Anthony Gaudioso:

I am an expert in clay soils - I've been living with 65% clay for 30+ years up here in the Willamette Valley. Regarding your plan to prepare for vegetable gardening:

• My recommendation, and my own practice has been to build raised beds and fill them with compost and other organic matter, and not try to garden in the native soil at all. Over time, the clay will be mixed in with the compost. You have to top up these raised beds over time.
• You'll also want to check for bedding mix pH and phosphorus over time as well. But it's easy to succeed in the first year of a new garden, so no need to run a soil test.
• If you purchase compost, ask the seller about laboratory analyses that may have been run; this should be available to you.
• If you add manure to the beds, be sure to get something that was kept covered. Weed seed blow into manure and germinate once you start watering.
  

The clay content of the soils on my lot is so high, that if we didn't apply wood chips every 3-5 years, we'd be walking in water in the winter time. I live in town and we have sidewalks and curbs, but we also garden all winter long. I have easily 9" of wood chips in the paths between the garden beds. I have mulched out the whole front yard. The house was a rental for years before we bought it; tenants don't want to pay the water bill to irrigate a nice lawn. Mulching was the most expedient way to manage the weeds without herbicides. We've planted native groundcovers and low-lying shrubs into the mulch. There are all kinds of grades of wood chips; shop with your budget in mind to get good value for the look you want.

Yes, you can garden throughout the winter. In fact, now is the perfect time to plant garlic. By mid-August, start kale and other cabbage family vegetables from seed. In September, you can put out scallions and start lettuce from seed. To have good lettuce through the fall, a hoop house is recommended.

Now let's talk about the soil and the soil test:

The construction may have compacted your soil, or it may just be high-clay content soil that hasn't seen rainfall in 3 months.
I'm interested to know whether you see cracks forming in the surface of the soil.
Soil test recommendations:

• I'm surprised at their phosphate recommendation. The soil already has a high Bray test result. I think for a lawn or vegetable garden, you could reasonably skip the phosphate for this year. When I saw the phosphorus level in the soil, I assumed it had been manured over a long time, but low organic matter and sodium suggest it has not been manured.
  
• If you do lime, apply only agricultural lime, and do not apply dolomitic lime. Your soil already has more than plenty of magnesium.
  
• Although 6.5 would be a respectable pH for a vegetable garden West of the Cascades, the soil has high iron and moderately high copper. Increasing the soil pH closer to 7 will force more of the metals into insoluble molecules. Metals can negatively impact plant roots.
• Once the summer growing season has ended, and as the autumn rains begin DO NOT apply nitrogen - whether as a fertilizer out of a bag or as manures. Nitrogen is highly soluble in water, and will run off your site, with potential water quality threats to nearby streams, or will leach below the level of plant roots and threaten groundwater.
• If you do plant cabbage family vegetables, they will benefit from the addition of boron. I recommend a product like 20 Mule Team Borax or similar as a boron source. I apply 1/2 teaspoon per 12' x 3' raised garden bed. It's easiest if you mix the boron source into sand, and apply by salt shaker to the bed. That sounds like a ridiculously low dose, but in the case of micro-nutrients, the dose makes the poison. The difference between not enough a too much is very fine.
• As for zinc, I suspect it's not so important for a winter vegetable garden. If you were growing sweet corn for your livelihood, it might be a different thing, but I've never applied zinc to my soil, the level is very slow to change. Save it for the moss that will grow on your roof.
  

I haven't answered your question about cover crops. There are many that will do well, but none of them are going to change the nature of a clay soil. Like diamonds, clay soils are forever.

NOTE that repeated applications of wood chips will breakdown over time. A year or two ago, we found a leak in the veg garden irrigation. Fixing it required hand-digging. I found almost 30 years of well-rotted wood. It inspired me to throw most of the wood breakdown products into the garden beds and start again with a fresh layer of chips on the soil.

Thanks for sending that photo of the soil test results. That, and knowing your soil texture really guided my responses to your questions.

Cracking clays are classified as Vertisols - one of the 12 soil orders. The cause of the cracking takes place at the molecular level, so although management practices like green manuring, mulching and adding organic matter improve the condition of the soil, they cannot change its essential nature.

Clays have a crystaline structure much like a diamond. A diamond is defined by the atoms making up its molecules (all carbons, in this case), a defined distance between the atoms, and a defined angle of intersection of the latices (90 degrees for a diamond.) The atoms, the distance, and the angles are different for the various clay minerals, but the principle is the same. For that reason, no amount of tillage of compost or other organic materials will change the clay over the long term. That's why I recommended that you build raised beds and fill them with materials brought in, rather than trying to amend the soil. Organic materials break down over the scale of a few years; those clays are the result of geologic processes taking place over many thousands of years.

These vertisols are organized into a structured layer, with each layer having a specific set of atoms, distances, and angles. Water molecules enter the space between the layers, and as the season and soil dry, they escape. Structural weaknesses in the layered structure give rise to the cracks.

Somewhere I have a photo of a yard stick lowered into one of the cracks in my yard. Only the last 5 inches are above the ground. I hadn't hit the bottom of the crack, rather I was afraid of losing the yard stick!