Compact, poorly aerated soil restricts oxygen, water, and nutrients to grass roots. Aerate it with a core aerator or a spading fork, making holes 4 inches deep in the soil.
Loosen soil by breaking the topsoil with a rototiller and composting the top 3 inches of soil with a rake. Do this before sowing your seeds, as this process kills growing grass.
If the soil in thinning areas is either dry or soggy, you’re under or overwatering. Also, water no more than twice a week, evenly and deeply to encourage deep roots.
Give your grass about 1 inch of water a week in spring and summer, or about 0.6 gallons of water per square foot. Water early in the morning, and water less during fall and winter.
Soil often turns acidic due to rainfall and ammonium-rich fertilizer. Most turfgrass thrives in slightly acidic soil with a pH of 6.5 to 7, deteriorating outside that range.
Increase the pH of acidic soil by evenly applying pulverized lime, then watering. You can also use wood ashes, but they’re less effective and can damage the roots of mature grass.
Decrease the pH of alkaline soil by adding aluminum sulfate or sulfur. Aluminum sulfate works faster and is safer for established lawns, as sulfur can cause leaf burn.
Insufficient sun can cause thinning grass, so reseed shady areas with shade-tolerant grass varieties like tall fescue, ryegrass, St. Augustine grass, and zoysia grass.
Encourage shade grass growth by avoiding foot traffic in those areas, pruning trees to allow more sunlight in, and letting the grass grow longer so that it gets more sun exposure.
In established lawns, a layer of undecomposed grass clippings, stems, and roots often forms between the grass and soil, preventing water and nutrients reaching the roots.
Dethatch during spring or fall by first mowing the grass, removing debris, and watering well. Then, use a dethatching rake, power rake, or vertical mower to remove the thatch.