Soil, Amendments, and Plant Nutrition
Landscape plants live in soil, and most landscape failures are soil failures wearing a plant's clothing. This chapter covers soil texture and what it means for irrigation, pH and the nutrient problems that follow from it, the amendments that do and do not work, fertilizer arithmetic, and MWELO's soil requirements. The next chapter covers the planting itself, turf, and the maintenance and repair content area that CSLB lists separately at about 11 percent of the examination.
Texture, Structure, and Water-Holding Capacity
Soil texture is the proportion of sand, silt, and clay particles, and it is fixed; you cannot change a soil's texture with a rototiller. A soil reported as 45 percent sand, 35 percent silt, and 20 percent clay is a loam, which holds moderate moisture and drains at a moderate rate, and that is the texture most landscape plants prefer. Sandy soil has large particles and large pores, so it drains very quickly, warms fast in spring, and holds little water; clay has tiny plate-shaped particles and fine pores, so it holds a great deal of water but releases it slowly and drains poorly. That is why two beds irrigated identically, one in sand and one in clay loam, behave so differently: the sand bed needs water more often because it holds less available water per foot of soil depth, even though it drains better. Two terms describe the water in a soil. Field capacity is the moisture a soil retains after free drainage has stopped, meaning after gravity has pulled out what it can, and it is the upper limit of what plants can draw on. Infiltration rate is how fast water enters the surface, which governs the application rate an irrigation system may use without producing runoff. Structure is different from texture and it is something you can change. Compaction destroys structure by crushing the pore space, and a heavy clay pad compacted by grading equipment cannot be fixed by adding amendment on top; the corrective measure that addresses the root cause is ripping the compacted layer and incorporating organic matter. A simple field test tells you when a bed is ready for finish raking and planting: the soil is moist but crumbles rather than smearing when squeezed. Working soil wet enough to smear destroys the structure you are trying to build.
pH, Salinity, and Sodium
Most landscape and turf plants grow best in soil with a pH of about 6.0 to 7.0, slightly acidic to neutral, because that is the range in which the widest set of nutrients stays soluble and available to roots. Much of California is naturally alkaline. A soil test returning a pH of 8.1 on a site where azaleas are specified presents a choice, and the practical response is to revise the plant list toward species tolerant of alkaline soil rather than to fight the site's chemistry indefinitely. Where lowering pH is worth doing, elemental sulfur is the standard amendment: soil bacteria oxidize it to sulfuric acid, which acidifies the soil gradually over months. In the other direction, using ammonium sulfate as the nitrogen source year after year drifts soil pH downward, because that material acidifies the soil as it nitrifies. High pH produces a specific and recognizable nutrient symptom. Iron becomes insoluble at high pH, and iron is immobile in the plant, so the deficiency shows first on the newest growth: young citrus leaves that are yellow between green veins while older leaves stay dark green is interveinal chlorosis from iron deficiency, common where soil pH is high. Contrast that with nitrogen, which is mobile and gets moved out of old tissue into new: a hedge whose older leaves are uniformly pale while new growth stays green is short of nitrogen. Salinity is a separate problem. High electrical conductivity in a soil analysis, especially with a history of irrigation with hard well water, means accumulated soluble salts, and the management practice is periodic deep irrigation to leach the salts below the root zone. Sodium is different again. Gypsum, calcium sulfate, works on sodic soils because the calcium displaces sodium from the clay particles so they can aggregate into structure, and the displaced sodium is then leached out. It follows that applying gypsum to a compacted clay pad with normal sodium levels produces little improvement, because the problem there is compaction, not sodium.
