Soil particle size analysis
You may wonder, if it rains so
much in Olympia, is it really necessary to water the roses?
Unfortunately, yes. Our summers are very dry, and our soil is
charitably described as 'excessively well drained'. That means the
soil dries out fast after the winter rains stop. Roses need about
1" of water a week, but we don't get anywhere near that much during the
growing season - May through September.
Some roses can go unwatered. Wild roses won't need watering.
Once-blooming roses won't need much water. Very large, old,
well-established roses can go a long time between waterings. Roses
that you don't care much about can go without watering. But for
most roses, if you want them to be healthy and to produce flowers,
you'll have to water at least once a week in the summer.
Different soil types have different water holding abilities. Soil
type refers to the particle sizes of soil, which determine the pore
sizes, which determine the water holding capacity and drainage. Soil
structure also affects water holding capacity. Structure means how the
particles stick together - grains, crumbs, blocks and hardpans. Loose
structure, called friable, like grains, crumbs and small blocks absorb
water easily while larger blocks and hardpans might not absorb water at
all, acting like rock or concrete and not like soil.
When you water your garden, youíre not watering your plants. What
youíre actually doing is refilling the soil water. Soil isnít just what
you stick the roots into. Soil is a living thing that acts like a sponge
or a reservoir, for water and nutrients for plants. When it rains, or
when you water the garden, water fills the pore spaces in the soil. Once
the pore spaces are full the extra water runs off or drains away. When
it first starts raining in fall, the soil is very dry so most all the
rain soaks in. Over winter the soil fills up with water, and normally by
spring the soil is at full capacity and then some, ready for plants to
start growing and pulling up water for their own use. As plants pull out
more water and rainfall decreases, the soil slowly dries out until we
have to start watering our roses, usually some time in May or June. Then
we continue to water them until they slow down growth as temperatures
cool in September.
There are three basic size categories of soil particles: sand, silt
and clay. Of course here we have a fourth category, rocks and gravel but
those arenít technically soil. And organic matter is critical to soil
health and water holding capacity but itís not included as a soil
particle. Most soils are a blend of all three particle sizes and the
ratio determines the soil type: loam, sandy loam, silty clay, whatever.
Particles leave spaces between them. These are the soil pores.
Sandy soils with large particles also have large pores. Clay soils with
very small particles also have very small pores. Large pores drain fast
and donít hold water, but let lots of air into the soil. Small pores
drain slower and hold water longer, but let in less air. Most soils
average about 50% pore space, so thatís the volume thatís available for
holding water and air.
Sandy and gravelly soils drain fast, absorb fast and donít hold much
water. They need to be watered more often but for less time as water
absorbs fast and travels through the large pores quickly. Water also
tends to travel straight down in sandy soils, not spread out. So if
youíre watering with a drip or a hose, you need to make sure you water
all around the plant or only the roots directly under the hose will get
Clay soils drain slowly, absorb slowly and hold water well. They need to
be watered less often, slower and for longer than sandy soils. Water
takes longer to soak in, but once itís in it stays in, and the small
pores hold a lot of water a long time. Water spreads out in clay soils
too so you donít need drip emitters as close together as in sandy soils.
Loam soils are those with a balance of sand, silt and clay. Theyíre the
best soils for gardening in because they balance drainage, infiltration
and water/air holding capacity. Clay is necessary to hold water, and
sand is necessary to allow in air.
So, in order to know how far apart to put emitters, and how often to
water for how long, you need to know what kind of soil you have. Hereís
how to find out.
Soil Particle Size Analysis - The mason jar test Fill up a quart size mason jar half way with
soil from the garden. You can remove sticks and leaves and larger rocks
first. Then fill the jar with water to the top and shake it up so all
the dirt is shaken around in the water. Set the jar down and grab a
ruler. At one minute the sand has settled out - measure it and write it
down. Allow 6 hours for the silt to settle out, measure and record.
Allow 3 days for the clay to settle out, measure and record. Whatís left
floating is the organic matter, which doesnít get measured even though
you want lots of it. Now divide each measurement by the total inches of
soil for the ratio of sand to silt to clay, and use the soil chart below
to find out what youíve got.
