Home & Garden

Silty Soil Suggestions: Amending & Improving Silt

Many of our world’s ancient civilizations were created along the banks of rivers. This is not a coincidence because rivers offered strategic resources. The ancient Chinese had the Yellow River, the Sumerians built along the Tigris and Euphrates rivers, the Indus river gave rise to the ancient Indian civilization and the name “India”, and the ancient Egyptians worshipped the Nile. These people thrived due to their access to abundant drinking water, ample irrigation, efficient transportation, and fertile, silty soil. 

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Rivers experience cyclical flooding which deposits a layer of mineral-rich silt along their banks. Ancient Egyptians called this annual event the “Gift of the Nile” and attributed it to the tears of goddess Isis for her husband Osiris. Early human societies were agrarian and relied on farming. The key to their success was founded on the ability to grow enough crops along these river valleys to eat and make into other products like papyrus. In many ways, humanity as we know it today was built upon silt.

Soil is a precious resource and some say that a garden is just as much about cultivating the soil as it is about growing plants. Of course, you are not expected to feed the world from your home garden, but understanding what silt is and how to work with this soil type can help with your production. 

What Is Silty Soil?

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Silty Soil
Silty soil is fine-grained and can erode easily. Source: wiremoons

Before we get into silty soil, we need to start with what exactly is soil. There is a whole field of science devoted to the study of soil called pedology. Soil is a mixture of mineral particles, organic matter, organisms, gas and water. Scientists have further developed three types of soil textures – sand, silt and clay – depending on the size of the solid mineral particles. Silt particles are between sand and clay particles both in size and physical properties. Soil texture determines how easily the soil can be worked, its ability to retain air and water and how quickly water can flow through it. 

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According to the USDA, silt particles must be between 0.002-0.05 mm, making them smaller than sand but larger than clay. These soil particles are deposited by wind, water, or ice through the erosion of larger rocks. Dry silt feels like baby powder and wet silt has a slippery and soapy texture. It does not clump easily like wet clay or crumble like wet sand.  In the United States, areas along the Mississippi and Missouri rivers have the highest concentration of silt. As glaciers retreated from this region at the end of the last Ice Age, they deposited sediments along their path. 

Soil texture is classified as silt soil if more than 80 percent of its composition is made up of silt. By this definition, pure silt soil is rare for the home garden. Your soil composition will most likely have a mixture of silt, clay and sand particles with varying levels of each. Many gardeners find loamy soil to be the best for growing most flowers and vegetables. Loam is an equal mixture of sand, silt and clay content. This type of soil can retain moisture during dry spells but still have enough drainage for healthy roots.

How To Improve Silt Soils

If you are planting containers or raised beds, there’s less reason to be concerned about silt, sand or clay if you’re buying or mixing your growing medium. However, if you are planting directly in the ground or filling beds with your native soil, you do need to consider soil particles and what you may need to add to improve your soil for plant growth. 

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Begin By Preventing Erosion

Silt runoff clouding water
Silty soil can easily cloud waterways when it’s disturbed. Source: NRCS Oregon

A productive garden starts with healthy and fertile soil. A key indication of soil health is its aggregate stability. Aggregates are soil clumps made from a combination of clay, sand, silt and organic matter. This structure protects microorganisms, allows water to permeate and can hold on to moisture. Silty soils made up of mostly mineral particles with little organic matter tend to be unstable aggregates and experience erosion and compaction. While you may not be able to control factors like rainfall and temperature, you can make smart design and cultivation choices to ensure a well cared for soil environment. 

If you have a sloped property, your soil will be at a greater risk of being washed away after heavy rains. Choose your gardening spot carefully and try to find an area that is level and dry. If that’s not possible, you can try to create small terraces along the slope. You can build these terraces using wood, brick, or stone material but you must make sure the terraces are well secured and have some drainage to prevent waterlogging. 

Using cover crops or green manures is another way to prevent erosion. Cover crops act as physical barriers on top of the soil, protect the soil particles from being carried off by the elements, and anchor the soil down with their roots. Buckwheat and berseem clover are two cover crops that are well suited for silty soil. 

On the other hand, you can accelerate erosion by frequently tilling your soil and physically breaking apart soil aggregates. Even though this type of soil treatment is pervasive in large scale agriculture, you would be better off practicing low tilling or no-till gardening in the home garden. 

Work In Lots Of Organic Material

Since pure silt is eroded mineral particles, it does not contain organic matter. Silt in flooded rivers is mixed with all sorts of naturally occurring organic materials which makes the sediment fertile. Your in-ground soil does not get this natural amendment. This is why it is important to manually condition your soil type with organic materials to create more stable soil aggregates. Your goal is to achieve good soil tilth with proper aeration, water filtration, and nutrition. The addition of soil microbes like beneficial bacteria and fungi will also help your plants take up nutrients more efficiently. 

