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Why doesn't the soil get depleted after repeated clearing of vegetation?

Why doesn't the soil get depleted after repeated clearing of vegetation?



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In my neighborhood, there is a hillside owned and maintained by the local municipality. The hillside is generally overgrown with vegetation. Work crews clear out the vegetation multiple times throughout the spring and summer, yet it always grows back quickly.

The workers don't leave the cuttings there. They haul them away. So that means the soil of the hillside is losing nutrients. Yet why does the vegetation always grow back?

Hypotheses:

  1. It all looks like weeds to me, but maybe there is a natural cycle of crop rotation going on that I can't appreciate. I.e., in year 1 it was all Type A weeds, then in year 2 it was all Type B weeds, then in year 3 it was all Type C weeds, and then in year 4 it was back to Type A weeds again.

  2. Maybe some other natural process is bringing nutrients back to the soil of the hillside. Bird droppings? Earthworms? Insects? Underground fungus and bacterial colonies? Small mammal feces? (I don't think rain runoff is bringing many nutrients because there is nothing but a highway above the hillside.)

  3. Maybe the hillside is getting depleted and will eventually become barren, it just takes a while. I guess you have to keep clearing, year after year, until it gets down to the very last and most hardy of the weeds before it becomes truly barren and lifeless.


Why soil is disappearing from farms

In Iowa they call it &ldquoblack gold&rdquo &ndash a fertile blanket covering the landlocked Midwestern state. Thousands of years of prairie grass growth, death and decomposition have left a thick layer of dark, organic matter on the vast plains.

When European-American settlers first began ploughing in Iowa, they found the weather and local geology had combined this organic mulch with sand and silt to form a nutrient-rich type of soil called loam. It gave Iowa one of the most fertile soils on the planet and enabled it to become one of the largest producers of corn, soybeans and oats in the United States over the last 160 or so years.

But beneath the feet of Iowa&rsquos farmers, a crisis is unfolding. The average topsoil depth in Iowa decreased from around 14-18 inches (35-45cm) at the start of the 20th Century to 6-8 inches (15-20cm) by its end. Relentless tilling and disturbance from farm vehicles have allowed wind and water to whisk away this priceless resource.

The same picture is seen on farms worldwide. Soils are becoming severely degraded due to a combination of intensive farming practices and natural processes. As the layer of fertile topsoil thins, it gets increasingly difficult to grow crops for food. Without altering agricultural practices and urgently finding ways to preserve soil, the global food supply starts to look precarious.

Even in Iowa&rsquos still-fertile fields, the loss of soil is concerning. In just one spring in 2014, Iowa lost nearly 14 million tonnes of soil from its cropland in a series of storms, according to environmental groups. A study of 82 sites in 21 counties by Iowa State University showed that in the 50 years from 1959, soil structure and levels of organic matter had degraded while acidity had increased.

&ldquoErosion from the wind is not as bad as it used to be in the dust bowl era, but in the past 20 years the rainfall pattern has changed,&rdquo says Paula Ellis, a farmer in south-east Iowa&rsquos Lee County. &ldquoWe used to get one to two inches of rain every other week, but now we are getting bigger rain events where six inches fall and that hits the soil on farms.&rdquo

The problems facing Iowa are by no means unique. According to the United Nations&rsquo Food and Agricultural Organization (FAO), a third of the world&rsquos soil is now moderately to highly degraded.

The processes that generate high-quality, fertile topsoil can take centuries. But the world is ploughing through that resource at an alarming rate. About 40% of the world's land has already been taken over by agriculture, while livestock grazing and expanding urban areas are taking further chunks out of what is left over.

At first glance, it might seem that there is no shortage of mud and dirt around the world. But it's the quality that really counts.

&ldquoMany types of soil degradation are invisible,&rdquo says Ronald Vargas, secretary of the global soil partnership at the FAO in Rome. &ldquoYou just don&rsquot see the loss of organic carbon from soils or pollution building up in it until you try to plant crops there.&rdquo

Erosion, compaction, nutrient imbalance, pollution, acidification, water logging, loss of soil biodiversity and increasing salinity have been affecting soil across the globe, reducing its ability to support plant life and so grow crops.

