Wow, thanks for the fantastic information everyone. It sounds like my plan of 1 cubic yard of compost + rabbit manure should do the job. I admit I was surprised to have such good results. I will be adding rabbit manure (either directly or through tea) throughout the season, so nitrogen definitely shouldn't be an issue. To be honest, I'm not sure if the area has been limed--this is our first year planting here. The existing bed hasn't been cared much in quite a few years, so if it was limed, it wasn't recently. I was honestly expecting to have to lime myself, since the whole area is bordered on one side by some sizable hemlocks. I'm actually interested to see if any of the seeds I randomly threw down among some bushes in the back will actually grow. Because it's wooded, it's never been mown, so there are lots of decaying leaves and what looks like very rich soil as a result. I figured some shade-tolerant plants (kale, collards, lettuce, spinach, peas) might be happy there. Best case, my bunnies will happily eat whatever is grown. Worst case, I used seeds which I already have too many of anyway. There's also a HUGE compost pile, but I have no idea what to do with it. It's all yard waste and hasn't been tended. Our landlords have just haphazardly thrown everything in there from leaves and grass (good) to gigantic branches (bad). Digging out the good stuff on the bottom might require a backhoe...

Mackel, it's easy to figure out exactly how much you are applying. Potassium Sulfate: K2SO4 Sulfur 18.401% Potassium 44.874% 20 pounds of 16-4-8 fertilizer is 1.6 lbs of potassium per 1,000 sq feet, or 3.5 pounds of the potassium sulfate. Multiple by 1.46 to get the quantity for the whole area. (5 lbs of K2SO4 scattered over the whole area would do it - the difficulty would be getting an even distribution and no hot spots) I should add 1/2 a pound of sulfur per 1000 sq feet the 3.5 pounds of K2SO4 has .63 pounds of sulfur in it (close enough for gardens) ============ Aw heck, this is too much finicky chemistry, and if you took different samples you would probably get slightly different results from the lab. The results aren't so far off "desirable" that it's worth the bother unless your plants start showing signs of a deficiency. Add a layer of compost and call it done and see what happens.

I'm a doofus. I missed the fact that the analysis was conducted on your 2/3 soil to 1/3 mushroom compost blend. Anyway, my comment about 0.0 ppm available nitrogen stands. I think it's a lab error. My comments about salinity also stand because we don't know the starting salinity of the compost or the soil. That being said, the final blend is 23% O.M. by dry weight, which is super high. If the mushroom compost was 1/3 of the final blend by volume, that would put you in the 7 to 8% organic range by dry weight if mixed with a mineral soil. And that's if the compost itself were high in organic material. (The analyses I looked at for the compost ranged from about 30 to 60% O.M. by dry weight). So, it would be a safe bet that your 4-way soil is probably around 15% O.M. by dry weight or a little higher. That means that it was probably about 50% to 60% organic material by volume and who the heck knows what that O.M. was. If it was raw wood chips, you would still have some available nitrogen in the analysis, but potentially not enough to balance carbon and it's feasible that you do indeed have a nitrogen draw issue. Or, if it has a heavy manure component, that could contribute to salts. (even a high quality greenwaste compost will often have a salinity level of 5 to 7 dS/m). Also consider that such a high amount of organic matter has the potential to hold too much water, especially if drainage isn't all that great at the bottom of the planters.

I also thought that lettuce was shallow rooted until I read this. LETTUCE < The development of the root system during the intervening period of 4 weeks was remarkable (Fig. 94). The taproots had extended their depths from about 3.5 feet to the 6- to 7.5-foot level. As before, however, relatively few long branches originated below the first 10 inches. The large laterals, which formerly had reached a depth of only about 2 feet, now extended well into the fourth foot of soil. Some occupied the fifth foot also, and a few of the longest reached a depthof over 6 feet. That growth was still occurring was shown by the long, unbranched, turgid root ends. The number of branches filling the soil was much greater than formerly and sublaterals were much more abundant. The total spread, however, had not been increased since the preceding examination. Thus a volume of soil, with a surface area of over 9 square feet and a depth of at least 5 feet-nearly 50 cubic feet-was thoroughly ramified by the roots of a single plant.> ROOT DEVELOPMENT OF VEGETABLE CROPS