Have you ever prepared a new space for planting, gotten your seeds or transplants started, and then seen new plants emerge from the soil .. plants that you had not planted, plants that weren't even there before you began your preparations? Yes, I know, many will call these unwanted plants 'weeds' - and that is the usual definition; a weed is a plant that you did not intend to grow, but grows anyway - but where did they come from? The answer is simple: the seeds were already in the soil, waiting for the right circumstances to germinate. Perhaps I should explain.
When a plant reproduces, and forms a seed, that seed will then make its way to the ground. there are a number of ways the seed can make this journey .. it might simply fall from the flower, or with the flower at the end of a growing season, it could fall with the fruit in the same way, or be taken by an animal, and deposited there at a later time. Of course, being on or within the ground is not enough to coax a seed to germinate; if it were the end of a growing season, for example, it would do a plant little good to have it's offspring try to emerge, just as winter is arriving - so seeds have dormancy periods, and special circumstances under which they can sprout and begin to grow.
What happens when the right circumstances for germination aren't met? Some seeds require light in order to germinate, and if they are covered with too much organic matter (or otherwise shaded) won't sprout. Others might need to absorb a certain amount of water within a specific time, or they will not germinate - and these are common and ordinary circumstances; there are plants with much much more interesting requirements. When the requirements aren't met within the growing season, and organic matter builds up over a seed, it may remain dormant, eventually becoming buried in the soil, waiting for its own special set of circumstances to make an appearance .. sometimes for hundreds of years.
When the ground is disturbed, long buried seeds come to light, and if their circumstances are met, will begin to germinate .. some need a certain amount of moisture, some a certain range of Ph. Some need levels of nitrogen to be high, or low, and the same can be said for other nutrients and minerals. It is a strange and curious thing, that many plants have as their germination circumstances the very conditions that those plants will alter, if they are permitted to grow. A dandelion seed, for example, is most likely to germinate in fertile soil with moderate to severe compaction, and a low amount of calcium. During it's lifespan, a dandelion plant will loosen soil with its deep taproots, and draw up calcium from the subsoil, depositing it on the surface when its leaves die back. When the surface soil calcium increases, and the soil loosens, even though there may be thousands of dandelion seeds that would otherwise be ready to germinate, only a few will .. the rest of them remain in the soil seed bank, awaiting the day when they are needed again.
The presence of one indicator species alone should not be one's only consideration when attempting to diagnose soil conditions. Sometimes, a species might simply be all that was available in the soil seed bank at the time, and the single most prevalent germination circumstance is 'the soil is bare, something needs to grow here'. Most annual crops are planted under that single most important circumstance, even if the soil conditions are otherwise not optimal. Annual crops typically do not have the same rigorous sets of accompanying requirements that wild plants do - they are accustomed to having their needs attended by the farmer or gardener to ensure their survival.
With every tillage, some of the seeds within the soil become exposed, some germinate, and then, should the ground be tilled again, as is done in modern agriculture, those plants die off before they can replenish the bank. After several years of work, the ground finally becomes 'tired', depleted of some resources, and becomes less useful for growing an annual crop - but now nature has to work longer, and harder to repair the damage. The same holds true for land that has been used for grazing, although pasture land can be rehabilitated in several year's less time. If, upon seeing a multiplication of indicators like Canadian thistle and pigweed, the animal handler moves the herd off of the land and has them forage elsewhere for a season or two, the repairs will be done, and the area can return to grazing rotation.
I like to think of ideas as being like seeds .. when the circumstances are right, they germinate and grow into something wonderful, or something terrible. I'm sure the reader can bring to mind an example or two of ideas both wonderous and horrific - and contemplate this: If the soil' were in different condition, if the circumstances were not as they were when those ideas germinated .. would they have grown? I have spent several years sowing the seeds of an idea - that idea that theft, and the initiation of violence against other human beings is wrong, and should not be tolerated. The soil is not quite right for this idea, but I believe that it is getting close. Perhaps another season of terrible ideas and the tyrannical fruits that they produce will be sufficient to create the right germination circumstances, so long as the seeds of liberty remain in the soil bank of the human consciousness.