Which Soil Type Holds Water? (Plus 4 Ways To Retain Water In Soil)


If you are having a tough time keeping the plants in your garden from wilting, don’t worry.  We’ve all been there – but there are ways to make things a little easier on you (and on your water meter!)

So, which soil type holds water?  Clay soil holds the most water, since clay particles are smaller than silt or sand particles. This gives clay soil more surface area, more pores, and thus more ability to hold water.  Clay also drains slowly, since it holds water more tightly in its tiny pores.

Of course, since clay soil particles are so small, there is less room for air between particles.  This can lead to a lack of air in clay soil, especially when this soil type is saturated with water.

However, even if you don’t have clay soil, there are still lots of ways to help your soil retain water.

In this article, we’ll talk about clay soil and why it retains so much water.  We’ll also take a look at some ways to prevent evaporation and keep more water in your soil for a longer time.

Let’s get started.

Which Soil Type Holds The Most Water?

Clay soil holds the most water of all three types (clay, silt, and sand).  This is true for several different reasons:

  • Clay particles are smaller than silt or sand particles (1/10 or less the size of silt and 1/1000 or less the size of sand). This smaller size gives clay soil more surface area – and thus more places for water to “stick” to the clay particles.
  • Clay soil has more pores than silt or sand particles.  Although the pore space* between 2 clay particles is smaller than that between sand particles, the total pore space is larger for clay than for sand.  This leaves more space for water between clay particles.
  • Clay particles hold water more tightly than silt or sand particles.  Due to the smaller pore size in clay, there is more negative pressure in the capillary tubes of clay particles. This allows clay to retain water better than other soil types.

*Remember that pore space is the space between soil particles.

The average soil is made up of the following percentages:

  • 45% to 49.5% mineral (for example, rock pulverized into tiny pieces)
  • 0.5% to 5% organic (for example, rotted leaves, grass, or plants)
  • 25% air (mainly nitrogen and oxygen)
  • 25% water
soil composition
Soil is typically 45 to 49.5% mineral, 0.5 to 5% organic matter, 25% water, and 25% air.

In other words, half of soil (about 50%) is not really soil at all, but rather, the space between soil particles.  Of this space, about half is air and the other half is water.

Clay soil is best for dry areas where it is difficult to keep plants watered. Sandy soil is best in rainy areas for plants that do not like wet soil.

Soil
Type
Info
&
Uses
ClaySmallest particles. Retains
the most water. Drains
slowly. Best for plants
that need wet soil.
SiltMedium particles. Retains
some water. Drains
moderately. Best for
plants that need neither
wet nor dry soil.
SandLargest particles. Retains
the least water. Drains
fast. Best for plants that
do not like wet soil.
This table summarizes three basic soil
types (clay, silt, and sand), along with
water retention, drainage, and uses.

Below, you can see pictures of various types of soil. Here is clay:

illite clay
Clay has the smallest particle size and the smallest pore size of the soil types, but it has the most surface area and holds the most water. It is also slow to drain.

Below is silt:

silt soil
Silt has a particle size that is somewhere between clay and sand. Its water retention ability is also somewhere between clay and sand.

Here is sandy soil:

sandy soil
Sandy soil is gritty and coarse, with the largest particle size of the soil types. It does not retain water well, and drains quickly.

Which Soil Type Holds The Least Water?

Sandy soil holds the least water of the three types.  This is true for several different reasons (basically the opposite of why clay holds the most water):

  • Sand particles are larger than silt or clay particles (10 or more times the size of silt and 1000 or more times the size of clay). This larger size gives sandy soil less surface area – and thus less places for water to “stick” to the sand particles.
  • Sandy soil has fewer pores than silt or clay particles.  Although the pore space* between 2 sand particles is larger than that between clay or silt particles, the total pore space is smaller for sand than for clay.  This leaves less space for water between sand particles.
  • Sand particles hold water less tightly than silt or clay particles.  Due to the larger pore size in sand, there is lower negative pressure in the capillary tubes of sand particles. This allows sand to drain faster than other soil types.
sandy soil
Sandy soil has less surface area than the same amount of clay or silt soil, and so it holds less water.