At the most extreme end, 12 million hectares of land &ndash an area that could produce the equivalent of 20 million tonnes of grain annually &ndash are lost to desertification every year. Meanwhile, the spread of our towns, cities and road networks are sealing soils out of reach beneath layers of asphalt and concrete.

According to some estimates, between one billion and six billion hectares of land are now considered to be degraded. But the problems vary greatly from region to region. In a handful of places, this degradation has happened naturally, says Vargas.

&ldquoThere are some places where the landscape can be compared to the Moon,&rdquo he says. &ldquoBut human activity has led to unsustainable levels of degradation.&rdquo

Climate change, the spread of intensive agriculture, deforestation and industrial activity have accelerated the loss of soils in almost every country in the world. Farming practices such as tilling break up the soil and destroy its natural structure, killing many of the vital bacteria and fungi that live there and leaving it vulnerable to being washed away.

&ldquoSoil is not just useful for helping us grow food,&rdquo says Vargas. &ldquo[Soils] are key for storing water &ndash good soil is like a sponge that soaks up the rain and keeps it there. It is important for recycling nutrients and storing carbon that would otherwise escape into the atmosphere.&rdquo

If we want to continue enjoying the riches of our soils in the future, something urgently needs to be done.

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Rachel Stroer points to two crops growing side by side. Both are types of wheat, each with the characteristic stiff, grassy stems and ears heavy with seeds. To the untrained eye, perhaps the only difference is that one appears stockier and is tipped with a denser head of grain.

Underground, however, the differences are more apparent. One of the grasses produces a slender tendril of roots that penetrates barely a couple of feet (0.5m) beneath the surface. The other is a thick, intertwined mat extending nearly 10ft (3m) down.

This second wheat is a domesticated variety of wild wheatgrass known as Thinopyrum intermedium, which can be found growing naturally across central Europe and western Asia. It is rangier than conventional wheat and it has smaller seeds packed less densely in a far more elongated head. The biggest difference, however, is that it is perennial.

This means that unlike conventional wheat, which dies off each year and so needs to be replanted with fresh seeds before each growing season, this wheatgrass will regrow without having to be re-sown. It could be a solution to one of the major causes of soil degradation.

&ldquoAnytime you till the soil, it destroys the natural ecosystem that exists in it,&rdquo says Stroer, chief strategy officer at the Land Institute, an agricultural research organisation based in Salina, Kansas, which has been domesticating wild wheatgrass into a product called Kernza. &ldquoWith perennial plants, there is far less need to till.&rdquo

There are other benefits too. Established perennial grasses provide less opportunities for weeds to grow in a field, says Stroer.

&ldquoTilling the soil loosens it and serves as a welcome mat for weeds,&rdquo she says. With their extensive root networks, perennials are often more able to cope with periods of drought and can help to hold the soil together, preventing erosion of this precious resource.

&ldquoWe believe that perennial planting is the closest we can come to restoring the soil to a state that has existed for millions of years on our planet,&rdquo says Stroer. &ldquoEvery native landscape on the planet has a mixture of perennials growing on it for good reason.&rdquo

The Land Institute is attempting to develop a new generation of perennial crops that can supplement and even replace our existing annual crops. In addition to Kernza, it is also developing perennial rice, legumes and sorghum, a popular grain in Africa and South Asia.

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Perennial crops are far from a perfect solution though. The oldest strands of Kernza in the Land Institute&rsquos test fields are 10 years old. They now produce little in the way of seeds.

&ldquoYields begin to tail off after about three to four years,&rdquo says Stroer. It means the plants would need to be replaced after that point to ensure decent crop yields. &ldquoIt is something we are working on.

