Is Silt Good for Farming? Water, Nutrients, Erosion

Yes, silt is good for farming, with two warnings. Silt-sized particles (0.002 to 0.05 mm) sit between sand and clay, and a silt loam holds about 2.40 inches of plant-available water in the top foot, against 0.60 inches for sand (SDSU Extension). But silt soils also wash away more easily than most other textures, and they crust and compact when worked wet. This guide gives the numbers, three charts and a calculator that shows how many days a silt soil can go between irrigations.

Whether you grow corn on Iowa loess, wheat in eastern Washington or vegetables on a river floodplain, the same rules apply: keep the surface covered, keep organic matter up and stay off wet fields.

What silt is and where it comes from

Soil scientists sort mineral particles by size. According to New Mexico State University, silt particles are 0.002 to 0.05 mm across and clay particles are smaller than 0.002 mm. Sand is larger than silt. Texture is the share of each: a lab or a hand test tells you the percentage of sand, silt and clay, and the USDA texture triangle turns that into a class such as silt loam, as NDSU Extension explains.

Silt mostly arrives by water or wind. Rivers lay it down on floodplains and in deltas. Wind lays down a silt called loess. The Iowa Geological Survey describes loess as wind-deposited silt made mostly of quartz grains, and says it is the parent material across broad areas of the nation’s most productive agricultural soils. Loess covers tens of thousands of square miles of the Midwest and also occurs along the lower Mississippi valley, the Platte River valley and the Palouse district of eastern Washington (University of Iowa, Iowa Geological Survey).

The scale can be large. In the Loess Hills of western Iowa the loess is generally more than 60 feet thick, and 150 to 200 feet has been recorded locally. It was blown out of Missouri River floodplains that were clogged with glacial outwash, mostly between about 31,000 and 12,500 years ago, according to the same survey. Deep, even silt gives roots room, but it also means gullies can cut deep and fast once the cover is gone.

In a well-balanced soil, mineral particles take up about 45% of the volume, air and water about 50% and organic matter about 5% (New Mexico State University). Silt shapes the pores in that 50%, and the pores decide how much water the soil can hold and how fast air and roots can move.

Is silt good for farming? The short answer

Silt is good for farming when you manage it, and the table shows why. Each strength has a matching risk, so the practices later in this guide are built around the risks.

Silt is not good everywhere. On flat land with a slow-draining layer below, silt soils can stay wet in spring and be late to plant. On slopes, bare silt can lose the topsoil that holds its value. Where loess is thick and wet, the bank itself can fail, as the Iowa Geological Survey describes for the Loess Hills. A silt loam that sits level, drains and carries a cover is the version people mean when they call silt a good soil.

How much water silt soils hold

This is the main reason silt is prized. Plant-available water is the water between field capacity and wilting point. South Dakota State University Extension reports that intermediate textures such as silt loam generally have the highest plant-available water, because they have a mix of pore sizes, and that more organic matter usually raises it. Sand drains fast and keeps little, while clay holds water so tightly that roots cannot use a large share of it (SDSU Extension).

The chart below uses SDSU’s estimates for soils with 2.5% organic matter. They are averages from many soils, so your field will differ with bulk density, clay type and organic matter. NDSU gives a wider range for loams and silt loams of 2.0 to 2.8 inches per foot (NDSU Extension). For your own field, use the county soil survey or a lab test.

Here is how to read those numbers. Field capacity is the moisture left after gravity has drained the soil, and the water between field capacity and wilting point is what plants can use, according to NDSU. Add the layers down to the bottom of the root zone. NDSU gives an example of fine sandy loam in the first foot (about 2 inches), loamy sand in the second (about 1 inch) and sand in the third (about 0.75 inch), which totals 3.75 inches for a crop rooted 3 feet deep. A uniform silt loam with 2.40 inches per foot would hold 7.2 inches over the same 3 feet, or about twice as much. Roots do not always reach the full depth, and the crop cannot use all of it before it stresses, so the number is a ceiling, not a target.

Chart

Plant-available water in the top foot of soil

Inches of water, estimates for soils with 2.5% organic matter

Silt and nutrients: what the numbers show

Many articles say silt has a large surface area and a high capacity to hold nutrients. The measurements say otherwise. Cation exchange capacity (CEC) is a soil’s ability to hold nutrients such as calcium, magnesium and potassium so they do not wash out. The Soil Quality Knowledge Base in Australia reports that the CEC of the sand and silt size fractions of most soils is negligible; it comes from clay and organic matter instead (Soil Quality Knowledge Base).

