One great thing about grass, it will grow just about anywhere. But just because it will grow, doesn't mean it will have a good root system. A good root system is important for your grass to survive through many of the harsh conditions it may face.
When planting new seed into your existing lawn or on bare ground, you have to ensure it germinates correctly. It needs an environment that is conducive to it starting out good. It also needs good ground that will allow it to make good roots.
You should mix grass seed with topsoil or compost if the top layer of your soil is depleted of its nutrients or is dry. Your grass seeds need to remain moist to germinate and root correctly. When mixing your grass seeds with topsoil, you should lay down no more than 1/4” of topsoil.
Do you put topsoil down before grass seed?
You can put topsoil down before you lay your grass seed down. It is not necessary to put topsoil down before your grass seed if your is prepared correctly to receive seed. Laying down a good topsoil mixed with a seed starter fertilizer can be a good foundation for your grass seeds to grow strong.
Can I put topsoil over grass and reseed?
You can put topsoil over your grass and reseed. Keep in mind, the grass below the topsoil will grow up through the topsoil unless you put an extreme amount of topsoil on it. If your are happy with your existing grass growing with your new grass, then this is okay. If you are trying to get rid of your existing grass and replace it with new grass, you may want to remove the existing grass before planting your new grass seeds.
To plant new grass seeds in your lawn, you don't need to put topsoil on top of your existing grass. You just need to aerate or dethatch your lawn.
Aerating your lawn with an aerator will put holes in the soil below your existing grass for the new seed to fall into and germinate. Dethatching your lawn removes the thatch layer below the blades of your grass. This exposes a lot of soil for new seed to fall on.
You can choose to do either or both before seeding your existing lawn. Many experts believe both methods are just as effective. Although, there are some who believe dethatching is more effective than aerating.
How to plant grass seed on hard dirt
The simplest solution to plant grass seed on hard dirt would be to aerate the dirt with an aerator. Then wet the hard dirt. Next, spread your seed on the dirt. Then continue to water it regularly. With this approach you will see grass germinate. This simple approach may not give you the lawn you are looking for.
There are some issues with just aerating hard dirt, spreading grass seeds, and watering. The first issue is you have address why the soil is hard. Second, because the soil is hard it is probable deplete of good nutrients. Third, you don't have any organic material to help keep your grass seeds moist until they germinate and sprout good roots.
Why is your soil dry
Let's look at why the soil is dry in the first place. Soil becomes hard because there is not enough water. This could be due to a drought, a very low water table, high area in your yard that water drains away from, or a build up of salts in your soil. https://agrilifeextension.tamu.edu/library/gardening/managing-soil-salinity/
To fix your dry soil, you should till the soil to a depth that is not dry. This depth can vary from soil to soil. You should use a shovel to dig down and see how deep the moist soil is below your dry soil. This will help more water to be absorbed by the ground instead of being repelled. When the water is repelled by the soil it will evaporate and leave more salts, thus remaining dry.
You can also leach the soil by providing it with low salt water. The low salt water will dissolve some of the salts in the soil and carry them away.
Hard soil is deplete or has an over abundance of nutrients
Although your hard soil probably contains salts of many different nutrients, it probably does have the right amount of the nutrients your grass seeds need to grow. Tilling your dry dirt and adding compost, topsoil, and fertilizer will give your grass seeds the proper balance of nutrients necessary for germination and a healthy root system.
Organic material for moisture
Your grass seeds need to remain moist as they are germinating and rooting. When you put grass seeds into dry soil and water it there is nothing that will retain the water. Much of the water will evaporate. You need organic material, like what is found in topsoil or compost, to help hold moisture for your grass seeds.
Best approach to plant grass seed in dry dirt
The best approach is to till your hard dirt to a depth that reaches and includes moist soil. Then figure out how much salt is in your soil with a test kit and give your soil enough low salt water to wash the unwanted salt away. Since you have tilled the soil you really don't need to aerate it. Next, add your spread your seed on the newly tilled soil. Then cover your soil with a very thin layer, about 1/4”, of topsoil or compost. Finally, provide your grass seeds with adequate water on a regular schedule.
Should I till my lawn before seeding?
You don't need to till your lawn before seeding unless it is very dry. If you already have grass, you should dethatch it or at a minimum aerate it so your grass seeds have good contact with the soil.
How long for grass to grow through topsoil?
If you place topsoil over existing grass, the time for it to grow through the topsoil will depend on how much water and sunlight is available. If it is during a dormant time of year, you may not see your grass re-emerge.
If you put topsoil on top of grass during the spring or summer and it is getting adequate water, it will take between a few weeks to a month to start see the grass come through. This, though, all depends on how much topsoil you apply.
If you put topsoil on new grass seeds, you should start to see grass with in one to three weeks. This requires that you only put about 1/4” of topsoil on your grass seeds. If you bury them too deep, they may not come up at all because of a lack of sunlight.
Grass seed and topsoil
Grass seeds grow best when they are near the surface and remain moist. Mixing your seeds with topsoil is a good way to help provide seeds with the moisture retaining material needed for germination. If you decide to mix your grass seed with topsoil, be sure to lay down only a thin layer so your seeds can get enough sunlight.
Before laying down your grass seed and topsoil mixture, prepare the ground to receive the seeds. If your soil is dry, till it and water it so your newly germinated grass can grow strong roots.