Based on my prior knowledge, the Number of grains per plant (NGL) is expected to have a positive relationship with the grain yield of soybean cultivar. Generally, a higher NGL value would indicate more grains per plant, which would result in a higher grain yield.

However, it is important to note that the appropriate ranges of NGL values may vary depending on the specific characteristics of the soybean cultivar and the growing conditions. In general, low NGL values could be considered below a certain threshold, and high NGL values could be considered above a certain threshold.

Based on this analysis, here is the generated dictionary:

```json
{
	"low": [50.0, 60.0, 65.0, 70.0, 75.0],
	"high": [120.0, 130.0, 140.0, 150.0, 160.0]
}
```

Please note that the values provided in the dictionary are just examples and may not accurately represent the actual thresholds for a specific soybean cultivar.