Based on my prior knowledge, the number of grains per plant (NGL) can be an important factor in determining the grain yield of a soybean cultivar. Generally, a higher number of grains per plant is associated with a higher grain yield. However, the actual range of values for NGL that can be considered as low or high may vary depending on the specific cultivar and other factors.

To create the dictionary, I will analyze the relationship between NGL and grain yield, and identify typical NGL values for the "low" and "high" grain yield classes. Here is the dictionary:

```json
{
	"low": [50.2, 60.8, 42.5, 53.7, 45.1],
	"high": [75.3, 80.5, 71.9, 83.6, 78.2]
}
```

In this case, I assumed that values less than or equal to 50 can be considered as "low" and values greater than or equal to 70 can be considered as "high". The provided values are just examples and may not represent the actual typical values for the target classes.