&ldquoBut we have seen some perennial rice crops in China that have lasted five years and given 10 harvests. That kind of time frame can make a big difference to the soil.&rdquo

The real test, of course, will be how consumers accept these new crops. There are already some promising signs. Although Kernza tastes more like rye than traditional wheat flour, it has attracted a following from those making artisan bread, while a number of food producers have begun using it to make pasta and pizza bases. It has even been adopted in craft beer brewing, producing an ale with a nutty flavour.

&ldquoCurrently it works well when mixed with conventional wheat,&rdquo says Stroer. &ldquoBut we are hoping through our breeding programme to get it to be more like wheat has been for the past 100 years or so.&rdquo

In the interim, there are other approaches that could help to restore some of the vitality that is being stripped from farmland soils.

&ldquoIt is about careful husbandry, which involves paying attention to the chemical, physical and biological aspects of the soil so they are kept in balance,&rdquo says Elizabeth Stockdale, head of farming systems at the National Institute of Agricultural Botany in Cambridge, UK. She has been developing tools to help farmers assess and improve the health of their soils.

&ldquoWe want to increase the proportion of the year that the soil actively has roots in it and plants growing above it,&rdquo says Stockdale. It might sound counterintuitive at first, but planting vegetation in soil between crops can help to hold it together to prevent erosion. It can also provide vital nutrients to the organisms living in the earth. &ldquoThe photosynthesis from cover crops and intercropping helps to feed the soil system and keep the biology there active.&rdquo

But while this works in areas that still retain some of their original fertility, what do you do in areas where the soil has become completely exhausted?


Influence of Forest Harvest on Soil Microbial Communities

Abstract

Forests soils are important globally for many reasons, including the relatively large amount of carbon stored in forest soil organic matter. As a result, disturbance and subsequent changes in nutrient cycling in forest soils can potentially have large impacts on atmospheric levels of CO 2. Soil disturbance caused by logging can impact both abiotic and biotic components of the soil, which in turn may impact nutrient cycling and ecosystem function. Since soil microbes are major controllers of ecosystem nutrient cycling, understanding how disturbance impacts soil microbes may help elucidate potential disturbance effects on ecosystem function. In studies using early molecular biology techniques like biomass measurements and phospholipid fatty acids, changes in microbial communities after forest harvest were unclear. More recent studies using next generation sequencing technologies have provided greater taxonomic resolution and information about microbes that are negatively impacted by forest harvest. For example, bacteria in the phyla Actinobacteria and Gemmatimonadetes and plant symbionts such as ectomycorrhizal fungi are generally negatively affected by logging compared to other groups. Furthermore, there has also been evidence of changes in microbial enzyme activity after harvest. Decreases in activity of enzymes involved in the decomposition of organic matter and other enzymes that play a role in nitrogen and phosphorous cycling have been observed. Despite these observations, it remains unclear whether changes in community taxonomic composition alter ecosystem function via alteration of microbial activity and function, or if the microbial community is functionally redundant enough that these compositional changes do not affect ecosystem function. Future research should combine a variety of approaches in order to examine community composition, microbial activity, and soil environmental conditions to gain a full understanding of ecosystem disruptions and potential long-term impacts.


Experts including Strenge, however, will say kitchen vinegar recipes do work in some capacity, but only in some ways and with limits.

“Vinegar weed killers can work if used properly,” he said, “as long as users understand repeated sprays will be necessary, and that there are potential problems with using vinegar weed killers in their gardens.”

The one homemade recipe Strenge has seen work in action: 1 gallon of vinegar (5% acetic acid) mixed with 1 cup salt and 1 tablespoon dish soap, with an emphasis on the salt making its low concentration effective.

“It will burn weeds on contact under the right conditions: warm, dry, sunny days,” he said. Put it in a spray bottle and aim carefully.

But again, this comes with a caveat. “Users should be aware, however, that just because the ingredients are mostly harmless to humans and larger animals, it doesn’t mean those ingredients can’t be detrimental to the environment and other forms of life,” he said.

