The nutrient cycle is very important for the continuation of life in any area. The loss of nutrients causes Deserts. Currently we believe that we can add any nutrients that plants need and do not really need a cycle to help us grow plants. This is a myth, as the inability to grow plants in barren lands shows.
Plants grow in the soil, which provides it several nutrients, and the major nutrients it gets from the air, oxygen, carbon, hydrogen and nitrogen. The micronutrients like calcium, magnesium etc, are not present in the air, and need to be recycled. In the prehistoric times there was no transportation. So plants and animals used to live in a geographically isolated areas. This tended to keep the nutrients in a balance.
But technological revolution has brought many improvements in transportation technology, and we get food from the world transported to every where. This has major implications for nutrient cycle.
Another major issue is understanding the nutrient cycle loops between plants and animals. Plants create most of the nutrients required for animals. When the plant dies the nutrients must be shunted back into the soil, so that it is available for the next generation of plants. Ultimately the decomposition happens through bacteria. But animals speed up the process quite substantially.
Bacteria need moisture for them to thrive, and break down substances. Moisture is not a problem in the sea. But its very important on the land. Animals provide a place for the bacteria to live, and provide them with the ideal breeding ground so that they can break down stuff much faster. Ultimately the plants need the nutrients the animal has absorbed from the plants. This is the reason plants thrive much better when they are grown together with animals.
Lets see what happens when we do not have animals where we grow plants. We need to put chemical fertilizers. These are only good to the point of our best understanding. Unfortunately, contrary to popular belief, scientists don't know all the nutrients that plants need. Also some of the nutrients, when created in factories are not nature identical but slightly different. This creates problem for the plants to utilize them.
Another big problem is that the major consumer of unrecycled nutrients is man. We do not feed the human waste back to the plants, because it is expensive, but we drain it out to the sea. We also drain out excess plant matter because we are not feeding them to the plants. This causes havoc in the sea, in the form of Algal blooms.