Letís assume weíre working with sandy loam soil. It will drain and
absorb pretty fast and the water wonít spread out far. WSU recommends
watering shrubs like roses enough to saturate the top 12-18 inches of
soil, then allowing the top few inches to dry. Sandy soils will
need 1 inch of irrigation or rain to soak that top 12 inches of soil,
and loams will need 2".
How long to water? If youíre using a spray like a lawn sprinkler, do the
tuna fish can test to find out how long it takes for your sprinkler to
put out 1" of water.
If youíre using a drip system or a hose end, itís easier to measure by
gallons over time than by inches. One inch of water roughly equals
one half gallon per square foot of garden, according to the ARS website,
and roses need about 5 gallons of water a week in summer. Your drip
emitters will be rated by gallons per hour so thatís easy to calculate.
Your hose end you will have to measure by timing how long it takes to
fill a one gallon bucket. If youíre using a soaker hose youíll have to
guesstimate how open the faucet is when the soaker is running full
blast, and see how long that takes to fill that one gallon bucket.
Your average hybrid tea rose takes up around 2 square feet of garden
space, up to 4 sqft for large shrubs or climbers; so, a smallish hybrid
tea in sandy loam soil will need about 1.5 gallons of water at a time,
and about 5 gallons a week in warm dry weather (June, July and August).
So, water 2 to 4 times a week, 1.5-2 gallons at a time, depending on
In loamy soil add twice as much water at a time, half as often, to get
the 5 gallon/week total. A larger rose will need more water, minis and
polyanthas less. And of course if it rains, you can deduct the
amount of rain from the one inch needed by the roses.
Roses can be watered by hand, by hose-end sprinkler, by drip system
or soaker hose, or by fixed-spray lawn-type sprinklers. Each has
its advantages and disadvantages.
Watering by hand is the cheapest and simplest way to water your
roses, and can be best when you have just a few plants. You get an
opportunity to spend some quality time with your roses when watering by
hand. But, it's boring - so boring that it's unlikely your roses
will get enough water, as you'll give up and go away before they're
Hose-end sprinklers are the next cheapest and simplest. Just
attach the sprinkler to the hose, set it in the garden, adjust the
location and the water pressure, and let it run. Use a
battery-powered faucet timer to shut the water off. The
disadvantage is it's not very precise, and foliage can block the water
spray. Watering the foliage can increase blackspot, but it also
Soaker hoses are very simple too. Lay the hose out in the
garden, attach the hose coming from the faucet, and let it run.
The disadvantage is that in sandy soils, the water doesn't travel very
far at all from the soaker hose - just a few inches at most, so you need
to just about cover the whole garden with hose going back and forth to
get good water coverage. The advantage is that you don't get the
foliage wet, so less blackspot, and you don't water the sidewalk trying
to cover the whole garden.
Drip systems take some time and some thought to lay out, but are
wonderful once installed. They're not terribly expensive either.
Drip components that spread water over a wide area are the best in sandy
soils - laser tubing for instance, like a miniature soaker hose; or
shrub bubblers that spray out small streams of water in a circle or
semicircle. Make a ring around each rose with the laser tubing, or
use 3 or more bubblers or emitters around each bush. You will want
to buy all your parts from the same brand to avoid getting parts that
should, but don't quite, fit together. Also, compression fittings
take some hand strength to put together. If you have arthritis or
tendonitis, look into fittings that twist together instead, like
DripWorks' Easy Loc?fittings. Drip systems can be easily
automated with either battery-operated faucet timers, or connected to a
lawn sprinkler timer that runs off of household current.
If you have an underground irrigation system, most brands include
shrub spray heads that will work well for watering roses. Make
sure your roses are on their own circuit however, not on the same
circuit as the lawn.
DripWorks is a great
source of drip irrigation expertise and supplies, highly recommended.
Their Easy Loc?fittings are wonderful, and can be reused when you want
to make changes to your drip system. They also carry fertilizer
injectors that work with irrigation systems.