Marin Master Gardeners
University of California
Marin Master Gardeners

Dig Your Soil

When it comes to gardening success, nothing is more important than soil. Truth be told, most soils in Marin County are not ideal for growing crops: they are too steep, or too rocky, or too sandy, or have too much clay. In suburban areas, much of the topsoil has been scraped off, compacted or contaminated with construction debris. The good news is that most poor soils can be corrected with amendments, and planting in boxed raised beds or containers resolves many soil issues. With thoughtful garden planning, educated use of amendments/fertilizers, and measured irrigation you’ll improve your soil and protect yourself from creating soil problems you didn’t initially have.

Most edible plants grow best in loamy soils—a mix of clay, sand, silt and organic matter. Marin’s predominately clay soils may be improved with cover crops; compost (adding 1-2” of compost twice a year, about two weeks before planting) and by adding other amendments such as kelp, feather meal, bone meal, organic matter etc. as needed.

For the purpose of planning a garden, these things keep in mind:

  • It’s not just soil, it’s the relationship of soil and water that will determine success in your garden. Clay soil that remains wet for too long will suffocate plants, while sandy soil may drain too quickly parching them. Both conditions inhibit nutrient absorption by plant roots.
  • Good garden soil will have enough organic matter to drain winter rains but hold moisture for several days in the heat of summer.
  • To test the drainage in your garden soil, dig a test pit about 1 foot deep, wide and long. This pit will reveal if there is standing water under the surface. It will also allow you to observe how the soil drains. To test, add ½ gallon of water, to an already damp pit, and time how long it takes to drain. If it takes a number of hours that’s OK, but if it takes days, water may pool undersurface in the summer when you irrigate, suffocating roots and creating anaerobic soil conditions.
  • To test the drainage in existing raised beds, fill a 1/2-gallon container with water, poke a few small holes in it, let the container drain, and note how deep and wide the water penetrates into the soil. Also note how long the soil takes to dry out. This will help you determine how much and how long to irrigate.
  • Test your soil for Nitrogen, Phosphorous and Potassium (N-P-K) and pH, at 5-6” deep, every 3 years, and more often if you are having issues with plant health. Amend as necessary.
    • Nitrogen encourages leafy green growth, and may be increased in soil by adding bone meal, blood meal, fishmeal, fish emulsion, manure or seasonal cover crops.
    • Phosphorous encourages blooming, fruiting and root development and may be increased by adding bone meal, bat guano, and rock phosphate.
    • Potassium is necessary for root development and plant vigor and may be increased by adding potash, wood ashes, greensand and kelp (which also contains small amounts of nitrogen and phosphorous). Add amendments 1-2 weeks before planting.
    • Soil pH may be lowered (made more neutral or acid) by adding agricultural sulfur, coffee grounds, peat moss, leaf mold or pine needles. Alternatively, pH may be raised (made more neutral or more alkaline) by adding ground limestone, bone meal, ground oyster shells, and hardwood ashes. Retest soil in six to eight weeks and expect changes in the range of .5 – 1.0 per application. For a list of pH tolerances for edible plants see “Vegetable Crop Soil pH Tolerances” by Sonoma Master Gardener, Steve Albert.
  • Mulch with straw, leaves, or compost to protect against moisture loss and erosion, and don’t walk on planting areas, or they will become compacted and not drain well.
  • More information about preparing and amending your soil is available on the Master Gardener Website

Read Gael’s First Edible Gardening Tidbit – "Choose Your Edible Garden Site"

Read Gael’s Second Edible Gardening Tidbit – "Create Your Edible Garden Plan"

Read Gael’s Third Edible Gardening Tidbit – "Select Your Plants"

View Gael's Crop Planning Guide for Planting by Season in Marin

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