One must think about soil microorganisms as well as plants and soil chemistry to properly understand soil fertility and anticipate results. Soil chemistry affects the variety and abundance of soil microorganisms, which in turn affect soil chemistry.

PH is an issue here too. Soil bacteria like the same sort of PH range as the plants and respond noticeably to changes. Soil fungi can be less affected by PH, doing well in acidic conditions, but the type of fungi changes with conditions.

Important soil microorganisms such as rhizobia and mycorrhizae that live cooperatively with plants prefer normal PH ranges but also prefer a certain amount of nutrient scarcity. Actually, it isn't that they prefer scarcity so much as that it is only in such conditions that their skills are in demand by plants. A clover that can get all of the nitrogen that it requires with its own naked roots from nearby soil has no incentive to trade its precious sugars to rhizobia for nitrogen. The clover continues to thrive but the rhizobia are out of work. Similarly, a plant that gets all of the phosphorus, water and nitrogen it needs on its own steam has no incentive to do business with mycorrhizae.

This can be wasteful. Plant roots only scour a portion of nearby soil for nutrients and so make less efficient use of the total amount of nutrients in a sward. It takes more total nutrients to satisfy demand, which is expensive and can lead to nutrient loss through leaching, lock up and denitrification. And when hard times come - when, not if - the population of soil microorganisms is not adequate to demand so the plants go hungry.

To get the most from a sward for the least expense it is necessary to consider the needs of soil macro and micro organisms as well as the crop plants. Since they all have slightly different and sometimes competing preferences it's a balancing act to keep them all tolerably happy.