Sandy soil isn’t necessarily a bad thing.  If you have plants that don’t like “wet feet” (soggy soil), then sandy soil is your best bet.

Also, if you get frequent heavy rain, then sandy soil will help you to avoid over watered plants.

rain
Sandy soil isn’t all bad – it drains quickly after a heavy rain so your plants are less likely to get root rot.

(You can learn more about over watering plants – and how to avoid it – here).

How To Retain Water In Soil (How To Keep Soil Wet)

If you don’t have clay soil, you might be looking for ways to keep more water in the soil.  Even if you do have clay soil, that alone might not be enough to retain enough water for your plants.

clay soil cracked
Clay soil alone might not be enough to retain water for your plants.

Here are a few ways to help you retain extra water in your soil to get your plants through those hot and dry summer days:

  • Add organic material to your soil – this basically means using well-rotted compost or aged manure (which also adds nutrients to soil).
  • Water deeply but infrequently – this will allow the water to really soak down deep into the soil.  Not to mention, it encourages stronger, deeper roots systems in plants, which helps them survive drought.
  • Put a layer of mulch over your soil – this will prevent evaporation by the sun and air, in addition to preventing wind erosion of soil if it does dry out.
  • Shade your soil – this will also prevent evaporation by the sun.  You can do this with a cover (such as plant netting, row covers, or shade cloth) or with plants (such as groundcover plants or plants with leaves that will not outgrow your crops).
compost bin
Adding compost to your soil is just one way to improve water retention.

Let’s take a closer look at each of these methods, starting with organic material.

Add Organic Material To Your Soil

Adding organic material to your soil helps to increase water retention (and also adds nutrients!)  According to the University of Nebraska-Lincoln, organic matter helps to form soil aggregates, which in turn help to hold water in soil.

According to the University of Florida Extension, soil with plenty of organic material acts like a sponge, soaking up water from rainfall and releasing it to plants when needed during dry spells.

Compost
Adding organic matter to soil helps to form soil aggregates, which in turn helps with water retention.

Well-rotted compost is one good source of organic material.  You can make your own compost from yard and kitchen waste, such as:

  • Grass clippings
  • Fallen leaves
  • Pulled weeds
  • Fruit and vegetable scraps
  • Coffee grounds
cutting grass
Grass clippings are just one ingredient you can use to make your own compost at home.

You can learn more about how to make your own compost here.

Aged manure is another good source of organic material.  You can source manure from anywhere near you that raises livestock, including:

  • Chickens
  • Cows
  • Horses
chickens
Chicken manure is one source of organic material for your garden, but make sure it is aged first!

It is true that compost and manure add organic material that helps to retain water in soil.  However, they also add important nutrients (including nitrogen, phosphorus, and potassium, or NPK) in forms that plants can use.

Water Deeply & Infrequently

The depth and frequency of your watering will determine how long the soil stays wet.  Watering very shallowly twice a day, every day, may still lead to most of the water evaporating.

watering can
Watering too often and too shallowly can lead to most of the water evaporating (not to mention weak, shallow plant roots).

This is because a shallow watering will not allow water to soak deep down into the soil.  You are better off with less frequent but deeper watering.

That way, more of the water will stay in the soil (it becomes more difficult for the sun and air to evaporate water once it gets further down into the soil).

You are also better off watering early in the day (when it is cooler and the sun is less intense).  This will also lead to less evaporation of the water you used for irrigation, saving you time and effort.

sunlight
Water early in the day, when it is cooler and the sun is lower in the sky, to avoid evaporation.

In addition, deep and infrequent watering encourages deeper and stronger root systems in plants.  When there is always a little water at the soil surface every day, plants respond by developing shallow roots at the soil surface.

