Based on my prior knowledge, the feature "PH" (which I assume represents the soil pH level) can have an impact on soybean grain yield. Generally, soybean plants prefer a slightly acidic to neutral pH level for optimal growth. Therefore, I will assume a pH range of 5.5 to 7.0 as appropriate for soybean cultivation.

Here's the analysis of the relationship between the "PH" feature and the task of determining grain yield:

- *Low yield*:
    - Typical PH values: 5.5, 5.7, 6.0, 6.2, 6.4
    - Soybeans tend to have lower grain yield when the soil pH level is below the optimal range. Acidic soil conditions can limit nutrient availability and affect root development, leading to reduced yields.

- *High yield*:
    - Typical PH values: 6.8, 6.9, 6.9, 6.7, 6.5
    - Soybeans thrive in slightly acidic to neutral soil pH conditions. Optimal pH levels ensure nutrient availability, microbial activity, and root development, resulting in higher grain yields.

Based on this analysis, here's the dictionary of "PH" values for each target class:

```json
{
	"low": [5.5, 5.7, 6.0, 6.2, 6.4],
	"high": [6.8, 6.9, 6.9, 6.7, 6.5]
}
```

Please note that these values are typical and can vary depending on various factors such as crop variety, environmental conditions, and other agronomic practices.