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

According to my calculations, you are short about 2 lb/k of P2O5 (phosphorus) and 4 lb/k of K2O (potassium). Probably the easiest way to get that 4 lb/k of potassium is to use a balanced NPK fertilizer, like 10-10-10 or 20-20-20. It would also give you 4 lb/k of nitrogen and 4 lb/k of phosphorus. You don't need that much phosphorus but it won't harm anything (except possibly the mycorrhizae). You want to avoid nitrogen in the summer. So if you went with the balanced fertilizer, you would be applying probably 3 applications, one a month for mid August, September, October. The 4th could be next year. Divide the NPK number into 100 to get the application rate (example, for 10-10-10, divide 10 into 100 to get 10 lb/k rate). An alternative is to find SOP, potassium sulfate, 0-0-50. You could apply that once a month, even in the summer, at 2 lb/k for four months. That would take care of the potassium. Two applications of starter fertilizer in the fall would take care of the phosphorus. Since you're planning to overseed, it would make sense to use this plan, as you could do an application of starter fertilizer at seeding and another a month later. Milorganite could substitute for the second application of starter in the fall. Use bag rate for starter or 25 lb/k of Milorganite at 5-4-0 to get 1 lb/k of phosphorus. As for nitrogen, the balanced fertilizer plan would give you 4 lb/k, which is more than you need for the year considering that you have already applied nitrogen this spring. If you use the alternative plan (which I think is better), you would get almost a lb/k with each application of starter fertilizer (1.25 with Milorganite at 25 lb/k) and then you could do a winterizer application of a quick release nitrogen in late fall after the grass has stopped growing but before the ground freezes. That would get you off to a good start next spring, when you could hold off on fertilizer until around Memorial Day. Your organic matter is low. It's good to leave clippings on the lawn when you mow. This fall mulch mow any leaves in your neighborhood that you can gather. If this is new construction, there may not be many trees but do what you can. The compost you're planning will help. Mulch mowing leaves is like making your own compost for the lawn --and you don't have to spread it! Water in your applications. Milorganite does not need to be watered in. Phosphorus tends to get tied with quickly with calcium in alkaline soil. It's there, just not available to the grass. Using organic forms of phosphorus, like Milorganite, enables a slow release of phosphorus. If you test next year and phosphorus is low, it's probably because of the alkaline soil. It's still there, just unavailable. Such is life for alkaline soil.

Wavering on replying, trying to be polite and stay on topic. I think a lot of regulars here have already read about NPK and know what a soil test is. You don't really have to reinvent the wheel, there's a balance between writing your own content and simply reading/sharing what has already been explained. Regarding the 'chemical push,' in short everything is case by case devil in the details in my view. It would depend on what soil test, what is measures, who does it, what they are recommending, have they demonstrated examples of their work, are they benefiting financially from what is happening, and so on. It's not that there is a chemical side and an anti-vaxxer side, it just actually depends on what it is. The science and scientific process is extremely important, but if you are getting so mired in the details and big words that you don't even understand what all the terms numbers and graphs mean, and it's blocking your view of just looking at the dirt, crumple the paper and bin it IMO.

I had to do that search to see what happened. It is what I thought it would be. I was a charter member in 2008 and still love that forum. I can provide a little history... The GardenWeb forum was created by a guy named Spike back in the 90s. Spike was the ultimate judge of what was acceptable here. For example, if you tried to post profanity, when you hit the Submit button, your post was erased and you found yourself at disneyland.com. It was hilarious until you found yourself "sent to Disney," (ahem). The forum was growing as more fora were added, and, in the early 2000s, Spike had an opportunity to sell. The new owners (not Houzz, yet) implemented Spike's vision but tried to use forum moderators. Trouble was nobody knew who the moderators were. These owners added sh-tton of advertising, which was a new and unwelcome idea at the time. The lawn care forum split off to have an organic lawn care part, because the arguments were intolerable. More forums were added and the community grew huge, fast. I don't remember the catalyst in the lawn forum, but it quickly grew into a dumpster fire. You could not get advice due to the megalomaniac personalities involved. A large group of us left GardenWeb and started an independent forum. A few of us/them returned regularly to GardenWeb to shanghai members here. Those members found themselves banned for life. The new forum thrived for awhile until the absolute power of forum ownership corrupted them absolutely. I personally had my comments changed to correct "errors" which were not errors but were my diverging opinion from the owner of the forum. A couple of the members on the new forum decided to start another new forum. I'm not sure how morpheuspa was not included in the charter membership of that forum but he came to it within a few weeks of the launch. I got the organic lawn care forum going and TexasWeed moderated the warm season lawns forum. TW had some issues with other members, and they found a way of booting him out. Again it is personalities and the ability to tolerate certain indignities. Morph and I both have been in and out of GardenWeb over the years, but that other forum is still going strong. Except for a very few glitches, it is a safe haven to discuss lawn care tuned to your situation. I think the participation has dropped, but the owner tells me it's all going about the same. But if you have a lawn problem, it's probably been discussed in the 10s of thousands of posts on there.