A silt loam still scores well, because it also contains some clay and organic matter. An NDSU presentation estimates silt loam at about 15 to 20 meq/100 g, above loam and sandy loam. The practical lesson is to test your own soil: two silt loams can differ a lot if one has more clay and organic matter.

Because nutrient holding comes from clay and organic matter, adding compost, manure or cover-crop residue is a direct way to raise it. Follow soil-test advice and the product label, and check local rules before applying manure or fertilizer.

Chart

Approximate cation exchange capacity by texture

meq/100 g, generalized ranges

Erosion is the weak spot of silt soils

Silt particles are small enough to detach and travel easily, and too fine to clump firmly without organic matter. The soil erodibility factor (K) in the Universal Soil Loss Equation measures this. Ontario’s agriculture ministry lists silt loam at 0.38, against 0.30 for loam, 0.22 for clay, 0.13 for sandy loam and 0.02 for sand; only the very fine sand classes are higher (OMAFA, Universal Soil Loss Equation). Soil with more than 2% organic matter scores slightly lower, 0.37 for silt loam.

Chart

Soil erodibility (K factor) by texture

Higher means easier for rain and runoff to detach and move soil; average organic matter

Loess shows what this means on a landscape. The Iowa Geological Survey says that soil erosion rates in the Loess Hills are very high and that the sediment carried by the streams draining the region is among the highest recorded in the United States. Loess is also vulnerable to collapse when wet. Erosion is not only a farm loss: a 2005 US Environmental Protection Agency fact sheet says the most prevalent source of agricultural water pollution is soil washed off fields, and that farmers can cut erosion and sedimentation by 20 to 90 percent with management practices (US EPA, Protecting Water Quality from Agricultural Runoff).

Organic matter helps here as well. In the Ontario table, silt loam with more than 2% organic matter scores 0.37 instead of 0.41, and every class with more organic matter scores equal or lower. The gain is small on paper, but organic matter also feeds the aggregates that resist raindrops, which is why residue and cover crops sit at the top of the management list.

Crusting and compaction in silt soils

Bare silt soil seals over. Raindrops break the surface aggregates, the fine particles settle into a skin, and seedlings struggle to emerge and water runs off. SARE’s Building Soils for Better Crops calls crusting a symptom of aggregate breakdown that occurs especially with intensively and clean-tilled soils. It says residue covering even 30% of the surface greatly reduces crusting and gives water a path in. A rotary hoe can break a crust as a short-term fix, but the long-term answer is less tillage, surface residue, cover crops and organic matter (SARE).

Compaction is the other risk. Iowa State’s extension specialist notes that loams, clay loams and silt loams are much less affected by wet conditions than silty clay and clay, but that compaction potential is highest when soil is just dry enough to work. He suggests a ribbon test: if a ribbon of soil breaks within 1 to 2 inches, compaction risk is lower; if it stretches 4 or 5 inches, wait (Iowa State University Extension). Alberta’s agriculture department adds that compaction can increase soil erosion and decrease crop production (Alberta Agriculture).

How to manage silt soils

• Keep the surface covered. Leave crop residue, use cover crops and cut tillage passes. This addresses both crusting and erosion.
• Add organic matter. Compost, manure and cover crops improve aggregate stability and raise water and nutrient holding.
• Stay off wet soil. Use the ribbon test above, run lighter equipment and limit passes to the ones you need.
• Test the soil. A soil test shows pH, nutrients and organic matter. Follow the lab advice, and follow the product label and local rules for lime, fertilizer or manure.
• Manage drainage. Silt soils on flat land or over a dense layer can stay wet, so maintain ditches or tile where they are installed.
• Protect slopes. On sloping silt soil use contour planting, terraces, grass waterways or buffer strips to slow runoff.
• Rotate crops. Mix deep-rooted and shallow-rooted crops to keep roots working at different depths.

▶ Seven simple ways to protect soil and water, from cover crops to less tillage. · 4:34

1. Before planting: take a soil sample, check the ribbon test and wait if the soil is plastic.
2. At planting: use the lightest equipment that does the job and keep seed-furrow sidewalls from smearing in wet soil, as Iowa State advises.
3. After heavy rain: look for crusts and ponding. Break a crust only if seedlings cannot get through, and note where water stood.
4. After harvest: leave residue or sow a cover crop so the surface is not bare through winter and spring rain.

