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Tips to evaluate soil fertilityTips to evaluate soil fertility

If you adjusted your soil fertility management to account for high prices, now is the time to see if it worked.

October 24, 2022

6 Min Read
Hand checking soil on ground
GET IN THE GROUND: Now that the growing season has wrapped up, it’s a great time to get into your soils and check how your fertility program worked.piyaset/Getty Images

As another growing season wraps up and fertilizer prices continue to be high, there are several things worth considering as you begin to manage soil fertility levels for next year's crops. 

First, consider how you managed your fertility this past season, and if you did things differently than normal, were the results satisfactory? Next, consider whether you need to collect a new set of soil samples this year.

Finally, consider ways to source nutrients as affordably as possible — importing manure, for example — and use those sources as efficiently as possible.

What was your approach?

At the beginning of this past growing season, fertilizer prices were at record-high levels, and we made several suggestions to producers to adjust their fertilizer management. One suggestion was to forego maintenance fertility applications if the soil test levels were already in the optimum range. Another suggestion was to apply only crop-removal levels of nutrients if the soil test level was below optimum. 

These suggestions represent a philosophical shift from the "build-and-maintain" approach to "sufficiency" approach of fertility management. If you made this shift, now is a good time to reflect on how your fertility program performed, and if you will continue with this same approach again.

Personally, I shifted to the sufficiency approach for P and K in most of my research trial fields and observed several things that are worth sharing.

On the phosphorus side, results turned out nicely. In below-optimum testing fields, I used approximately crop-removal rates of P2O5, applied as a combination of 4 gallons per acre of 10-34-0 starter fertilizer and a supplemental broadcast of triple-super-phosphate. In optimal-testing fields, I still used starter fertilizer at planting but did not apply any broadcast fertilizer.

On the potassium side, I did run into some deficiency symptoms, both where I did not apply any fertilizer to optimal testing fields and where I applied only crop-removal rates to below-optimum testing fields.

So, what explains the difference between the relative success with P and the issues with K? First, I didn't strictly follow the sufficiency approach with P; I still used starter fertilizer on the optimal-testing soils because we were set up with it on the planter, and it made sense to just keep rolling with it.

Second, applying P in a band, as with starter fertilizer, enhances the efficiency because it is placed near the roots and limits mixing of fertilizer with soil mineral surfaces that can render it less available. 

Third, P is a relatively well-buffered nutrient in the soil, meaning that soil test levels close to optimal still have a relatively large pool of phosphorus that can dissolve into soil solution.

Finally, about 80% of P taken up by the crop ends up in the grain, so the crop removal levels of P2O5, based on grain harvests are relatively close to the whole-plant P uptake requirements.

On the potassium side, several factors ended up working against me when implementing sufficiency-based fertility management. First, on fields that tested optimum and where I did not make a K2O application, I ended up seeing patches of potassium deficiency in various areas.

What occurred to me after seeing this is that the soil test measurement is just an average for a field; within fields there are likely to be areas that fall above and below the average level for the overall field. If the average is right at optimum level, the below-average areas are going to be below optimum in nutrient levels. These were the areas I saw potassium deficiency.

When making fertility recommendations using the build-and-maintain philosophy, these areas of fields would still receive fertilizer at approximately crop-removal levels, potentially masking the below-optimum status of these areas. On one field that tested severely below optimum on potassium, I applied 90 pounds an acre K2O fertilizer, which was above the crop-removal rate of 54 pounds an acre K2O for our typical yield of 180 bushels an acre corn grain.

However, despite this K2O application, I still saw potassium deficiency symptoms appear. One possible reason is that crop-removal rates of K2O for grain are much lower than the actual K2O uptake by the plant; only about 30% of the K2O taken up ends up in the grain.

So, the total K2O uptake requirement of this corn would have been closer to 180 pounds an acre K2O (54 pounds an acre K2O in grain divided by 0.30 K2O proportion in grain per whole crop).

Consider soil testing

The lessons learned from my experience with potassium are to consider continuing maintenance fertilizer applications when soil test levels are near the bottom of optimal range. When soil test levels are well-below optimal, consider rates closer to whole-plant uptake vs. just grain removal rates when making sufficiency-based recommendations.

After reflecting on your experiences, consider whether you should collect new soil samples this fall. Typically, soil sample results are valid for three years, assuming the fertilizer recommendations derived from the soil testing are followed. If you made significant adjustments to recommended fertilizer rates from prior soil test levels, it may be worthwhile to collect new soil samples this fall. 

This would be particularly important if your soil test levels are close to or below the bottom of optimum range for nutrient levels, and if you applied significantly less fertilizer than recommended last year. Updated soil test levels will help you to identify any fields that have fallen below or are near the bottom of optimum range.

Don’t forget manure

Finally, consider sourcing nutrients as affordably as possible. Manure is something many growers are turning to because it is a valuable source of N, P and K, and is abundant in some parts of Pennsylvania. Efforts should be made to maximize the efficiency of manure nutrients to reduce the amount of supplemental fertilizer required to meet crop needs. 

To increase the efficiency of manure N, consider injecting liquid manure to reduce volatilization. Alternatively, volatilization can be reduced by applying liquid or solid manures when air temperatures are cold (less than 40 degrees F). Late-fall and early-spring application (mid-November or mid-March) provides a good balance between realizing cold temperatures at the time of application and mitigating nutrient losses that can occur during winter. 

To reduce the risk for nutrient runoff, avoid spreading manure on fields with steep slopes, high erosion rates, poorly drained soils or that are close to water sources. Manure applications should also be prioritized on fields that require N, P and K to maximize utilization of all three nutrients.

Soil fertility management should be an adaptive process of monitoring, making adjustments in response to weather and market factors, more monitoring, and learning from the performance of adjustments to help inform future responses.

While fertilizer prices remain high, take adaptive steps and learn from the outcomes of any adjustments you have made.

White is assistant professor of soil fertility and nutrient management with Penn State University.

Source: Penn State University Cooperative Extension

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