Based on prior knowledge, the number of legumes per plant (NGP) can be considered an important feature for predicting the grain yield of soybean cultivars. Generally, a higher number of legumes per plant is associated with a higher grain yield.

Here is the analysis of the relationship between NGP and the task of predicting the grain yield:

- Low grain yield: This target class represents cultivars that are expected to have a lower grain yield. Typical NGP values for this class may range from 2 to 8 legumes per plant. Some possible values could be [2.5, 3.2, 4.1, 5.8, 7.4].

- High grain yield: This target class represents cultivars that are expected to have a higher grain yield. Typical NGP values for this class may range from 6 to 12 legumes per plant. Some possible values could be [6.7, 8.3, 9.9, 11.2, 12.6].

Based on this analysis, here is the dictionary with the specific details:

```json
{
	"low": [2.5, 3.2, 4.1, 5.8, 7.4],
	"high": [6.7, 8.3, 9.9, 11.2, 12.6]
}
```