Based on prior knowledge, we can assume that the plant height (IFP) will have an impact on the grain yield of the soybean cultivar.

To analyze the relationship between plant height (IFP) and grain yield, we can consider the following:

1. Low grain yield: If the plant height is too short, it may indicate stunted growth and insufficient nutrient uptake, resulting in a low grain yield. Additionally, excessively tall plants can lead to competition for resources and reduced yield.

2. High grain yield: Optimal plant height, which allows for efficient sunlight absorption and nutrient uptake, is likely to result in a high grain yield.

Now, let's generate the dictionary with specific details based on this analysis:

```json
{
  "low": [15.2, 16.5, 17.1, 14.9, 15.7], 
  "high": [19.3, 18.7, 20.1, 19.9, 21.5]
}
```

Please note that the provided values are just examples and may not represent the actual range of values for plant height (IFP) and grain yield of a soybean cultivar accurately.