Tips for Your Fall Soil Fertility Check-Up

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, for instance through imported manure, and use those sources as efficiently as possible.

At the beginning of this past growing season, fertilizer prices were at record high levels, and we made several suggestions about how producers might want to adjust their fertilizer management. One of the suggestions 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 the "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 would continue with this same management again.

Personally, I shifted to the "sufficiency" approach of fertility management 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 gal/ac 10-34-0 starter fertilizer and a supplemental broadcasting of triple-super-phosphate. In optimum testing fields, I still used the 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 optimum testing fields, and where I applied only crop removal rates to below optimum testing fields.

What would explain 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 optimum 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 fertilizer in a band, as with starter fertilizer, enhances the fertilizer efficiency because it is placed near the roots, and limits mixing of the 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 optimum still have a relatively large pool of phosphorus that can dissolve into soil solution. Finally, about 80% of the 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 just at the optimum level, and where I did not make a K₂O application, I ended up seeing patches of potassium deficiency appear in various areas of the fields. What occurred to me after seeing this, is that the soil test measurement is an average for a field, and within fields there is likely to be areas that fall both above and below the average level for the field. If the average is right at the optimum level, then the below average areas of fields are going to be below optimum in nutrient levels. These were the areas of fields where I saw the potassium deficiency symptoms appear. 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 (64 ppm Mehlich 3 K), I applied 90 lbs/ac K₂O fertilizer, which was above the crop removal rate of 54 lbs/ac K₂O for our typical yield of 180 bu/ac corn grain. However, despite the K₂O application, I still saw potassium deficiency symptoms appear in this field. One of the possible reasons for this is that crop removal rates of K₂O for grain crops are much lower than the actual K₂O uptake by the plant, because only about 30% of the K₂O taken up ends up in the grain. So the total K₂O uptake requirement of this corn crop would have been closer to 180 lbs/ac K₂O (54 lbs/ac K₂O in grain divided by 0.30 K₂O proportion in grain per whole crop). The lessons learned from the experience with potassium are to consider continuing maintenance fertilizer applications when soil test levels are near the bottom of the optimum range and when soil test levels are well below optimum, consider rates closer to whole plant uptake requirements versus just grain removal rates when making sufficiency based recommendations.

After reflecting on your experiences with your fertility management program this past season, 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 the 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 were close to or below the bottom of the 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 if any fields have fallen below or are near the bottom of the optimum range.

Finally, consider sourcing nutrients as affordably as possible.  One such source that many are turning to now is manure, because it is a valuable source of N, P, and K and is abundantly available in some parts of Pennsylvania. Efforts should be made to maximize the efficiency of the nutrients contained in manure to reduce the amount of supplemental fertilizer required to meet crop needs.  To increase the efficiency of manure N, considering injecting liquid manure to reduce volatilization. Alternatively, volatilization can be reduced by applying liquid or solid manures when air temperatures are cold (<40 degrees F). Late-fall and early-spring (e.g., mid-November or mid-March) application timing provides a good balance between realizing cold temperatures at the time of application and mitigating nutrient losses that can occur during the winter. To reduce the risk for nutrient runoff these times of year, 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, it is even more important to take adaptive steps and learn from the outcomes of any adjustments that are made. Reviewing the results from your management last season, collecting new soil samples if necessary, and sourcing the most affordable nutrient inputs will put you in the best position for success next season.