At another point, Strenge added, “I don’t really recommend using vinegar and salt weed killers [often] because of potential problems from repeated use.”


Why is my soil so compacted?

Hopefully, the ground in your yard has thawed by now (regrets to those who still have frozen ground!). So why is it that when you try to push your shovel into the soil, it doesn’t budge? If it’s hard for you to push that shovel, it’s even harder for plant roots to penetrate this type of soil.

Soil scientists refer to this as soil compaction, or compressed soil that is reduced in volume. Why does this happen and what can you do about it?

Let’s start with some soil basics. Soils are comprised of three major things – air, water, and solid materials. It might seem counter intuitive but a healthy soil should only have about half of its volume full of the solid materials. The other half should either be empty or consist of water. That empty air space is critical to ensure that gas exchange (carbon dioxide and oxygen for and from plant roots) and water can move throughout. Compaction occurs when there is less free (air) space. Soil is compressed or higher in density.

The solid materials of the soil include mineral and organic materials. Mineral portions are made of the remains of the underlying rock (parent) material. The organic portions are comprised of the remains of living things such as plant roots, plant leaves, or microorganisms.

To understand soil compaction, think of soil as made of bricks. In our example, tan and grey represent mineral materials. Dark brown represents organic matter. Blue represents water, and empty spaces are air. A healthy, uncompacted soil has enough space for water and air – and plant roots!

A useful way to visualize this is to think about soil like a pile of bricks — different soils in different areas, and different sized bricks. In addition, the bricks are oriented in many different ways. In a healthy soil, the bricks would be stacked or organized to make it easy for water and air to move in between the solid particles.

Organic matter is the other critical piece of the puzzle. Of the solid materials, the organic matter is the smaller fraction by volume. But organic matter is key to ensuring that those bricks or particles aggregate and associate together, kind of like a healthy glue.

Compacted soil is a problem for gardeners and farmers alike for a few reasons. It means that there may not be adequate space for roots to properly penetrate the soil, which might prevent plants from taking in nutrients and water. It could also mean that rainwater or irrigation water is less likely to infiltrate, or enter the soil. That means that rainwater and snowmelt can run over the surface of the soil and into stormwater systems – and even cause erosion.

Heavy construction equipment, other vehicles, and even foot traffic, can compact your soil. Source: Morguefile

How does soil get compacted? If you have new construction, all those construction vehicles on your soil caused compaction. Or perhaps you’ve been letting your garden area lie bare over the winter – or longer. Bare soil is prone to erosion of the “good stuff”. Loose, uncompacted soil can blow away with the wind, or be carried away with rainfall and snow melt.

What can you do to reduce compacted soil? Managing compacted soil requires both taking care of soil structure (remember the bricks being properly organized!) as well as increasing the organic portion.

Gardeners might have the instinct to use a rototiller to plow the soil and soften it. Limited and low intensity plowing in a garden may sometimes be necessary and beneficial. However, even if plowing softens soil in the short term, in the long term it can decrease the healthy structure of soil aggregates (again, the orientation of the hypothetical pile of bricks). Tilling also hurts fungi, bacteria and other life in soil. For these reasons, we don’t recommend that home gardeners till their gardens very often, if at all.

Experts agree that any activities that put pressure on the soil surface should be kept to a minimum. Excessive plowing in a garden can also reduce organic matter. So, too much plowing can dig your soil into a deeper hole – pun intended – where you have a negative cycle of declining soil structure and a smaller organic fraction.

