Pulse Beat Individual Articles

Soil Health in Soybeans

Dr. Stephen Crittenden, Research Scientist, Agriculture and Agri-Food – Brandon – Spring 2021 Pulse Beat

The common definition of soil health is “the capacity of soil to function.” Personally, I find this definition a little confusing — while soil is complex, soil health doesn’t need to be complicated. What’s going on in soil is consistently dynamic and always changing, but weather is the main player. Temperature and precipitation drive the biological, chemical and physical processes in soil, and soil health is all of these processes put together.

We can measure soil health in several ways. Traditionally, we have used soil fertility tests to look at snapshots of nutrients (like nitrogen, phosphorus, potassium and sulphur) before applying fertilizer to help plan application rates. After harvest, we can take soil samples to see what nutrients are left and how efficient our crop has been. Both of these measurements are chemical indicators of soil health that the producer can influence directly through fertilization.

Soil biology includes everything from bacteria and fungi to earthworms. Soil physical properties are things like aggregation (sand, silt and clay particles stuck together) and infiltration (the ability for water to move into soil). The biological and physical indicators of soil health are mostly influenced indirectly. Tillage, for example, changes soil structure and thus the environment in which soil biology lives. Cover cropping and manuring have a good chance of influencing soil health, but all aspects of soil health interact with each other.

If you are tracking your soil health, in addition to the traditional soil fertility, I recommend “digging in” to your soil organic matter. Organic matter is the lynchpin that many soil processes hinge on, such as nitrogen mineralization (the process where nitrogen becomes available from organic matter). Additionally, higher soil organic matter often means better soil structure. Your soil health or soil fertility reports may also include an “index.” A soil health index is a way of putting all these measurements together in a meaningful way. First, you choose a scale, like 1 to 100, then rank the values from your measurements on this scale. The problem with ranking is that it can be subjective — “healthy” soil may be in the eyes of the beholder. There is no definitive soil health score and how these scores are calculated changes depending on where your soil health report is from; however, the calculations behind these indexes should be available upon request.

One thing I want to point out is the difference between lab and field measurements. Some soil health indicators cannot be assessed in a “grab” or regular soil sample. For some soil properties, we must take specialized equipment to the field, for example, infiltration, penetration resistance and earthworms. Some of my favourite soil properties are those that cannot be tested from a grab sample but rather must be done in person or with intact soil cores.

Infiltration is often a focus of discussions on soil health. We assume that adding carbon improves soil structure and should also improve the way water moves through it. We want water to move into soil and away from the rooting zone when we have too much, but we also want soil to hold water when it is dry (water retention). My assertion is that we need both lab and field measurements of the soil chemical, biological and physical properties to get the most complete picture of soil health that we can.

Some of the new soil health indicators we’re looking at are the Haney test (a series of chemical extractions from soil), Solvita (CO2 burst from soil), ACE protein (N in soil organic matter) and POXC (reactive soil carbon). We’re evaluating the usefulness of these new indicators relative to traditional soil tests that have been calibrated for Manitoba and have local research behind them. First, we’re looking at crop rotations and soil tillage. The crop rotation combinations include soybean, corn, canola, wheat and alfalfa (Photo 1). Tillage systems include vertical tillage, subsoiling and raised beds compared to conventional tillage with a cultivator (Photo 2).

The twist we’ve added to these experiments is that of climate change. Climate change could bring more variability in weather patterns, lengthy droughts and intense rainfalls. We’ve included some simulated climate change in our experiments where we either irrigate heavily to simulate rainfall or exclude rainfall with shelters to simulate drought (Photo 3). The objective is to look at the utility of these soil health indicators and decipher which management combinations will result in better crop performance and how this might holdup under climate change. This work will contribute to the development of a soil health index for Manitoba.

Our soil health work with Manitoba Pulse & Soybean Growers should help Manitoba farmers interpret what soil health means and understand what soil measurements are useful for their farms —  and of course, we’re always looking for soil management practices that work well. As a scientist, I rely on evidence-based solutions, so my message to you about soil health is to define your soil health goals, keep track and think critically.