Fertilizer Arithmetic, Worked
A fertilizer label carries three numbers, and they are always in the same order: percentage of nitrogen, percentage of available phosphate, and percentage of soluble potash, commonly stated as nitrogen, phosphorus, and potassium. So a 16-6-8 product is 16 percent nitrogen, 6 percent phosphorus, and 8 percent potassium by weight. Actual nutrient in a bag is the bag weight times the percentage. A 50-pound bag of 20-0-10 contains 50 times 0.20, which is 10 pounds of actual nitrogen. A 50-pound bag of 15-5-10 contains 50 times 0.15, which is 7.5 pounds of actual nitrogen. Application rates run the same arithmetic backward. A specification calling for one pound of actual nitrogen per 1,000 square feet, using a 20-0-10 product, requires 1 divided by 0.20, which is 5 pounds of product per 1,000 square feet. A soil test reporting very low phosphorus is best corrected with a product whose middle number is high and whose others are low, such as 0-45-0. Two behavioral properties matter as much as the numbers. Slow-release or controlled-release fertilizers feed gradually, which cuts both foliar burn and leaching, and that is exactly why a controlled-release product is the right choice on a slope above a creek where a soluble product would move with the first rain. And nitrogen drawdown is the trap: fresh wood chips tilled into a planting bed cause plants to yellow within weeks, because the microbes decomposing the high-carbon chips consume the available soil nitrogen to do it, tying it up temporarily. The same chips used correctly, spread on the surface as mulch rather than incorporated, cause no such problem, because decomposition happens at the soil surface interface rather than throughout the root zone.
Compost, Mulch, and What MWELO Requires
Compost is added to soil to improve structure and add organic matter, and its long-term benefit is that it builds aggregate structure, which improves both water-holding capacity and aeration at the same time, an unusual combination. MWELO makes compost and mulch mandatory rather than optional on covered projects. The ordinance requires a soil management report, meaning soil sampling and analysis with amendment recommendations, and the report's recommendations govern the amendment plan. Where the report does not specify otherwise, MWELO gives a default: for landscape installations, compost at a rate of a minimum of four cubic yards per 1,000 square feet of permeable area, incorporated to a depth of six inches into the soil, with soils exceeding a stated organic matter content in the top six inches exempt from the requirement. Mulch is required too. MWELO requires a minimum three-inch layer of mulch on all exposed soil surfaces of planting areas, excepting turf areas, creeping or rooting groundcovers, and direct seeding applications where mulch is contraindicated, and it allows a small percentage of the landscape area to be left unmulched as designated habitat when that is shown on the landscape design plan. Mulch is also the cheapest water conservation measure available: a three-inch layer of organic mulch conserves soil moisture by reducing evaporation and suppresses weeds by blocking light. Two placement details matter. Organic mulch must be pulled back from the trunk of a newly planted tree, because constant moisture held against the bark at the root collar encourages crown and collar rot. And on slopes, stabilizing mulching products are used so the mulch stays where it was placed. Because MWELO is a model ordinance adopted locally and was reorganized by a Department of Water Resources rulemaking effective in early 2025, confirm the compost rate, the mulch depth, and the exemptions against the ordinance your jurisdiction has adopted.
Drainage Problems That Look Like Plant Problems
A very large share of declining plants are drowning rather than starving, and the diagnostic clues are consistent. A shrub showing wilted foliage in soil that is wet and smells sour is displaying root rot from saturated soil: the roots have run out of oxygen, they cannot take up water even though water surrounds them, and the plant wilts. A homeowner reporting a yellowing new lawn watered for 30 minutes every evening is describing the same mechanism, and the first thing to check is whether the soil is staying saturated and starving the roots of air. A percolation test tells you before planting rather than after. Fill a proposed tree pit with water and check it later; if water is still standing in the hole 24 hours after filling, drainage is inadequate and the planting detail must be changed, whether by raising the plant, adding subsurface drainage, or selecting a different species. Improving drainage in a heavy clay planting bed is done by incorporating organic matter such as compost throughout the bed and improving the surface grading so water leaves, not by digging a deeper hole and backfilling it with sand, which creates a bathtub. The general principle that ties this section together is that deep, infrequent irrigation produces healthier established plants than daily light watering, because it wets the full root zone and encourages roots to grow deeper toward the moisture, while frequent shallow watering keeps the surface wet and the roots shallow. A soil probe or auger is the practical tool for checking that: it tells you how deep applied water actually penetrated, which no controller can tell you. And a newly planted landscape put on the same schedule as the mature planting next door will struggle, because new root balls are small, sit above the surrounding soil's moisture reserve, and dry faster; they need their own establishment schedule.
Last updated: September 2026