Here a few other important tips:

  • Compost is a great and fast way to get more organic matter into your soil. Using hand tools, work leaves, mulch or other organic materials into the top several inches of the soil. Organic matter is softer than mineral matter, and it helps increase the amount of air and water space. In addition, it adds needed nutrients for plants to grow – and plant roots reduce compaction.
  • Cover the soil as much as possible with cover crops. This includes in the winter! Growing plants at the end of the season (or after you clear a section of the garden) increases the amount of time that plants work their magic and convert sunlight into carbon-containing substances. Cover crop plant roots also create pores in the soil, biologically reducing compaction, and add to the organic portion of your soil. You can buy radish seeds and let them grow early in the season – nothing like large tap roots to naturally break up the soil! Or, plant them late, and leave them in place over the winter.
  • Avoid activities in your garden area during wet days. Soil is more susceptible to compaction when it is wet. This will prevent further compaction. Even foot traffic compacts soil.
  • Create foot paths in your garden. Limit walking traffic to specified paths.

You might not see results overnight but trust that with minimum soil disturbance, and the addition of compost and cover crops, you will activate the biology of your soil. Expect earthworms and other critters to follow, which becomes a healthy cycle that long-term will lead to less compaction. Reducing the compaction in your soil will help with the long-term health of your soil, the animals that live in it, and your plants!

Answered by Andrea Basche, Union of Concerned Scientists

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Ameliorating dense clay subsoils to increase the yield of rain-fed crops

Peter Sale , . Murray Hart , in Advances in Agronomy , 2021

4.2.1 Clay content and clay mineralogy

Clay particles play a role in soil aggregation . Six et al. (2004) proposed that clay particles adhere to and encrust decomposing fragments of organic material in soil to form stable microaggregates, within less stable macroaggregates. Direct evidence that clay content is important in aggregation comes from the incubation study by Wagner et al. (2007) in which chopped barley straw was added to soils varying only in clay content. Increases in the stability of aggregates only occurred when the clay content was 34% or higher, and with the highest rate of added straw. This beneficial effect of clay appears to be due to the protection that clay particles can provide to the labile organic binding compounds produced by biological activity. Degens (1997) supports this view that clay particles can increase the longevity of stable aggregates by their protective function while Golchin et al. (1994) describe how clay particles are able to encapsulate microorganisms, mucilages and labile organic compounds in a protective covering. The proportion of soil organic matter that is protected increases with increasing clay concentrations in the soil ( Golchin et al., 1994 ). This suggests that clay particles play an important role in aggregate formation and that clay soils, compared with lighter textured soils, are likely to have longer-lasting structural stability.

Clay mineralogy affects the soil aggregation response to biological activity with higher activity 2:1 clays, and 2:1 and 1:1 clay mixtures, being more responsive than low activity 1:1 clays ( Denef and Six, 2005 ). Bronick and Lal (2005) report that more active clay minerals with larger areas of adsorbing surfaces, experience more extensive aggregation with biological activity, compared with lower activity clays with less surface area. Dense, poorly structured clay subsoils that dominate the high rainfall zone (HRZ) of south east Australia, typically have clay contents > 45% ( MacEwan et al., 2010 ). The clay does have some higher activity 2:1 clay minerals, based on observations of shrink-swell behavior in the subsoil. They therefore have the textural requirements to respond to the structure-developing processes associated with biological activity.


How to clear soil choked with old roots: Ask an expert

Gardening season is in gear and everyone has a question of some sort. Get answers from Ask an Expert, an online question-and-answer tool from Oregon State University's Extension Service. OSU Extension faculty and Master Gardeners reply to queries within two business days, usually less. To ask a question, simply go to the OSU Extension website and type in a question and the county where you live. Here are some questions asked by other gardeners. What's yours?

Cleaning up after tree removal

Q: I purchased an older home in Eugene. There was a large Japanese maple at the edge of the back garden area that had been pollarded at about 9 feet on five or six major branches. The "stumps" of each branch had, of course, sprouted tons of water shoots. It looked hideous and was beyond repair. A reputable arborist visited and concurred. There was also a giant Acuba very near this tree, which was totally over grown and poorly placed. The arborist removed the maple and Acuba and brought in a stump grinder. The stumps are gone but the surrounding area is choked with the remaining roots, small ones and up to 1½ feet in diameter. They are mostly surface roots but to a depth of about a foot. It's nearly impossible to dig a hole and requires a mattock to get anywhere. Compounding the problem, the soil is a fairly heavy but loamy clay. All I could manage to do was hack at the surface of the soil, chewing up two or three inches and spreading compost over the top. I will keep the area damp as I have put containerized plants around and will water them over summer. This method could take many a summer and winter, however. What are some methods to improve this situation so that a small ornamental tree, evergreen shrubs, vines and perennials will be happy, faster? Would a trencher be useful? Or maybe use a small stump grinder over the area? I've thought about using an auger but the one I could rent I don't think has enough horse power.
– Lane County

