When you use the adjective, 'rich' in conjunction with soil, what exactly do you mean?
If you are wondering about the soil beneath the decomposing grass clippings, I would say it is full of life. As those clippings decompose (they have lots of nitrogen to feed and promote the decomposition process, decomposers primarily use nitrogen for 'energy'. That is why when using legumes for instance, as a cover crop,they have enough nitrogen sequestered to feed the decomposers. Quicker decomposition. But there is little to none left for crops, ornamentals in the soil.
That material, in this case the clippings, as they decompose quickly, there is always some decomposed material available to the soil organisms during the process. Feeding the soil bacteria, fungi, nematodes, earthworms, insects...that soil will be in my opinion RICH.
My definition of a rich soil is one full of life and organic material that this life ate and pooped out in the lower soil profile...6" plus or minus? Soil that has been fluffed up, that easily warms in the spring...no matter if that soil is made up of sand particles, silt, loam-sized particles or pure clay, flat, tiny tiny tiny particles.
Decomposed organic matter just dumped on the surface of the soil will feed the soil life, promote more life, and will be mixed into the top 6" of soil for you by these happy organisms in the soil that can only eat decomposed organic matter. This is a very big deal. This makes great 'tilth'.
But never imagine this decomposed organic matter or compost or mulch is ever meant to add NPK or calcium, or molybdenum, magnesium, sulfur, iron etc. etc etc...the recipe for plants to make their own food is kinda complicated. Every species is different. If we humans imagine success with crops, pretty landscapes with healthy plants and lawns, vibrant potted plants...we need to understand we have to be in charge of everything to include proper chemistry, proper ingredients to add to our artificial gardens for success, health, a decent crops.
Plants need tilth, proper water and drainage, proper sunlight either full sun, partial sun, total shade or artificial lighting, ventilation by fans annd pumps in tighter spaces to blow off the O2 around the leaves suck in fresh air that has CO2 that the plants have to have to make their own food, fertilizer, compost, fish emulsion whatever that we all see called 'plant food' on the labels is NOT PLANT FOOD. CO2 plus H20 plus nitrogen, phosphorus, potassium, iron, sulfur, calcium, etc. MAKE food for the plant.
A rich soil is full of decomposed organic matter making it dark and more friable. Easier to grow great root systems, improves drainage and 'buffers' pH...making a little too alkaline soil neutral, a little too acid soil more neutral. But organic matter does not add the chemistry to fulfill the recipe requirements for photosynthesis...a little too much and you'll have death or vulnerable plants. A little too little and you'll have wimpy, vulnerable plants. We have to add that chemistry.
The worst soils for chemistry or chemicals necessary for photosynthesis for new plants is old forest soils. All of the chemistry, literally almost all of the chemistry necessary for NEW plants is up in the biomass, not available for new plants (artificial gardens). Part of population control. To be able to grow crops that are completely artificial, We HAVE to add fertilizer/chemistry to our artificial gardens. We have to pay attention to this very important aspect of chemistry.
Rich soil does not have fertilizer or the chemical ingredients necessary for success with plants and crops. Yes, there might be some chemistry in the soil but that is not a definition for rich soil...whatever cover crop, fertilizers, compost one has added need to be taken into consideration when deciding what your plants need for fertilizer BEFORE they show symptoms. Less is best but knowing the chemistry between soil and plants and climate will cause success.