Based on prior knowledge, PH is a measure of soil acidity or alkalinity. For the task of predicting grain yield of a soybean cultivar, it is likely that the soil pH would impact the yield. 

Analysis:

1. Low Grain Yield: Soybeans generally prefer slightly acidic to neutral soil pH. In conditions where the soil pH is high (alkaline), the grain yield of the soybean cultivar is expected to be low.

2. High Grain Yield: Soybeans thrive in slightly acidic to neutral soil conditions. Therefore, a relatively lower pH (slightly acidic) is preferred for achieving high grain yield. Higher soil pH levels (alkaline) can hinder nutrient availability and affect the soybean plant's ability to take up essential elements, leading to decreased yield.

Based on this analysis, the dictionary can be created as follows:

```json
{
  "low": [7.8, 7.9, 8.0, 8.1, 8.2],
  "high": [6.0, 6.1, 6.2, 6.3, 6.4]
}
```

The "low" list contains typical pH values for low grain yield, while the "high" list contains typical pH values for high grain yield. These values are provided as floats in the lists.