Weeds: the plants no one wants. They have names too though we may not know it. They are also telling us stories if we pay attention close enough. Dandelions try to break up compaction - they drill deep into the soil: that's why they are hard to pull out. Bermuda grass holds together loose soil, so when you rototilled it so it was "perfect", it was perfect for that weed to come and help hold together that loosened soil. After fire, phosphorous is burned out of the topsoil and that's why we see a special set of plants arrive after fire: they're returning the phosphorous and generating the mulch that will recalibrate the soil. Each plant in addition to physically interacting with the soil is also creating a unique response to the soil's and soil life's succession. Every plant accumulates a spectrum of nutrients that remain when that plant breaks down to soil - it does range depending on the soil and conditions, but those are ranges within an established spectrum or unique circumstances like toxins being taken up by plants. Rice for instance is an aquatic filter as are all aquatic plants to a degree, so the unsafe arsenic levels we are finding are simply the plant trying to clean the water and/or soil. It may be the case that years of using arsenic-laden pesticides have imbued the soil with arsenic loads even in now established organic paddies.
Listen to your weeds: they are trying to tell you about your soil conditions and what needs to happen to improve them. You can pre-empt your weeds, chop & drop them before they set seed so you return the nutrients to the soil, and then plant something more vigorous and desirable that does the same function in their wake. If your weeds are thistles, plant artichokes or cardoon! If your weeds are dandelions, plant daikon radish! If you have bermuda grass, plant white clover and smother it: you can plant directly in clover or chop and drop it to the nubs and then throw sow plant in it. I've done it following Masanobu Fukuoka's example: it works. It's a really easy and fast pathway to healing a landscape. Listen to your plants, your birds, your soil! They are all trying to tell you something if you just give them the time and attention.