, increased crop productivity can increase acidity by the aforementioned cation removal in plant biomass and increasing root respiration that forms carbonic acids.

Impact of soil pH on fertility management

Since natural acid-forming processes happen faster in regions with warmer temperature and greater rainfall, low pH soils are common in many cotton production areas. Acid soil conditions can significantly reduce yield, largely by reducing nutrient availability but also due to the proliferation of certain weed species, reduced microbial activity, toxic elements such as aluminum, and slowed nitrification processes. The optimum soil pH range for cotton is between 6.0 and 7.0 which allows for maximum nutrient availability.

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Any liming decision should be based on the results of an appropriate soil analysis. Grid sampling has shown great success as soil pH can vary greatly across a field. However, it takes very large lime volumes to correct soil acidity, and surface applied lime can take several years to move through the soil profile. This can be a cumbersome management strategy for cotton growing areas. Although lime is the only method to correct a soil pH problem, luckily there are different ways to manage soil pH to reduce acidification potential and lime costs.

Using the right N-source to manage pH

Fertilizer source matters when it comes to the impact of soil pH. Any N fertilizer that requires nitrification to form available N will lower soil pH. The application of animal organic matter, urea, ammonium nitrate, ammonium sulfate, and ammonium phosphates all acidify the soil. Lime is needed to compensate for acidification effect of ammonium-based nitrogen fertilizers. This means that the cost of lime to maintain soil pH should automatically be factored into fertilizer cost. Conversely, when nitrate- N is used as the N source there is no effect on soil pH. In addition, when nitrate is taken up by the plant a hydroxyl anion is released to maintain neutrality and in turn, maintains pH. The use of calcium nitrate-based fertilizers can substantially reduce liming demand compared to other nitrogen sources. This saves additional input costs while increasing nutrient balance, agronomic benefits, and return for your cotton crop.

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Trey Cutts
Trey Cutts
Market Development Director