Based on prior knowledge, the number of legumes per plant (NGP) can be a crucial factor in determining the grain yield of a soybean cultivar. A higher number of legumes per plant generally indicates a higher grain yield. 

To create the dictionary, we need to analyze the relationship between NGP and the grain yield target variable (low or high). Let's assume that a low grain yield corresponds to a value of 0, and a high grain yield corresponds to a value of 1.

```json
{
	"low": [2.5, 3.0, 3.5, 4.0, 4.5],
	"high": [5.0, 5.5, 6.0, 6.5, 7.0]
}
```

In this example, we have included 5 typical NGP values for each target class. The values provided are just for demonstration purposes and can be adjusted based on domain knowledge or further data analysis.