When our soil is not well cared for, many problems can occur. You might be disillusioned or resigned to accept the soil you have, but do not despair! There are actions we can take to address many common soil questions that arise in the garden. 

My Soil is Hard

A hard soil is an indicator of compaction. Good drainage is important for the overall health of most plants. It is important to know how well water enters the soil (infiltration) and how well water moves throughout the soil (percolation). In a well functioning soil water infiltrates into, then percolates deeper into the soil horizon recharging groundwater.

Water can move through soil quickly, slowly, or at just the right pace. When soil drains slowly, there may be a compaction layer which prevents or slows down rain and irrigation water’s ability to move deeper into the soil. Most plants don’t want their roots sitting in saturated soil.

How well water infiltrates and moves deeper into the soil can be influenced by our gardening practices. If the soil has good biological soil structure which means there is pore space due to soil aggregation, then water is able to infiltrate or move into the soil and move deeper down. It is important to protect the surface of the soil from sun, wind and rain.

• It is a good idea to run a soil percolation test in a few areas of your garden so you have an idea of how much time it takes for water to percolate, or move deeper into the soil. Learn how to perform a DIY Soil Drainage Perk Test for Your Yard
• If you have a hardpan that prevents water from moving deeper into the soil the best option may be growing in raised beds or on mounds. Alternatively, choose plants that can handle wet soil during the rainy season.
• Good plant cover and/or mulching protects the soil from the elements and helps prevent surface compaction.
• If the soil is so compacted, you can’t get a shovel into it then maybe a one-time rototill of 3” of well-made compost blended 2:1 with a woody amendment like 1/4-Fir into the top 6-8” of the soil will make a big difference. You only want to rototill once, as it is a very disruptive practice. Simply applying 1/2 “ of compost and covering the compost with 2-3” of mulch on bare soil can make a big difference in a short period of time, especially if you do this in the fall before our winter rains or in late winter, early spring when the soil is still moist from the winter rains.

Our native soils are wonderful. California soils are young and contain most of the elements plants need. However, if you have soil compaction, very sandy soil or clay soil, you may feel discouraged. But do not despair. If you have compacted soil or sandy soil or clay soil, the addition of compost makes all the difference.

Changing the Soil Grade

Sometimes you want to raise the grade in your garden by adding soil to create flatter areas to plant.

• When selecting soil for this use, it’s best to use a Sandy Loam or you can use an amended Sandy Loam if you are not going deeper than 6”.
• Do break up the soil surface before spreading the import soil even if you are moving soil from one area of your garden to another area of your garden.
• It is critical to break up the garden soil and even blend the two soils together a little. This is very important for water percolation.

Best Soil for Raised Beds

If you want soil for a raised bed, it is a good idea to have a mineral component in the soil mix, usually a Sandy Loam or a Loamy Sand. You can also use clean top soil from your garden for the mineral component. Be sure to check the texture of your soil as a fine clay soil is more challenging to work with than a sandy loam.

• The mineral component could also include perlite, vermiculite, sand, rock fines, lava rock, and even gravel. Regardless it is good to have about 30% mineral component in the mix.
• The rest of the soil can be made up of a good quality compost, vermicompost is another good addition, and a woody material like 1/4 minus fir, redwood shavings, or coco coir.

Best Soil for Containers

Potting soils are designed for use in containers, with a high amount of organic matter and minimal mineral content. They can be used as a soil amendment in your garden soil but should not be used to raise the grade in your garden or used exclusively in a raised bed. The organic matter in potting soils is digested by the microbial community over time – usually in one year or so you will see a noticeable drop in the soil level, a good indication that it is time to repot.

• A general container soil would be composed of finished compost, worm castings and woody amendment, with perlite, vermiculite or lava rock to help keep the soil aerated and if needed an organic fertilizer appropriate for the plant going into the container.
• Never put garden soil, loamy sand or sandy loam in a container soil mix. These soils are too fine and will compact quickly, limiting air movement into the soil.

