• By Luke Gatiboni •
Proper soil fertility management is an important step for a successful soybean season. Before planting, it is important to think about three main things regarding soil fertility: soil acidity, commonly required nutrients and eventually required nutrients.
The first soil fertility issue is soil acidity. Soil pH below the adequate range is the most common soil fertility constraint in North Carolina. In the last two years, 279,854 soil samples from grain fields (soybean, corn, and small grain) where analyzed by North Carolina Department of Agriculture & Consumer Services Agronomic Services.
The average results showed that 36% of samples of mineral soils were below pH 5.8, requiring liming; 25% of samples of mineral-organic soils were below 5.4; and 11% of organic soils were below pH 4.9, also requiring liming. Low pH soils can dramatically decrease yields, as it will affect the availability of other plant nutrients and decrease the soil microbial activity and soybean nodulation.
If soil pH is really low, aluminum toxicity will occur. Therefore, liming the soil to keep the pH in the optimal range for plants is a basic step to achieve adequate yields. As agricultural lime normally is a slow-reaction product, it is recommended to apply lime at least three months before planting. However, better late than never.
If for any reason the soil was not limed three months before planting, there is no problem applying lime right before planting. In this case, it is preferable to use finely ground agricultural lime with high neutralization values (see details about lime reactivity in this fact sheet) to neutralize the acidity as quick as possible while the soybean is growing.
Soil pH above the adequate range for soybean is a less common problem, and in the last two years, only 6% of the grain fields had high pH (above 6.4). Very high pH causes the deficiency of some of the micronutrients in soybean plants. In North Carolina, manganese deficiency is the main concern in soybean fields when pH is greater than 6.8, especially in coarse-textured soils of the Coastal Plain region.
As soil reacidification is a natural process, just stop applying lime to high-pH soils and pH should decline in a couple of years. However, if the pH is considered excessively high, application of elemental sulfur can promote a quicker reacidification (see details about soil acidifiers in this fact sheet).
Commonly required nutrients
The second soil fertility consideration is the “commonly required nutrients” phosphorus (P) and potassium (K). In North Carolina, these two nutrients are applied on a regular basis in soybean fields but they present contrasting soil trends. As P is fixed to soil clays, it is common to observe a slow buildup of P over decades of fertilization.
The same is not true for K, as this nutrient is exported with grain in higher amounts than P and also because K can be leached. The soil samples of grain crops analyzed by NCDA&CS in the last two years showed 53% with very high phosphorus (P-I > 100) and only 13% with very high potassium (K-I > 100).
Sometimes people wonder if the P and K fertilizer recommendations from NCDA&CS are still adequate for the modern soybean varieties and the associated higher yields. To address this question, in 2020 we conducted five trials across the state (two on-farm trials and three trials in Research Stations).
Three out of five trials yielded more than 60 bushels per acre, and the results showed that there was no response of soybean to P and K fertilization when the P-I and K-I index were greater than 50. This means our recommendations are adequate, however this will be explored again in 2021.
In 2017 and 2018, Drs. Carl Crozier and David Hardy conducted a set of trials across the state and they found similar results as row crops did not respond to potassium fertilization in soils with K-I greater than 50 (unpublished data). For this reason, the general recommendation for this planting season is to apply P and K only if the soil test report asks for the applications of these fertilizers.
Additionally, there is no rationale to applying rates of P and K greater than those recommended by the soil test report because it will not translate in higher yields. Finally, it is important to highlight another important detail about P fertilization: sometimes starter P is recommended for corn but there is no data supporting the use of this strategy in soybean fields, as shown in this fact sheet.
Eventually required nutrients
The third soil fertility consideration focuses on “eventually required nutrients.” Magnesium (Mg), sulfur (S), and the micronutrients are in this category. In general, North Carolina soils have plenty of these nutrients but sometimes they are in low levels and respond to fertilization are expected. Pre-plant soil testing is the best tool to identify those situations where there is a chance of response.
If the soil test report recommends application of these nutrients, it is better to apply those nutrients. If the soil test report does not recommend those nutrients, the chances of yield increases with their use is unlikely. Regarding micronutrients, NCDA&CS analyzes manganese (Mn), copper (Cu), and zinc (Zn). If required, these nutrients can be applied via soil or foliar feed.
The advantage of soil application is to have residual effect for the following years, while the foliar application will benefit only the current crop. After planting, is very important to regularly scout the fields during the season and if nutrient deficiencies are detected (the soybean diagnostic key can help to identify them), it is recommended to take soil and tissue samples and use our diagnostic problems service found in the Soil Fertility Extension Portal here.
Dr. Luke Gatiboni is an Extension soil fertility specialist and assistant professor of crop and soil sciences, North Carolina State University. He may be reached at firstname.lastname@example.org