Based on prior knowledge, the number of grains per plant (NGL) is expected to have a positive relationship with the grain yield of a soybean cultivar. Typically, as the number of grains per plant increases, the grain yield is also expected to increase.

To generate the dictionary with specific details, I will assume the following ranges for the number of grains per plant:

- Low: NGL values ranging from 0 to 500
- High: NGL values ranging from 1000 to 2000

Based on these assumptions, the dictionary can be constructed as follows:

```json
{
	"low": [50.2, 100.4, 150.6, 200.1, 300.3],
	"high": [1100.5, 1300.2, 1500.7, 1700.3, 1900.9]
}
```

Please note that the NGL values provided in the dictionary are just examples and may not represent the actual values for the given soybean cultivar. The actual values would require further analysis and experimentation.