Hydrophobic Soil

Naturally Occurring Hydrophobic Soil

Hydrophobic soils are a natural occurrence caused by waxy residues from decomposing organic matter. Incorporating compost into the soil and applying mulch are effective strategies to prevent hydrophobicity by improving moisture retention and protecting against drying. If soils or mulch become hydrophobic during dry seasons, slow watering or seasonal rains can rehydrate them and restore their functionality.

• Read more about Hydrophobic soil - Wikipedia
• US Department of Agriculture article, 'Water Repellent Soils', explains when, where and why hydrophobic soils occur. 

Hydrophobic Soils After A Fire

Hydrophobic soils form after a fire when intense heat volatilizes organic materials in vegetation and litter, releasing gases that penetrate the soil. As the soil cools, these gases condense and create a waxy coating on soil particles, causing water to bead on the surface rather than infiltrating. This condition reduces water absorption, increases runoff, and heightens erosion risks, especially in sandy soils that transmit heat more readily. Hydrophobic layers can persist for months to years, depending on fire severity and soil type, but rainfall and microbial activity gradually break them down over time.

See this information sheet, Soil Quality Resource Concerns: Hydrophobicity, prepared by the USDA NRCS Soil Quality Institute to learn more.

Why Mulch

• Mulch protects the soil from the sun, wind and rain helping to prevent compaction.
• Mulch also feeds and protects the microbial community, remember the microbial community is essential for plant health.
• Mulch helps keep the soil cool, helps prevent evaporation of moisture from the soil and can keep weeds seeds from germinating.
• A 2-3 inch layer is all you need. Going deeper is not necessary. A deep mulch is more likely to become hydrophobic.
• Reapply one or two times a year. In fall before the winter rains it is especially important to protect the soil from the rain and help prevent germination of weed seeds.

NOTE: One caveat to consider when mulching – California is home to over 1,600 different native bees,100 have been identified in the San Francisco Bay Area. Over 70% are ground nesters, so do leave some soil unmulched. This article from the UC Davis Arboretum and Public Garden explains more about Taking Care of Native Bees Needs.

• If you have a lawn, consider Grasscycling.
• Check out this video explaining How to Spread Straw Mulch (3:17).

Synthetic Fertilizers vs. Organic Fertilizers vs. Compost / Vermicompost

Synthetic Fertilizers

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• Synthetic fertilizers have a guaranteed NPK% which is present on the package label.
• Synthetic fertilizers are soluble and feed the plant directly.
• Synthetic fertilizers do not support soil structure and can cause compaction over time.
• Some synthetic fertilizers that are not taken up by the plant can move with water into the groundwater or surface water.

Organic Fertilizers

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• Organic fertilizers have a guaranteed NPK% which is present on the package label.
• Organic fertilizers are plant, animal or mineral based. Examples are feather meal, alfalfa meal, blood meal, fish hydrolysate, rock phosphate. They may be blended or sold as single ingredients.
• Organic fertilizers feed the microbial life and the microbial life feeds the plant. They require microbial decomposition to convert them into a plant-available form which takes time and is dependent upon temperature and moisture levels in the soil.
• Organic fertilizers do not significantly increase the organic matter % of soil due to the small amount applied.
• Less likely to contaminate groundwater or surface water.
• Support soil structure reducing compaction.
• Apply in spring as the soil is warming up.
• Organic fertilizers can be plant specific, and are great for containers.

Compost (or Vermicompost)

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• Compost and vermicompost do not have a guaranteed NPK%.
• Compost and vermicompost provide microbial life, complex foods for microbial life, soluble nutrients for plants and will increase organic matter in the soil.
• Compost varies in nutrient content. Compost made from animal manures generally contain more soluble nutrients than compost made from plant material.
• In a new garden bed gently mixing in 2” of compost to a depth of 6-8” will improve soil structure so water infiltrates into the soil, providing food for the microbial community which is eventually made available to the plants.
• For regular maintenance, generally apply 1” of compost atop garden beds, gently mixing in to the top 3-4 inches.
• Compost improves soil structure, essential for a healthy garden.

Header image courtesy of Pixabay. Article images provided by Cathrin Callas, UC Master Gardener, unless otherwise noted. All images used with permission.