Does Soybean Respond to an Increased Seeding Rate?

Main hypothesis. I based myself on the unique crop development features: soybeans branch well and form a bush up to a meter tall. Presumably, the plants will branch more actively in high-productivity areas. This may cause the plants to interfere with each other's development, meaning the seeding rate can be decreased in these areas. The opposite works for low-productivity areas: we'll increase the soybean planting rate.

Alternative hypothesis. Let's assume that soil productivity affects the number of seedlings, not stem branching. High-productivity areas will provide nutrients to more plants, while low-productivity areas will provide to fewer plants. To test this hypothesis, I increased the planting rate in high-productivity areas and reduced it in low-productivity ones.

An increase in the field's yield would confirm either hypothesis.

I delimited productivity zones in the OneSoil web app. The app analyzes the field's vegetation data for the previous four years and automatically generates a final productivity map based on how the field developed during this period.

To find the limiting factor for crop yields, I studied the field’s relief, soil brightness, and vegetation data for the previous four years.

The field's organic nutrient content varies quite a bit. Humus distribution reflects the field's productivity zones but has no connection to the relief. In cases like this, I always recommend testing the soil acidity. The higher acidity accelerates the mineralization of organic nutrients in the soil and limits productivity. It turned out that two factors — humus content and the pH level — were beginning to affect the field's yield.

To test both hypotheses, I had to use three seeding rates in low-, moderate- and high-productivity areas. These are the rates I used: 240 kg/ha, 212 kg/ha, and 160 kg/ha. I left a strip with the farm's standard soybean seeding rate, 212 kg/ha, as a control strip.

The soybeans were sown in May, and the field was harvested in September 2019 using a combine harvester with yield monitoring. I downloaded the yield file from the combine's onboard computer and visualized the map in the OneSoil web app. This can be done in the "Field data" section.

The first thing that stands out is the way the yield zones are distributed. It changed compared to the productivity zone map. For example, the moderate-productivity zone in the center of the field became a high-productivity area, and the high-productivity zone partially became a moderate-productivity area. The weather conditions in 2019 (it was a dry year) and the unique features of the crop itself could explain the changes.

The second thing that stands out is that there are no obvious yield increases despite the wide range of seeding rates (160−240 kg/ha). I identified homogeneous zones in the field to reliably assess the seeding rate’s impact on yield. These are areas of the field with the same seeding rate at each point and approximately the same yield. I’ve put the average values for homogeneous zones in areas with varying productivity in the table.

The experiment was a success. I confirmed the first hypothesis. In high-productivity areas, yield increased when cutting the seeding rate. In low-productivity areas, yield rose when increasing the seeding rate.

Even though I proved the seeding rate's impact on yield, I won't risk making any recommendations based on the results. I studied how soybeans react to variable-rate seeding in just one field. That's not enough for overarching conclusions. In addition, the experiment's outcome may have been affected by weather conditions. Remember that soybeans are sensitive to moisture. The field where I conducted the experiment was not irrigated.

This year, I'm continuing to study variable-rate seeding. In particular, I'm researching the differences within and between spring and winter crops and the factors that may affect their yield. I'll definitely share my findings with you once I have them.