Site Assessment, Soil, and Water Management
Domain 1 tests whether you can read a site before drawing a plan: topography and grading, soil texture and structure, infiltration and drainage, existing vegetation, utilities and microclimate. Expect numeric questions - slope percent, percolation rate, pH, bulk density - so practise interpreting them, not just reciting them.
Slope, soil and water
Slope is percent, not degrees: rise ÷ run × 100, so a 1 m rise over 10 m is 10%. Ground must fall at least 2% away from a structure for the first 3 m; 5% drains faster and is preferred. Above roughly 10%, plan terraces, swales or retaining structures rather than trying to grass a steep bank. Texture is estimated by ribbon length and feel; a ribbon of 25–50 mm suggests clay loam. Most landscape plants grow best between pH 6.0 and 7.0, so test first and amend second. Bulk density above about 1.6 g/cm³ in clay indicates compaction that restricts roots and infiltration. For a simple percolation check, dig a 300 mm deep hole, pre-soak, refill and time the fall. Below about 12 mm/h, expect perched water: raised beds, subsurface drains or wet-tolerant species. Contact the utility locator before any excavation, and record aspect, frost pockets and reflected heat from walls.
Worked example
A 400 m² garden falls 8% toward the house on clay loam and percolates 6 mm/h. Regrade the first 3 m to 2% away from the wall. Keep the remainder at 4–6% with a shallow swale feeding a perforated drain laid at 1% minimum in gravel wrapped in filter fabric. Choose species tolerant of periodic saturation. Never rototill sand into clay - it sets like concrete; add organic matter instead and stay off the soil when wet.
Traps that cost marks
Treating 45° as a 45% slope (it is 100%). Prescribing sand to 'open up' clay. Relying on a colour-kit pH reading instead of a laboratory test. Forgetting the utility locate.