When you purchase land to build a home, the soil is more important than you might expect. The composition of your soil can affect everything from plumbing to foundations. Because of this, there are many soil tests to conduct as part of the home construction process.

Some tests, like perc (short for percolation) tests, are essential to installing properly functioning septic systems. Other tests inform the construction of the house itself.

What Is Soil Testing?

Soil testing checks a number of things to see if land is suitable for construction. Some tests analyze the soil’s composition and texture to determine its structural stability. Others involve testing for contaminants. Here’s what to know about soil contamination in your yard.

Soil engineers “usually work these tests before, during, and after the construction process to analyze if the soil composition is compatible for construction,” says Chuck Naish, owner of City Residential Foundation Repair. Test results identify if the soil might cause problems for foundation work, or septic efficiency, or other items. Then they recommend solutions, such as soil grading, extra drainage and foundation supports.

“Engineers can devise remediation strategies that mitigate risks and environmental liability to ensure the construction site is pollutant-free and safe for the community,” Naish says.

To test the bearing capacity of the soil (how much load the soil can support), Blueprint Homes CEO Scott Cam says the “physical and chemical composition of the soil is checked.” Some of these tests are liquid limit, plastic limit and shrinkage limit. These tests determine “the length and depth of the pillars put in the soil to lay the foundation of the building,” says Cam.

You may also have soil permeability and drainage tested with percolation, deep hole and seasonal high water table tests. These tests are most common in areas without municipal sewage systems as they aid in the design and function of septic systems.

“Engineers also test for toxins in the soil that might be hazardous to the health of humans,” says Naish. When present in the soil in high quantities, some naturally occurring heavy metals pose a health threat.

The University of New Hampshire Cooperative Extension Soil Testing Program tests for cadmium, chromium, copper, nickel, lead and zinc, as well as arsenic, mercury, molybdenum and selenium. The Environmental Protection Agency (EPA) sets standards for these contaminant levels. While at it, also learn how much soil you need for your garden.

Can You DIY Soil Testing?

Many home gardeners test their soil pH themselves. You can also collect a soil sample to be analyzed by your local extension office. These DIY tests are useful when amending soil for a vegetable or flower garden. Soil testing for construction, however, is left to the professionals.

“Since land contamination has adverse impacts on building structures, human health, the environment, and ecosystems,” Naish says, “professionals can conduct site assessments that consider the land history and existing conditions.”

Hiring a soil engineer ensures your soil tests are completed according to local rules and regulations, which prevents violations of building codes and barriers to getting permits.

What if Your Site Doesn’t Pass?

Many soil tests simply provide information, not a passing or failing grade. Some local authorities and codes require a particular test result before approving construction plans. For example, a perc test result must be within the area’s designated percolation rate for a traditional septic system to be approved.

In some areas, perc test results aren’t pass or fail, but dictate design and installation parameters of a septic system. Drainage and permeability test results are also used when deciding where to dig a well.

If you don’t get the necessary soil test results for your site, you’ll have to modify your building plan — and budget. For this reason, some closing agreements are contingent on soil tests.