Let's look at some more examples of how certain things are absolutely needed.
At the left is a structural representation of the chlorophyll a molecule. There are two kinds of chlorophyll, a and b, and their chemical formulas are the same, except that in chlorophyll b the CH3 at the top, right of the formula is replaced with a CHO group. What's important to notice here is that at the very center of the chlorophyll molecule there resides a magnesium atom, Mg. In other words, this mineral element, magnesium, is absolutely necessary for chlorophyll to exist. And without chlorophyll there cannot be photosynthesis in green plants, and without photosynthesis in green plants, there cannot be animals such as us humans, who are powered by energy provided to us by green plants, who capture it from sunlight.
The meat in your hamburger comes from cows who eat grass and grain, which cannot exist without photosynthesis, which cannot be accomplished without chlorophyll, which cannot exist without modest and underappreciated magnesium.
Again, the point is to demonstrate how certain underrated things can be absolutely necessary for life. We are not trying to stir up hysteria about a looming crises because of magnesium shortages.
Along this same vein I just snipped the Sweetgum leaf you see at the right from a tree right outside my door. This young leaf has problems. It should be dark green, not yellowish, and its dark green color should be evenly displayed across the leafblade, not restricted to vein areas. The tree bearing this leaf is suffering from iron deficiency, and that is serious because chlorophyll cannot be made in the absence of iron. Obviously this leaf had a little iron, but not enough to make all the chlorophyll it needed.
The point being made now is that sometimes absolutely necessary things may be present, but not in great enough quantities for life to proceed as it should. The tree bearing the above leaf will be stunted and vulnerable to diseases. What could have become a mighty tree may now wither and die simply because it doesn't have the iron it needed.
Throughout nature you see this kind of thing all the time, and that includes nature's human corner.
Yet, today at an accelerating rate the Earth's non-renewable resources are being squandered. More and more people are wanting and needing more and more, yet the Earth remains the same size, and its resources are diminishing. One of the worst instances of our societies accepting the ongoing losses of irreplaceable resources absolutely needed for life on Earth is the loss of mineral elements needed for life, through soil erosion. Already there is talk of the Green Revolution coming to an end because in critical areas of the world there is not enough phosphorus to continue fertilizing the soil on a large scale.
You might be interested in taking a look at the Essential Elements for Plant Growth page from the University of Wisconsin. Notice at the bottom of each page there's a "next page." By following the links and going to the "next page" you can learn a lot there!
But don't think that the only losses are taking place in the Earth's soils. Entire plant and animals species are being lost to extinction. Great ecosystems such as the rainforests are being threatened. The oceans' fisheries are being grossly overfished. The air we breathe is growing dirtier every day. More and more chemicals and radioactivity are entering the biosphere threatening the genetic integrity of all living things.
Don't forget about our Web Links for Learning about the Earth's Endangered Ecosystem