Micromanaging pH for High Soybean Yields

In today’s agricultural production environment, high yield is the number one priority of just about every soybean producer. This is obvious from the various high yield contests in many states, as well as the funding of projects that are designed to elucidate the myriad factors that contribute to stable high yields.

Soil is typically tested for nutrient content by collecting samples from the top 6-8 inches of soil and then compositing samples from locations that represent a similar micro-environment in the field. Thus, the nutrient and pH analyses results are effectively average values for the depth represented in the collective cores.

An article entitled “Depth Matters: Soil pH and Dilution Effects in the Northern Great Plains” [Soil Sci. Soc. J. 80: 1424 (2016)] presents results that are a case in point for considering micromanagement opportunities for ensuring that the highest possible yields are achieved from a production site. The objective of the research was to explore how soil sampling depth can affect soil pH results using data from two long-term [16-19 yr] cropping studies in the Northern Great Plains.

The authors hypothesized that pH values used to make amendment decisions will be diluted by samples taken from deeper soil depths, thus affecting decisions that are made to remedy low soil pH.

The question that needs to be answered before assessing the results from the above study is “Why does it matter if a pH analysis of a particular field or micro-environment within a field is from samples that were taken from the top 4, 6, or 8 inches, or possibly even deeper?” Below are some reasons this can matter on sites where ultra-high yields are the goal.

•    The majority of nodules are located on soybean roots in the top 4-6 inches. pH affects the nitrogen-fixing activity of the bacteria in the nodules.

•    Several soil-applied herbicides have rate and rotation restrictions based on soil pH. Since these herbicides have the greatest activity and effect in the shallow soil depths, it is important to know the pH in their zone of activity. This is especially important for the “Black Belt” soils of East Mississippi.

•    Realizing consistent ultra-high yields requires that all controllable variables that affect soybean yield must be micromanaged.

Now to the details of and results from the above study.

•    The soils at the study site were silt loams and clay loams.

•    In a conservation tillage–cropping systems experiment, average soil pH at the 0-3, 0-6, and 0-12 inch soil depths was 5.84, 6.10, and 6.45, respectively.

•    In a soil quality management experiment, average soil pH at the 0-3, 0-6, and 0-12 inch soil depths was 5.45, 5.71, and 6.13, respectively.

•    These results show that samples from all three sampling depths had significantly different average pH values, and final pH values were higher as sampling depth increased. Thus, sampling depth can affect pH values.

•    Increased sampling depth for standard fertility testing will likely lead to confounding (increased) pH values because acidification is most pronounced near the soil surface.

•    The above results lead to the conclusions that pH stratification with soil depth should be considered when taking samples for standard soil fertility testing. Also, lime applications should be made to address the potential greater soil acidification in the near-surface soil depths to ensure more efficient use of liming materials.

•    These results support a shallower sampling depth for pH determination. The authors recommend a sampling depth of 0-4 inches or shallower to avoid the dilution effects measured in their study with deeper sampling depths.

I recognize that these results are from a much more northerly environment than the Midsouth. However, the results from this study do suggest that pH stratification with soil depth should at least be checked to ensure that this is not a limitation in fields where all inputs are geared to ultra-high yield.

So, at the very least, to ensure that pH in the shallow soil zones of these fields is not a limiting factor, or rather, is amended appropriately, check this by taking shallow soil samples that are separate from standard soil samples from deeper depths. This can be done on a smaller field scale that the sampling for soil fertility, but doing this will determine if this is in fact something that should be addressed.

In my opinion, striving for consistently high yields [80-100 bu/acre] will require attention to these otherwise insignificant details. Again, consistent ultra-high yields will likely require more attention to micromanagement [defined as “to manage or control with excessive attention to details”] of inputs such as this.

Composed by Larry G. Heatherly, Nov. 2016, larryheatherly@bellsouth.net