Okay, welcome back. So in the previous section, we've established the importance of what farmers call healthy soil for public health, ecosystems and agriculture. In this section, we'll talk about some of the ways that agricultural practices can degrade soil quality compromising the long-term viability of our food supply. Specifically, I'll talk about soil compaction, salinization and soil erosion. Soil compaction, usually occurs when heavy machinery such as the harvesters pictured here, compact the porous spaces in soil. Compaction makes it harder for water and root systems to infiltrate soil, and as I’ll talk about later, it can also contribute to soil erosion. Compaction is also a common problem in urban settings, any guesses as to why that might the case? The reason being that many of the vacant lots where urban agricultural is taking place were formerly home to buildings. Which, as you would expect, are heavy and compacted the soil. I'll illustrate the problem of salinization using the Colorado River as a case study. From its source high in the Rocky Mountains, the Colorado and its tributaries flow through seven US states, supplying water to over 30 million people and over 4 million acres of crop land. And by the time that river reaches Mexico, it is so depleted that at times during the year it no longer reaches the sea. The photo on the left shows the Colorado River near the US, Mexico border. A once thriving ecosystem at the river's delta has given way to mud flats and the regions crop production as you would expect has suffered without a reliable source of irrigation water. And while there has been attempt to fairly allocate water use rights between the US and Mexico, the Colorado remains among the most contested waters in the world. The photo on the right shows a Mexican farmer, what do you think is wrong with his soil? What are those white deposits? As you may have guessed, those deposits are salt. But why do you think the soil has become so saline? Well freshwater generally contains some amount of dissolved salts. And that's only a problem if the concentration of those salts is high enough, as to be toxic to crops. Now, on its way to Mexico, the Colorado River's waters are withdrawn, as we discussed earlier, for irrigation, and that water is sprayed on crops. Much of that irrigation water evaporates into the atmosphere, and is transpired through plants. As the water evaporates, what's left behind is water with a higher concentration of dissolved minerals. That remaining saltier water then return to the Colorado River via a run off in groundwater channels, thus increasing the salinity of the river. And so by the time the Colorado reaches Mexico, that water is often too saline for agricultural use. Now on the bright side, the US has built desalinization plants at the border, and this has helped the issue somewhat. And there have been some solid efforts underway to restore the once thriving ecosystem at the Rivers Delta. But these challenges are far from solved. Particularly, in light of climate change and its anticipated effects on fresh water reserves. To give another case study of the problem of salinization, this one harkening back many millenia. The ancient civilization of Sumeria faced a similar problem. The Sumerians channeled fresh water along aqueducts such as this one and then flooded their fields with water to irrigate them. The problem was that transporting fresh water over such long distances in this manner meant that much of the water evaporated along the way. And when that water evaporates, it leaves behind a higher concentration of dissolved salts and other minerals. Thus, the more the Sumerian farmers irrigated their fields, the more saline their soil became. Eventually to the point where it was no longer possible to grow wheat, their staple crop. And this salinization is thought to have been the linchpin in the fall of the Sumerian civilization. And history is full of cautionary tales about soil such as this one. Case studies that should remind us of the importance of soil and water quality. Soil erosion is the removal of top soil usually by wind or rain. This photo shows erosion taking place on a wheat field. Erosion is problematic because the layer of top soil contains the highest percentage of fertile organic matter. Which as you may recall from the previous sections, is largely responsible for soil's ecosystem services. Tillage defined broadly is preparation of agriculture land and for planting crops. Tillage can involve tools such as the plow, disc or rototiller, or just humble handheld implements such as the shovel, hoe and rake. Now tilling or tillage, depending on how it's done, is often a major contributor to soil erosion because it breaks up soil and disrupts surface vegetation leaving soil exposed to the elements. Erosion can also contribute to water pollution when run off transports pesticides excess nutrients and sediments into nearby streams and rivers. The Dust Bowl of the mid 1930's is perhaps one of the more sobering demonstrations of the devastating potential of soil erosion. Eager to cultivate the fertile land of the American Midwest, farmers plowed up prairie grasses with deep root systems that held soil in place. Subsequent high winds, following a prolonged drought, then swept away hundreds of millions of tons of topsoil. Dust storms, such as this one pictured here in Texas, moved up to 100 miles an hour, choking people and killing livestock in their path. These storms traveled as far as New York, where parts of the state went dark at noon. The loss of top soil associated with Dust Bowl decimated an estimated 15 million acres of farmland, an area the size of South Dakota, leaving a wake of widespread hunger and poverty, and costing over a billion dollars in federal relief. Geologist, David R Montgomery has said quote, that in the history of civilization, the plow has been far more destructive than the sword. And despite numerous lessons from history about the pitfall of soil erosion, we're still losing soil much faster than it can be restored. A 2007 study analyzed soil erosion rates from sites around the world and found that rates of erosion associative agriculture, were typically ten to 100 times faster than the rates at which soil is naturally restored. So what's causing soil erosion? Well these are some of the agricultural contributors. The first is heavy tillage, and I think that would be a great name for a metal band. The second is leaving soil exposed to the elements after harvesting crops. Another is compaction by heavy machinery. The reason being that runoff is accelerated over a compacted soil and when runoff moves faster, it's going to carry away more of that top soil. Another contributor is the loss of organic matter. And this can be a vicious cycle because remember, soil organic matter is porous, and it slows runoff by allowing water to infiltrate soil. Certain microorganisms in soil organic matter also exude substances that slow and adhere to water, further slowing runoff. If runoff is slower, it's potential for eroding soil is diminished. Conversely, if there is less organic matter, runoff moves faster, increasing it's potential for erosion. Finally, when soil equality suffers, many farmers have come to rely on synthetic fertilizers and other technological fixes to improve fertility in the short term. Studies suggest, however, that applications of synthetic nitrogen actually deplete soil organic matter over time thus, increasing the potential for erosion. Furthermore, a narrow focus on synthetic fertilizers as a quick technological fix for fertility often means that, that farmer is neglecting to add organic matter back into their soil. Because top soil is vital to agricultural productivity, erosion and other forms of soil degradation represent a major threat to long term food security. When we come back in the next section, we'll talk about some interventions that can be applied to prevent soil degradation.
