Ask the Agronomist

Phosphorus on Corn for 2020 Success

Dr. Cristie Preston (Progressive Farmer image provided by eKonomics)

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Q: At what corn growth stage is phosphorus needed most, and how critical is soil pH for phosphorus uptake?

Dr. Cristie Preston: When I talk to farmers about phosphorus, my first question is, "What's your soil pH?" Phosphorus plant availability can be significantly affected by the soil pH. Areas where the soil pH is below 5.5, phosphorus availability is controlled by iron and aluminum. Above 7.5, phosphorus is controlled by calcium. Greatest phosphorus availability is between 6.5 and 7.5. If your pH is outside the optimum range, your soil test level may not accurately reflect phosphorus availability to the plant. A high or low pH can potentially affect the phosphorus that you apply. So, it's difficult to predict if you would see a response.

Phosphorus uptake really begins to increase as the crop moves from the V6 to V8 growth stages.

Q: Focusing on phosphorus in starter fertilizer, what conditions offer the most significant crop response?

Preston: First, response to fertilizer phosphorus, especially starter, might be even greater at non optimal pH because of reduced availability of soil phosphorus. Second, when the phosphorus soil test level is below critical, you'll see the highest corn crop response. Even when test levels are above critical, there can still be a positive response to phosphorus in starter fertilizer. Third, if you plant in any conditions that may restrict early root growth such as soils that are compacted or too wet/dry, cool spring conditions, you want phosphorus closest to the plant for the best root growth benefit, such as in a 2x2 or 2x2x2 band, since these conditions reduce uptake.

Q: Can phosphorus help fields that suffered Prevent Plant in 2019?

Preston: Farmers who suffered through Prevent Plant acres in 2019 can benefit from phosphorus in starter fertilizer. If there were no living roots in a field in 2019, phosphorus can help achieve quicker root establishment to improve mycorrhizal relationships with corn root growth to improve uptake. It is similar to the scenario when corn follows sugar beets or canola; there is a greater chance of phosphorus deficiency.

Q: Using the eKonomics ROI calculator tool, how can a farmer maximize yield with phosphorus applications?

Preston: The ROI tool allows farmers to input site-specific farm data on phosphorus and potassium to make fertilizer decisions. Fertilizer recommendations are based on years of university research trials. Results take into account spatial variability. If the soil test level is right at or below the critical level, since it is an average, there will be areas within the 2.5 acres that could be yield-limiting due to low phosphorus levels. When soil test levels drop below critical, yields and returns are potentially limited.

Q: The Nutrient Removal calculator helps farmers grasp pounds and dollars removed by grain yield. How do growers compare the Build-Maintain and Sufficiency approach?

Preston: It is critical to calculate what nutrients have been removed. The Sufficiency approach takes into account the economic optimum fertilizer rate for that year. If soil test levels are above the critical level, the recommendation would be zero pounds of fertilizer.

The Build-Maintain approach ensures soil test levels are built up above critical levels, and removal rates are applied. Since soil sampling methods are an average of an area, there are locations within that area that are above and below the reported value. If the soil test levels are close to the critical level, yields may decrease due to a lack of optimum phosphorus or potassium levels. The Build-Maintain approach increases soil test levels above the critical and maintains them to ensure maximum yield. This method is more costly, however it means a farmer can hedge his bets on not having yield-limiting conditions in the field.

The Build-Maintain strategy may be preferred if the farmer owns the land or has a long-term lease since it's based on banking some phosphorus and potassium in the soil for future years. In contrast, the Sufficiency level strategy may be more suitable for short-term land tenure or in cases of limited available capital, as it's based on applying just what is needed for the current crop.

For More Information:

> Visit Nutrien-eKonomics.comfor news and fertility management information. It also contains valuable tools like a rainfall tracker and Growing Degree Day calculator to help farmers assess possible fertility loss and plant development needs.

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