Based on prior knowledge, the number of grains per pod (NS) can be an indicator of grain yield in soybean cultivars. Higher values of NS are typically associated with higher grain yields, while lower values are associated with lower grain yields.

To create the dictionary, we will identify typical NS values for each target class of "low" and "high" grain yield.

Here is the dictionary with typical NS values for each target class:

```json
{
	"low": [2.5, 3.0, 3.5, 4.0, 4.5],
	"high": [5.5, 6.0, 6.5, 7.0, 7.5]
}
```

These values are just examples and can vary depending on the specific soybean cultivar and other factors. It is important to note that this analysis is based on general knowledge and there might be variations and exceptions in real-world data.