There are few more fundamental aspects of life than eating. Life does not spontaneously generate energy or material, and therefore all living things must pull nutrients from their surroundings in order to grow, survive and reproduce. On a personal, visceral level, every human understands this concept.
The early biologists (i.e., farmers) understood it too, and they understood that plants, as well as animals, needed to pull in nutrients to survive. Generally speaking, the first crops that would be planted on a plot of land would yield better than subsequent plantings. At least from the times of the early Egyptians, but probably long before that, the use of animal and plant waste products as fertilizers was common. There was very little understanding from those early farmers about what was actually being replaced with the additions, but whatever it was seemed to work. Adding manure or other fertilizers (dead plants, ashes, crushed seashells, etc.) prior to planting would cause greater yields.
Figuring out exactly what was happening had to wait until adequate understanding of chemistry was developed, and that happened during the lifetime of Justus von Liebig (mid 1800s). At the time, a lot of focus was given to the organic character of the soil. Much stock was put in the amount and quality of humus within the soil (Humus is degraded plant or animal matter that gives soil a dark brown or black color). Liebig, on the other hand, thought that humus was an essentially meaningless product for increasing plant yields, and argued instead that a single, inorganic component (ammonium) was far more important.
What Liebig understood is that a plant is made up of a few dozen elements. Agricultural plants needed all these elements to survive, but the repeatedly used soil was only deficient in one of them. When farmers of his day added manure or humic soil to their fields, they were adding a mix that contained all the elements, but all that mattered is that they were adding the 1 element that was missing. Liebig explained this by using a visual metaphor of a bucket constructed with staves of unequal length. Each stave represents a single nutrient that the plant needed, and the amount of water that the bucket can hold represents the plant's yield.
What Liebig explained is that the bucket would only hold as much water as the shortest stave. That is, a plant will only grow until it runs out of the nutrient that is least available. Liebig argued that for the agricultural systems in his neck of the woods (Europe) the nutrient in least supply was nitrogen, and that in order to increase yields, all one needed to do was add nitrogen (in the form of ammonia). Although there was some hiccups in the implementation of this concept, it has certainly proven to be correct. So much nitrogen fertilizer is used in the Mississippi basin, for instance, that it is causing the infamous Gulf of Mexico Hypoxic Zone.
However, what's true for plants is actually true for all living organisms. Liebig's Law states : growth is controlled not by the total of resources available, but by the scarcest resource (limiting factor). Notice the word resource and not element or nutrient here. That's because sometimes the limiting resource is not chemical. For instance, the growth of a population of hermit crabs may be limited by the availability of shells rather than any particular nutrient.
Liebig wasn't the first one to propose this idea, but he certainly made it popular. The effect on agriculture was slow but astounding. Fertilizers that targeted the missing nutrients in the soil (usually nitrogen) became extremely important to the boom in agriculture in the 1900s.
This idea of a limiting factor has really fascinated ecologists. Experiments have been on plant and animal species from aquatic and terrestrial habitats all over the world trying to get an idea of what the most likely limiting nutrients are. In general, important limiting factors for plants have included light, nitrogen, phosophorus, and the availability of pollinators, while larger animals (like humans) generally seem to be limited by food energy (the # of calories in the available food).
Limiting factors are an important component of evolutionary theory as well. This is, essentially, where evolution occurs. If a particular resource is limiting, and everyone in a population is competing for that resource, then individuals who can exploit that resource more efficiently will be more successful and more likely to pass on their genetic material.
Ok, I think that's enough for this week.
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