Based on prior knowledge, the relationship between the feature "Number of legumes per plant (NGP)" and the target variable "grain yield of soybean cultivar" could be as follows:

High NGP values are expected to be positively correlated with high grain yield, while low NGP values may be negatively correlated with grain yield.

Based on this analysis, let's create the dictionary:

```json
{
	"low": [0.0, 1.0, 2.0, 3.0, 4.0],
	"high": [8.0, 9.0, 10.0, 11.0, 12.0]
}
```

These are just example values. The actual ranges and typical values for low and high NGP values will depend on the specific dataset and the domain knowledge.