Perhaps the most important prepping guideline is to ensure that your sprinklers give an even spread of water across your entire lawn. You can test this by running your sprinklers on bare dirt and comparing the lighter and darker areas. Adjust your sprinklers until these areas are an even color.
Properly prepared soil will directly reflect the health and appearance of the lawn for years to come. At the very least, we need a loosely raked surface, free of existing vegetation and debris, to apply the hydroseed mix.
It is imperative that the soil's surface is bare and that all weeds are removed before we hydroseed. It is best to rake up all debris after weed removal and bag it so that you don't leave weed seed on the ground. The hydroseeding mixture (called a slurry) comes out of the hose with the consistency of a thick pea soup, and it needs to make contact with the soil for the grass seed to germinate and grow.
A high percentage of turf problems can be traced to - or caused by - a poor soil environment. Construction sites, for instance, can become severely compacted by machinery resulting in a loss of space between soil particles which impedes the flow of air and water along with the grassroots' ability to penetrate the soil. In these cases, the addition of a few inches of topsoil over the top of compacted soils does little to improve the situation, resulting in a shallow-rooted lawn that is highly susceptible to drought during the summer and flooded roots during the winter.
The best approach to correct this situation is to till decomposed organic matter into the existing soil (e.g., compost, peat moss, etc.). If this approach is not possible, the second option would be to import at least 6" of topsoil.
Proper soil preparation before any planting will make it easier for the grassroots to penetrate deeply and evenly. Deep roots will make your lawn more drought resistant - meaning more efficient at absorbing water and nutrients - resulting in a thicker, lusher lawn as well-fed grass shoots eagerly emerge.
When preparing your soil, be sure to fill in with topsoil any dips or low-lying areas where water may collect. As you loosen up compacted areas, you may uncover more debris and rocks, be sure to remove these as well.
When adding topsoil, if possible, till the compacted soil first to a depth of at least 2 inches. This will alleviate subsoil compaction, foster a bonding of the topsoil to the subsoil, and improve root penetration and air and water movement.
To Till or Not To Till
If your soil is hard-packed and you cannot dig a hole in it with your heel, you probably need to either rototill it or bring in at least 4 inches of new topsoil. This will improve drainage and aeration allowing baby grassroots to work their way unencumbered into the soil. There is a reason why grass won't grow on concrete, and if your soil feels like concrete, you probably won't have the beautiful lawn you imagined. When rototilling, try to till down at least 6" (minimum of 4") while being mindful of PVC and sprinkler piping. In fact, it is usually better to do this before installing your sprinkler system, but if you already have irrigation in place, you can locate the pipes and mark them with flags before you till. If you are going to add a soil amendment (sand, compost, etc), you will want to add it first, spread it around evenly, and then till it into the soil as deeply as you can, mixing thoroughly.
If you fail to till the amendment into the soil, the roots will want to stay in the top area where the nutrients and drainage are optimal, resulting in a shallow root system that will not produce a healthy and durable plant. Once tilling is complete, it is a good idea to rent a lawn roller, fill it halfway full with water, and us it over your yard in two different directions to firm it up. This will even up the soft areas and make them consistent, resulting in an ideal smooth surface bereft of hills and valleys. If you can step on the soil and sink in a couple of inches, you need to firm it up some more.
Tilling and rolling can sometimes be challenging steps that you may be tempted to skip, but you cannot do much about over-compaction once the lawn is planted, and your grass will be much healthier if you do them so roots can breathe, eat, and have room to grow.