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There are many sources of organic materials that you can use to condition the soil. For example, fallen leaves or yard trimmings are often free and abundant for the home gardener. You can also use regular plant-based compost, mushroom compost, or composted horse manure or cow manure as organic material. 

Amending your soil can take some time, so be patient with the process. A good rule of thumb is to start with two inches of organic material and work it into 6 inches of existing topsoil. Do not add your amendments when the soil is wet to prevent accidentally compacting your soil.

Mulch Heavily

Hay covered silt
A mulch like hay can reduce erosion and add organic matter to silty soil. Source: prefers salt marsh

Similar to cover crops, mulch is also an effective way to protect your soil from erosion. Furthermore, mulch can help the soil retain more water by preventing excess evaporation and moderate soil temperature. 

Many organic materials that we discussed above can also make great mulch. Leaves or compost will continue to break down over time and add more nutrients back into the soil. Straw and wood chips can also be used as a nice mulch layer. If you want to create a more finished look to your garden, you can apply bark or composted bark mulch. Apply a generous 3-6 inches layer of mulch for optimal results. 

The best time to apply mulch is at the start of the growing season after the ground has fully thawed. Applying mulch too early may slow seed germination and plant growth by keeping the soil too cold or too damp. If you are using mulch to help insulate the soil and keep it warm in the winter, use a thick layer of material like straw so that it’s less likely to get compacted under snow and ice. 

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Fertilizing Silt

Make sure to get a soil test for a baseline understanding of your soil condition. This step is especially important if it’s your first year of planting. Gather soil samples from multiple parts of your garden, mix the samples together, and send it to your local soil testing laboratory. 

Using organic matter like manure and compost will help to fertilize your soil but the nutrients may not be available to your plants immediately. Calculate the square footage of your garden to see how much additional fertilizer you’ll need and closely follow the instructions on the fertilizer’s packaging. More is not always better. You should adhere to the recommended amount for your space. Excess chemical fertilizers can dissolve and enter water runoff, leading to eutrophication of your local watershed. 

Nitrogen (N), phosphorous (P) and potassium (K) are the main nutrients that plants need for healthy growth. However, different crops may have different nutrient requirements so that’s another important factor to keep in mind. Fertilize at the beginning of each planting season and side-dress later in the year if you have heavy feeders. 

How To Water Silt

Silt flowing from hand
Silt is so fine that it can briefly float in the air. Source: stephmotogirl

The type of soil texture you have will determine how often you need to water your plants. Fine soils like silt or clay particles have larger surface areas than sand, rock, or gravel. This allows silt and clay soils to retain more water and have a high water holding capacity. You may need to water silty soils less frequently than sandy soil even if the total amount of water given is the same.

Remember that many watering best practices still apply regardless of soil texture. Drip irrigation is a great way to water without waste. Try not to water during the heat of the day or wet the leaves too much. And as always, use a mulch to preserve soil moisture and prevent evaporation. 

Good Plants For Silt Soil Types

Plants generally prefer silty soil to clay or sandy oil. With the addition of organic materials, you should be able to grow many types of plants in your garden except for root vegetables that specifically prefer loose and sandy soil.  

Vegetables that grow well in clay soil will thrive in silty soil because both soil types can retain moisture. Dry silt will not crack in the same way as clay and cause additional stress to the plants. Shallow-rooted vegetables like lettuce, onion, broccoli, and other related brassicas are good options. Nitrogen-fixing vegetables like peas and legumes are also great to further amend your soil. 

A plot with silty soil would be well suited for a Three Sisters garden with corn, beans, and squash. Corn is a shallow-rooted vegetable and the corn stalks would act as a natural trellis for the beans. The beans would add nitrogen to the soil and the squash would provide groundcover. This type of companion planting was developed by Native Americans and these plants played a central role in their culinary traditions.


About the writer, Huan Song:

Hello there, I’m Huan.

My love for gardening was inevitable having been raised by two grandmothers with epically green thumbs. One was a paleobotanist who had all sorts of weird and cools seeds around the house and the other was a farmer-turned-dumpling-entrepreneur who prized the freshest ingredients.

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I studied environmental science and marketing in college and was miraculously able to combine these interests into a career in nonprofits focused on social justice, food security and sustainability issues. In one of these roles, I drove a big green pickup truck planted with fruits and vegetables to help educate kids on where their food comes from and how to eat healthier.

I currently work at a Big Ten university in science communications and spend my free time gardening, binging gardening videos, and trying to keep my cat Nevis from destroying everything in sight.


The Green Thumbs Behind This Article:


Lorin Nielsen
Lifetime Gardener

This post was first published here

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