Is this June going to be a tough month for soybeans? In this part of the world, June usually challenges our crop but we’ve had a run of warm weather that makes it feel like we’re in North Carolina. So far, provided the crop got off to a good start, crop development has been rapid. But things change in a hurry and crop yellowing can will likely occur along with periods of slow or stunted plant growth. What’s normal and transitory and what should we worry about? Let’s look at a few issues.
The Period of N Starvation – Often Forgotten
All soybean plants, to a greater or lesser extent, go through a period of nitrogen starvation early in their development. This occurs at the point where the nutrition within the cotyledons, that fueled the plant up until that point, is running out and future growth will depend on the plant’s ability supply itself with nitrogen via nitrogen fixation. What is often forgotten is that nodule formation and the start up of Nitrogen (N) fixation demands a lot of energy from the plant. Energy that the young plant with its relatively small root system, can be hard pressed to supply depending on growing conditions. General yellowing of plant leaves and slow plant growth during this time is not unusual and will be transitory as N fixation takes off. However, there is something that should be monitored…
Has Nodulation Begun?
By the time your soybean plant gets to the V2 growth stage, nodule formation should be starting unless the field you are in has very high residual soil N levels that likely should have been avoided. Bear in mind that the objective here is only to verify that nodulation is starting. A later evaluation of how well the plants nodulated should best carried out at R1. For this first early season evaluation, follow these steps:
- Assess root nodules on five to 10 plants from two to th different areas of the field.
- Use a shovel to gently dig out each root system. At this stage the few nodules that are on the plant are very small and can be easily torn off. If soil cannot be gently removed from the plant root, soak the roots in a pail of water to remove soil. Make sure to inspect the root base or crown area as this is where nodules usually form first, particularly if a seed applied inoculant product was used.
- Verify that nodules are starting to form on the roots. Basically, you are only looking for little bumps starting to form on the roots as root hairs are transformed into nodules through the interaction of the plant and the Rhizobia bacteria. Some of these bumps may be starting to look a bit spherical. Our staff often bring along a magnifying glass to make this inspection easier. You may notice that some of the nodules are displaying a bit of pinkish-red colour, particularly if you squash them. That tell us that N fixation is starting.
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