Good soil is the gardener’s best asset.

Gardening is not just about plants, landscapes, flowers and vegetables. There is always the basis of a great garden—the soil to consider. I always recommend to people who want to get back to the land or want to commit to serious gardening to check out the soil before they buy property. That said, I recently bought property

Black adobe clay soil makes these hard to break clods when soil is worked too dry or too wet.

without following my own advice. I got black adobe clay which is the soil I had for my first home in Stockton purchased in 1968. Had I known that was what I was buying, I might have had a few second thoughts.

Black adobe is a heavy black clay soil common to North Stockton. It is rich and fertile, but it is also tillable at noon on one day of the year, so some say. The rest of the year it is either too dry and unbreakable or too muddy and gummy to deal with. Clay soils have relatively low pore space compared to sand soils, hence they drain very slowly and if compacted, drainage is much reduced. However, I have dealt with black adobe before and can deal with it again. One way around it is to use raised beds filled with other soil as I mentioned in last week’s article and/or add lots of compost to help

Clay soils crack open in the dry season.

loosen it up.

Why are clay soils rich and fertile?  Soil can be clay, silt or sand particles or some combination of these particles. The combinations are loams and named according to the dominant particle type and thus we have: sandy loams, clay loams and silt loams. Loams are considered more ideal because they are easier to till and they drain faster than pure clay soils and less rapidly than pure sand soils.

Clay particles are the smallest of the three types and are bound end to end and side to side in extensive planes in a sandwich like matrix held together by electrochemical charges. A clay particle is about 250 times smaller than a medium grain of sand to give you an idea of how different these soils are.

Individual clay particles are negatively charged so they have the capacity to attract positively charged elements (cations) such as ammonium, potassium, calcium, magnesium and other trace elements that are necessary for plant growth. Sand, on the other hand, has single uncharged particles that do not attract cations and hence it is not a fertile soil. It is also why fertilizer leaches more readily out of sandy soil as there are no electrochemical charges to hold the cations.

While soil structure, the way soil particles are assembled as aggregates, is an inherent property of the soil type that cannot be changed, it can be improved. Clay soils consolidate over time into tight and sticky stuff. To achieve a more granular looser crumbly soil with ease of tilth, the addition of compost and organic matter is necessary. Cover crops which send roots deep into the soil and also add organic matter when tilled in also helpful. For more information on improving clay soils for gardens see:

One recommendation is to use a spading fork instead of a shovel to till clay soils and not to till when the soil is too moist, which will result in large clods and greater compaction.  Application of gypsum may help break up clay soils. It should be applied at the rate of 1 pound per 5 square feet. However, the benefits obtained, if any, will likely be temporary, because irrigation water will gradually dissolve the gypsum out of the soil. Applying lots of organic matter such as manures, compost and green cover crops will be a more long term fix.

Lawns in clay soil may suffer from shallow roots and compaction of the soil which may retard water penetration. Aeration of the lawn with an aerifier that removes soil core plugs 3 to 4 inches deep is a good way to improve the drainage and getting air to the turf roots. Fill the plug holes with fine compost or other fine organic material after aeration.

Soil is more than the mineral particles. Soil is not dirt; that’s the stuff you pick up with a vacuum cleaner. Soil is a living substance with an entire ecosystem of bacteria, fungi, protozoans and invertebrates that include nematodes, earthworms. These living parts of the soil have symbiotic relationships with plants that help keep them nourished and disease free as well as keeping the soil healthy. So feed and treat your soil well as it is a living thing that makes your garden grow and shine.

If you have a gardening related question you can contact the UC Master Gardeners at 209-953-6112. More information can be found on our website:

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