A beautiful garden starts with healthy soil. Enhancing your soil is the most important part of good design and planning. To get to know your property’s soil, start with a basic test and analysis, which will reveal nutrient deficiencies, the soil’s pH, and recommendations for amending your soil. Have the test done at a local soil laboratory. The lab will tell you where and how to collect the soil samples to test.

Garden Soil

Think of soil as a sponge with pore space containing water, air, mineral particles, and living and dead organic matter. Roots grow in the topsoil layer (topsoil). A good garden soil has uniform texture, neutral pH, and the ability to hold water for root nourishment (water-holding capacity) while allowing excess water to drain (soil permeability).

A good garden soil.

Soil pH

Soil with a pH of 7 is neutral (neither acid nor alkaline). A pH below 7 is acidic and above 7 is alkaline. Most soil in Fresno County is alkaline with a soil pH of about 8.0. Soil pH affects nutrient availability: macronutrients tend to be less available in soils with low pH and micronutrients tend to be less available in soils with high pH. Test your soil to determine its pH. Here are some suggestions for soils that are not neutral:

  • Add lime (calcium carbonate) to acid soil to raise pH (you can use your test lab application rates as a guideline). Acid soil is most common in areas of heavy rainfall, sandy soils, and high organic matter.
  • Add sulfur to lower pH. Alkaline soil is high in calcium carbonate. Clay soil tends to be alkaline.

Soil Type

Clay, sand, and loam make up the three soil types. Clay has small, thin, disk-shaped particles. Sand has large rounded particles and silt particles that are medium sized. Loam is a mix of clay, silt, and sand and is the ideal garden soil. With a combination of large and small pore spaces, it drains well and contains enough air for healthy root growth, and nutrient loss is moderate.

To figure out your garden soil type, drench a patch of soil and then let it dry out for a day. Pick up a handful of the soil and squeeze it firmly. If it forms a tight ball and has a slippery feel, it’s clay. If it feels gritty and doesn't hold its shape (crumbles apart when you open your hand), it’s sandy. If it is slightly crumbly but still holds a loose ball, it’s closer to loam.

Improving Your Soil

Clay soil has small pore spaces, poor drainage, is heavy and compacts easily (which makes it difficult to dig). Improve soil structure by following these steps:

  1. Work in plenty of organic matter (see Soil Amendments below) to improve drainage by tilling the amendment about 8-12 inches deep and top-dress with a good biodegradable mulch.
  2. Organic mulch
    Organic mulch
  3. Avoid overhead spray irrigation, where possible. The large droplets of water lead to compaction of the soil and, therefore, reduced drainage.
  4. Allow time between watering so air can return to the root zone.
  5. Make raised beds and fill with top soil or a mix of 50% compost and 50% top soil.


A hardpan soil layer causes gardeners much grief. Hardpan is a naturally-impervious layer that ranges from 6” to 6’ below the surface. Hardpan is composed of a mass of soil grains firmly cemented by iron-silica and serves as a barrier to water percolating down from the surface. Impervious layers of soil similar to hardpan are also created when builders spread excavated subsoil over the soil surface and repeatedly drive heavy equipment over it. A hardpan layer close to the surface may be broken up with these steps:

  1. Till the soil to a depth of 1 foot or more.
  2. If tilling is not possible, drill through the hardpan with a soil auger to the porous soil below.
  3. A hardpan layer may require the installation of a French drain.
  4. Use raised beds in the garden and fill with top soil mix.

Sandy Soil

Sandy soil drains quickly because it has large pore spaces between the sand grains. It needs frequent watering because the porous spaces don’t hold water, and it needs fertilizer added frequently because the water washes the nutrients below the root zone and makes it unavailable to the plants. Plants have deeper root growth in sandy soil because it can easily be penetrated. Add organic amendments (compost or humus) to improve the water-holding capacity of sandy soil.

Soil Amendments

Organic matter is vital to soil fertility and is especially necessary for clay and sandy soils. Amend your soil with organic matter because it improves the soil structure while releasing nutrients. Amendments are beneficial in breaking up clay soils to allow oxygen and water to enter the root zone and they help sandy soils with moisture retention and nutrient-holding capacity. Below are some successful methods used by landscape professionals in improving soil.

Amending Soil

For clay soils:
Till in about 11/2 inches of pumice or gypsum and compost to the entire area to be landscaped to a depth of 12 inches. Then top with compost or biodegradable mulch after planting.

For sandy soils:

Incorporate compost or biodegradable mulch to a depth of 8-12" (worm castings may also be used).

Soil Solarization

An environmentally friendly method of using solar power for controlling disease agents and weed seed in the soil is done by covering the soil with a clear polyethylene sheeting. When covering the soil the sheeting must be sealed to the ground so the underlying soil can be heated up by sun for a period of four to six weeks.

For additional information see:

Soil Solarization for Gardens and Landscapes (PDF)


Mulch should not be confused with compost, which is a soil amendment. Mulch is used as a top dressing after plants and irrigation have been installed. Mulch covers drip irrigation, keeps the soil cooler, reduces evaporation, and makes the garden look neat and tidy.

Mulch Benefits

  • Moderates the soil temperature aiding development of healthier root systems.
  • Retains moisture during dry weather, reducing the need for watering.
  • Prevents germination of many weed seeds & reduces need for herbicides.
  • Protects the soil from the impact of raindrops that can cause crusting.
  • Adds organic matter to the soil as the mulch decomposes.
  • Encourages the growth of worms and other beneficial soil organisms.
  • Enhances the appearance of your garden.
  • Helps keep plants and house siding clean by reducing the splash of soil during rainstorms.
  • Helps infiltration of the rainfall into the garden.

General tips

  • Purchase organic mulches that are guaranteed weed-free. Straw, hay, and many types of manure, for instance, may contain weed seeds.
  • Remove weeds before spreading mulch. A pre-emergent herbicide may also be applied after weed removal and prior to spreading mulch.
  • To protect plants from insects and rot, which thrive in moist conditions, keep organic mulches a few inches away from the crowns and trunks of woody plants, like trees, California native plants, and water-wise shrubs.