Mixed farming also feeds the soil: animal manure and crop leftovers are routes to more organic matter, as the video below explains.

Silt, silt loam and other textures compared

The table puts the three numbers from this guide side by side, each from a different published source. Values are guides, not measurements for your field.

Sources: SDSU Extension (water, 2.5% organic matter), Ontario OMAFA (K factor, average organic matter), NDSU presentation and Soil Quality Knowledge Base (CEC).

▶ Mixed farming: manure as fertilizer and crop leftovers as feed, two routes to more organic matter. · 2:55

How often will your silt soil need water?

Water storage decides how long a crop can go between irrigations or dry spells. The calculator below turns your root depth, your soil’s available water and your crop’s daily water use into a number of days. It also shows the same root zone on a sandy soil, using SDSU’s 0.60 inches per foot.

Try the numbers for your own farm below.

Enter four numbers. Root depth is how deep the crop actually draws water, which may be less than the soil depth. Available water per foot comes from your soil survey, a lab report or the chart above. Allowed depletion is the share of that water you let the crop use before you irrigate or accept stress; the 50% shown is an example, not a rule. Daily crop water use depends on the crop, growth stage and weather, so use a local figure from your extension service or weather station. The result is a planning guide. Check it against soil moisture sensors or the feel of the soil in the root zone.

Checking silt fields from space

Satellites cannot measure soil texture, but they do show where crops grow unevenly. Farmonaut Satellite + AI Farm Monitoring System draws vegetation index maps for fields you outline, so you can see which parts of a field run ahead or behind after a dry spell. Patches that fade first on a silt field are good places to take a soil sample, check for compaction or look at drainage. Maps point you to where to look; the cause needs a spade, a soil test or an agronomist. A simple routine is to compare the same field on several dates after rain and after a dry week. If the same strip fades first each time, dig there, check for a compacted layer, and compare the soil texture with the rest of the field.

Related pages on our site: large-farm admin app, carbon footprint tracking, crop loan and insurance verification, Farmonaut API, satellite and weather API documentation, farm machinery fleet tracking, product traceability.

▶ How satellite NDVI maps show crop stress before you can see it on the ground. · 4:09

How we checked the numbers

Every figure above comes from a page we opened: SDSU Extension for plant-available water, the Ontario agriculture ministry for erodibility, an NDSU presentation and the Soil Quality Knowledge Base for CEC, and the Iowa Geological Survey, SARE, Iowa State, Alberta Agriculture and the US EPA for the loess, crusting, compaction and runoff points. We took the particle sizes from New Mexico State University. Values differ between institutions because the soils and methods differ, so we show ranges where a source gives one. Unsourced percentages that appeared in an earlier version of this page, such as claimed gains in nutrient retention and root growth, have been removed.

FAQ

Is silt good for farming?

Yes, with care. Silt loam holds about 2.40 inches of plant-available water in the top foot (SDSU Extension) and is easy to work. The risks are erosion, crusting and compaction, which cover, organic matter and staying off wet soil reduce.

Why is silt important in soil?

Silt sets much of the pore structure that controls water and air. It is also the main building block of loess, which the Iowa Geological Survey says is the parent material across broad areas of the nation’s most productive agricultural soils.

Does silt hold water well?

Silt and silt loam hold more plant-available water than sand and more than clay, because clay keeps much of its water too tightly for roots (SDSU Extension). Water that arrives faster than the soil can take it in will still run off, especially on crusted ground.

Where is silt commonly found?

On river floodplains and deltas, and in wind-blown loess. In the United States loess is extensive in western Iowa, along the lower Mississippi valley, the Platte River valley and the Palouse of eastern Washington. It also occurs in central Europe and northern China (Iowa Geological Survey).

What is the difference between silt and silt loam?

Silt describes a particle size. Silt loam is a soil texture class in which silt is the leading fraction, mixed with some sand and clay (USDA texture triangle, as explained by NDSU).

What does silting mean?

Silting is the build-up of fine soil particles in streams, ponds and ditches. The EPA says soil washed off fields clouds water, cuts the sunlight reaching aquatic plants and can clog waterways. Keeping fields covered reduces it.