Reversing Soil Degradation of Agricultural Lands
Once I was wandering in a field which had been sumer-followed. I began comparing the soil in the field to the soil right beside it in an area of trees. There was no comparison. The soil in the field was hard and compact due to it's clay base and years of cultivation which had a very negative impact on it's soil structure. The soil in the field when wet could turn into what we called “gumbo” almost clay that a potter might use and terrible stuff to get stuck in with equipment. In the trees I found I could easily dig down with my hand right to my shoulder. Old and decaying roots left a network that allowed many pockets of air to develop, almost like a sponge. To do the same thing in the field I would need a shovel. I also saw that the soil in the field was a fairly light gray. The soil in the trees was much darker, although this may have been due in part to higher moisture content which would have been because of the trees shading the ground and preventing evaporation of water. A dark color usually indicates much more productive soil. Also in the field there were low areas where salt had settled, produced as a result of using fertilizers. Enough salt will make an area completely unable to grow crops. I knew that each fall truckloads of grain were hauled off the fields and the nutrients necessary to produce the grain, obtained from the soil, went with it. I reflected that we were really mining the soil, not using it in a way that could continue indefinitely. In human history many civilizations have fallen because of a collapse of agriculture caused by soil degradation.
In nature nutrients are recycled. Plants grow, die and are decomposed by bacteria returning the nutrients to the soil. Anything eaten by animals is returned in animal waste close to where it was consumed or by the death and decomposition of the animal itself. In traditional agriculture, human and animal waste was often used as fertilizer but in modern times this practice is hazardous because of heavy metals, pesticides and residues of drugs used on livestock such as antibiotics which are used in high dosages routinely on healthy cattle to improve weight gain. Use of human waste is especially problematical because of the possibility that human diseases caused by viruses and bacteria will cycle through the crops.
Animal and human waste probably can be recycled if it is done in a careful and controlled manner. Perhaps the heavy metals can be removed by a process similar to electroplating which would allow the metals to be removed in solid form, although this is more of a problem to refer to a chemical engineer. The problem of bacteria and viruses can probably be dealt with by irradiation. An electrically produced radiation source is preferable since it does not require the production, transportation and safe reclamation of radioactive substances. Hopefully this would not require too much energy.
Nutrients that have already been removed from the land ultimately end up in the ocean. We should probably go to the ocean and remove some of it's sediment in order to return it to the land. Since sea life exists at most depths, even below the level where sunlight can penetrate in any quantity, we need to be careful about the effect we are having on the local marine life and about stirring up the sediment and overly muddying the water. Since the sea contains some amount of just about everything, we need to be careful what we are bringing back. Heavy metals and disease organisms are again a problem but can probably be dealt with as above but chemical pollutants of every kind also contaminate the ocean. I am not sure if these can be properly removed or not but this is a research problem for a chemist. Care would have to be taken to make sure every undesirable chemical was removed. If it is necessary to remove mass in order to make transportation back to the field less energy intensive then perhaps a more complex processing process could achieve this.
There are many other interesting ways of improving soil fertility. I remember reading of a practice used by indigenous cultures in South America whereby they would take a small amount of soil from a particularly productive area of soil and take it to a new location and spread it around. After a few years the high productivity would extend to the new area. Unfortunately I cannot find a link to a description of this on the internet. I believe what they were doing in this way was transplanting microorganisms that were freeing nutrients useful to plants from the soil and rock. The use of fertilizer trees (https://en.wikipedia.org/wiki/Fertilizer_tree) is also particularly interesting. This practice is mostly used in Africa but perhaps suitable species of tree could be found to do the same thing in other regions of the world.
Ultimately food can be produced wherever we can bring together water, light and appropriate nutrients at a reasonable temperature. Urban Agriculture and Hydroponics are examples of this. There are many new and promising ideas being generated in the area of agriculture and food production and I believe we are on the cusp of a second Agricultural Revolution such as the one we had in the 1970s that will likely allow us to sustain a growing world population which is expected to peak at about 10 billion by 2050 (https://en.wikipedia.org/wiki/World_population) although I think the true number will be determined by government policy in each country concerning things such as taxation and contraceptive availability and promotion.