Enriched substrates. I post this information often answering questions about soil tanks because I have a few of them and like the results.
Miracle Grow Organic Potting Soil has cow manure listed on all the bags I've seen. Miracle Grow Organic Choice Potting Mix (exact wording on the label) has chicken waste added in the mix and has less tendency to spike parameters when first submerged. I use this product because even though it varies on the contents found in the bag it's been consistent enough I trust repeated good results. The potting mix is claimed to be 0.10 - 0.05 - 0.05 so the ratio is low but fairly balanced. Currently I have been setting up tanks with it for 3yrs.
To you're question/comments on sub and lighting I've have done exactly what you are thinking about more than once. Placing up to a 2" layer of soil under the cap works very well for me. A 2" layer of Eco Complete should hold it down fine as a capping material. Another option would be to use it (Eco) and add some coal slag blasting media as a filler with the Eco to help close gaps in the capping material and keep a black appearance. Using a mixture of Black Beauty and Eco 1.5" held the lighter organics of the potting mix great here. Even thinner on the cap will work using sand size grained materials. 1" holds very well in my experience. The mixed size of the Eco Complete I have handled 2" would be my minimum capping thickness.
No matter what you put in a tank be it too much or too little without light energy plants don't grow. Extremely high nutrient levels won't cause algae to grow. But intense light without all nutrients at good levels will. Even non injected at medium/low light energy lacking a macro or micro algae will grow and higher plants fail. Following many threads algae in a planted aquarium is caused by imbalance. All planted aquariums are limited by one factor or another and always out of balance one way or another so it's a tricky answer to find.
Light (imo) is easiest to control. Floaters filter the water column removing excess NPK and can greatly help controlling light rather than changing photo period as another option. Good growth both in rooted plants and emerged without dosing the water column for 3 years is my greatest success to date. Water column dosing of trace/Fe with a soil substrate seems to benefit my tanks the most.
Low and medium light with a potting mix base like I'm discussing using phosphate and nitrate on weekly parameter checks will be zero or a static value most of the time without additions. Recorded periods of over 2 months without change on tanks following the Walstad outlined method. TDS showed a gradual increase but other readings remained static. Even with no PO4 or NO3 tested in the water column plant growth was good and little or no algae in these tanks.
Reading the opinions on algae issues I find them all across the extremes. Lacking phosphate causes GSA. Shortage of nitrate or CO2 swings are blamed for BBA outbreaks. High organic content (DOC) is blamed by many yet the Walstad outlined method can be followed and adjusted to great success. Growth of plants always has a limiting factor based on what I've read. What I have seen succeed here is what I'll post. If it works here I'll repeat it in a post once I've tried it seeing results.
All my new tank starts now include floaters. Added starting out many are cleared completely after a few months.
With my belief that light energy is easiest to understand and adjust without changing the tank in any major way. Great at filtering the water too goes almost without saying. Ricca, Frogbit, Moss or Subwassertang, Salvinia cheap easy plants with great value (imo).
Low tech having a number of soil based tanks all containing a high percentage of organics adding macro nutrients (NPK) to the water is only done at start up and water changes. Trace is more liberally applied.
Higher lighting levels and CO2 injected I'll hold a parameter range in the water tests. PO4 is what bottoms out on my tanks when things shift almost every time.
Flourite, Eco Complete and gravel mixes release to the water more during the first few months. Using sand or a mixture of sand and frag my water parameter always test a lower nutrient content.
I don't care for traditional river gravel very much as it doesn't produce a very good boundary layer.
I have included some clays a couple times now to increase Fe and used the additions listed for mixing when setting up MTS tanks.
That said having a tank set up with dirt alone MGOCPM and nothing else for over a year I saw nothing to indicate the plants were lacking any nutrient at all and growth never stumbled. Beyond 3yrs. now and still no additives or changes made. Growth has slowed but the tank is still stable and growing plants.
still here, flooded 4/30/2009 still growing plants.
No changes made, no daily or even weekly ferts, no root tabs ticktock ticktock
hope my thoughts posted here make some sense LOL.
I'm sold on using soils containing organics when it comes to planted tanks.