When water arrives less frequently and goes deeper into the soil, plants respond by forming deeper root systems to access water below the surface.

drip irrigation emitter
Drip irrigation is one way to conserve water and give plants water where they need it (the root zone).

You can learn more about time-saving ways to water a garden (and when to do it) here.

Put A Layer Of Mulch Over Your Soil

Putting a layer of mulch over your soil is another way to help retain moisture in the garden.  The idea is that the water in the soil (below the mulch) will be trapped and less likely to evaporate in the sun and air.

red cedar mulch
A layer of mulch over soil will help to retain water – and there are lots of things you can use as mulch.

Depending on what you have available, you could use the following as mulch:

  • Well-rotted compost or aged manure
  • Grass clippings (avoid grass from lawns treated with pesticides or herbicides – this “killer compost” can hurt plants or bees!)
  • Straw (hay has seeds that can grow into weeds, so be careful and learn the difference)
  • Pine needles (also called pine straw)
  • Newspaper
  • Cardboard
  • Wood chips
  • Sawdust (leave it on the top – don’t mix it into the soil, or the carbon in the wood will tie up nitrogen as it decomposes)
  • Plastic tarp (this can insulate the soil quite a bit and trap heat via the greenhouse effect, so be careful!)
straw
Straw is another type of mulch you can use to cover the soil in your garden (it’s great for potatoes!)

When applying mulch, use only 2 inches or so in most cases.  Avoid piling it up around the base of plants, which can cause damage or disease (there are some exceptions, such as tomatoes).

Remember that too much mulch can kill plants – you can learn more here.

Shade Your Soil

Finally, giving your soil some shade can really help with water retention on the hottest, driest days of summer.  When less sun reaches the soil, it doesn’t heat up as much, and less water will evaporate.

green shade cloth
Putting shade cloth over plants will keep the soil cooler and wetter.
Image from:
Wikimedia Commons,
courtesy of user:
T. R. Shankar Raman: https://commons.
wikimedia.org/wiki/File:RJDRP_Nursery_
DSC0282.jpg

One good way to shade your soil is with a covering, such as:

  • Plant netting – this is used to keep garden pests at bay, but it can also provide shade.  The size of the openings in the netting can vary, as can the capacity to provide shade and the material it is made of.
  • Row Covers – this is a lightweight sheet of fabric that is used to protect plants from cold, wind, and pests.  However, it also provides some shade, depending on the thickness and type.  Row covers are often made of polyethylene, polyester, or polypropylene.
  • Shade cloth – this is a knitted or woven material that filters out sunlight and protects plants from sunburn.  It can reduce temperature by 10 degrees Fahrenheit or more.  Shade cloth can block as little as 20% of light up to as much as 90% of light.
row cover
Row covers protect plants from cold, wind, and pests, but they can also provide some shade and help with water retention. You can also water plants through a row cover.

One drawback of these covers is that they can prevent some of your plants from getting the sunlight that they need.  Lettuce and spinach might be fine, since they tolerate shade, but tomatoes, potatoes, etc. might not do as well under shade cloth.

Another way to shade the soil for sun-loving plants is to use other plants!  Find some groundcover or low-growing plants with shady leaves to cover the soil near your tomatoes, potatoes, and other tall plants.

green lettuce
Plant some lettuce near your tomatoes. The lettuce will shade the soil, helping to retain water, without taking sunlight away from the tomato plant.

The leaves of these shorter plants will shade the soil and keep it cool to prevent evaporation, retaining some moisture.  Since these groundcover or low-growing plants are not too tall, they shouldn’t block out any light from your crops (like tomatoes, potatoes, etc.)

According to the University of Michigan Extension, cover crops (such as alfalfa) will also help to retain water in soil.

You can learn more about cover crops (green manure) for your garden (and why they are useful) here.

Conclusion

Now you know that clay soil holds the most water and drains slowest, while sandy soil holds the least water and drains fastest.  You also have some ways to keep your garden soil wet when the hottest dry days of summer approach.

I hope you found this article helpful.  If so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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