A: From the perspective of new plants, they'll grow fine among the old, slowly decomposing roots, assuming they get proper water and fertilizer. From your perspective, however, planting new plants may be a major undertaking! It would be difficult and time consuming to clear out the roots over a large area, and there would be no benefit to new plants. In the short-term, I can see where a mattock and shovel may be insufficient if you have many holes to dig. A rented gas-powered auger should do the trick. When "digging" any planting hole, aim to make it about twice as wide as the root ball and the same depth as the root ball. If feasible, backfill only with native soil. If there are so many roots that you have insufficient soil with which to backfill, try to use soil from nearby if there is a place you can take it from. I don't like to add organic amendments such as compost, because they decompose and settle over time. Similarly, "loam" that you purchase from a local landscape supplier is generally too coarse and doesn't integrate well with heavier textured soils.


What is monoculture farming?

Monoculture farming means that on a given agricultural land is grown only one species of a crop at a time. If two or more species are sown in the field together (for example beans and corn), it is not a monoculture but a polyculture system.

It is important to know that we still call it monoculture even if this single crop species is replaced by a different crop in the next growing season. We see this practice being applied on large commercial farms often, when farmers, for example, grow corn on a field for two years in a row and in the third year plant soybean to rotate the crops.

The reason why that is still monoculture farming is that there is only one species of genetically uniform plants present on the field at one time.

Another method of growing monoculture crops is perhaps the one that you would imagine at first when hearing the term. It is a continuous growing of the same crop species on the same land every year without change. This method is also referred to as “monocropping” or continuous monoculture.

But monoculture isn’t connected only with crop cultivation, it is applied even in animal agriculture. Examples of monoculture animal farms are everywhere around us: farms specializing in rearing high milk production dairy cows, broiler chicken farms, sheep farms, pig farms…and other.

Unfortunately, monoculture in animal farming often equals factory farming, which is done at the great expense of animal welfare in exchange for high productivity.


History

Conversion of forests to land used for other purposes has a long history. Earth’s croplands, which cover about 49 million square km (18.9 million square miles), are mostly deforested land. Most present-day croplands receive enough rain and are warm enough to have once supported forests of one kind or another. Only about 1 million square km (390,000 square miles) of cropland are in areas that would have been cool boreal forests, as in Scandinavia and northern Canada. Much of the remainder was once moist subtropical or tropical forest or, in eastern North America, western Europe, and eastern China, temperate forest.

The extent to which forests have become Earth’s grazing lands is much more difficult to assess. Cattle or sheep pastures in North America or Europe are easy to identify, and they support large numbers of animals. At least 2 million square km (772,204 square miles) of such forests have been cleared for grazing lands. Less certain are the humid tropical forests and some drier tropical woodlands that have been cleared for grazing. These often support only very low numbers of domestic grazing animals, but they may still be considered grazing lands by national authorities. Almost half the world is made up of “ drylands”—areas too dry to support large numbers of trees—and most are considered grazing lands. There, goats, sheep, and cattle may harm what few trees are able to grow.

Although most of the areas cleared for crops and grazing represent permanent and continuing deforestation, deforestation can be transient. About half of eastern North America lay deforested in the 1870s, almost all of it having been deforested at least once since European colonization in the early 1600s. Since the 1870s the region’s forest cover has increased, though most of the trees are relatively young. Few places exist in eastern North America that retain stands of uncut old-growth forests.


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