They should have the same, well draining, acidic soil that they used to have before you transplanted them. Whatever soil you provided for them before should be fine now. If concerned, conduct a soil test to determine the existing pH and nutrient levels. This will then help you make more targeted amendments, tailored to your specific soil conditions. Ensure high nitrogen fertilizer pellets used for the grass maintain a safe distance away from hydrangeas or their high nitrogen fertilizer content could start to influence/reduce the number of flower buds in future years. For sandy soils, consider annual 1/4 to 1/2" layers of organic compost to improve nutrient deficiencies in the sandy soils. Supplemental waterings (drip irrigation or soaker hoses) may be useful in the summer months, when very windy or dry (including some winters).
