Based on prior knowledge, the number of stems (NLP) in soybean cultivars can generally be considered as a contributing factor to grain yield. More stems usually lead to a higher yield, while fewer stems tend to result in a lower yield.

However, without specific information about the ranges of values for NLP, it is challenging to provide precise cutoff points for low and high grain yield. For this analysis, let's assume that the range of NLP values is between 0 and 20.

Here is the dictionary with example NLP values for each target class:

```json
{
	"low": [1.5, 3.2, 2.0, 1.9, 4.8],
	"high": [12.3, 10.5, 14.2, 16.8, 13.7]
}
```

Please note that these example values are arbitrary and should be considered as illustrative. Actual values and their ranges may vary based on the specific dataset and characteristics of the soybean cultivars being analyzed.