watering: when how much
SOIL SPECIALIST, RUTGERS UNIVERSITY
A LAWN OR GARDEN is tricky business. Knowing when and
how much is two-thirds of the job. When depends upon
the soil and the rooting characteristics of the plant
involved, how much will be governed by the amount of
soil that is dried out.
actual inspection of the soil in the root zone of the
plant is the best way to tell when to irrigate. Shallow
rooted plants suffer from drought first because they
cannot exploit a very large volume of soil. Deep-rooted
plants can withstand prolonged dry spells since they
have a large reservoir of soil water to draw on.
plants are probably the most shallow-rooted plants.
Many of these plants never root more than 3 inches deep.
Leafy vegetables probably come next with root systems
from 6 to 10 inches deep. The bean family will use about
a foot of soil, potatoes around 18 inches, sweet corn
probably 2 feet, and such crops as tomatoes and as paragus
at least 3 feet.
these figures are the characteristic rooting depth of
the plant. A soil restriction such as high water table,
hardpan or poor aeration can decrease this depth of
rooting there are a lot of tomato plants growing in
this country with root systems less than a foot deep.
Trees and woody plants. for instance, growing in their
natural location generally have very extensive and deep
root systems, but when transplanted they must frequently
exist on smaller and unnatural root systems.
the soil surrounding the root systems is pumped dry
the plant will, of course, die. The trick is to let
the soil dry down just so far and then completely recharge
it. It is important not to water too often since this
keeps the soil too wet and soggy. A soil that is kept
too wet does not contain enough air and root troubles
can get started quite easily.
can tell the moisture status of a soil by examination.
A soil that breaks easily, with shiny, glistening edges
is near or above its water-holding capacity. A soil
that will hold the cast of your hand after being tightly
squeezed contains ample moisture. A soil that will not
hold the cast is dry enough to be recharged. Soil more
than 2 inches deep will never get dry enough to be dusty.
Plants can die for want of water in soils with as much
as 6 per cent moisture because it is held so tightly
that the roots cannot pry it loose.
soil will dry out first at the surface; then the dryness
will progress down thrqugh the root zone as the dry
weather persists. When working with shallow-rooted crops,
such as lawn grasses and leafy vegetables, examine the
entire root zone. On deeper rooted crops a depth of
9 inches seems to be the most indicative. This is also
true of trees and woody shrubs.
are periods during the life of any plant when shortages
of water seem to be more critical, generally at pollination
and when setting fruit. Many plants can survive very
prolonged dry spells if they have ample water during
also use much more water during July and August than
at any other time because the days are longer and temperatures
higher. A twenty day drought in May or October is equivalent
to only ten days of drought in July and August.
enough water to recharge the entire root system. Here
again the best way to ascertain when this has been accomplished
is to inspect the soil. The same standards hold as when
examining it before watering. In case of water shortages
one may recharge what is estimated to be two-thirds
of the requirement and then pray for rain to finish
the root zone of plants requires rather substantial
amounts of water. Sandy soils hold 1/2 inch of water
per foot of depth, loamy sands 1 inch per foot, and
loams and clays about 2 inches. In other words, a loamy
soil can go four times as long without water but requires
four times as much to recharge it as a sandy soil. It
will take 350 gallons of water to recharge 1,000 square
feet of sandy soil and 1400 gallons for 1,000 square
feet of a loam soil one foot deep.
long does it take to apply this? The only way to tell
is to measure the rate that the equipment being used
will discharge the water and then plan on the approximate
amount of time to cover the area involved. After the
equipment has operated the calculated length of time
examine the soil to see if results have been obtained.
the water in any manner that will give an even distribution
and at rates no faster than the soil will accept it.
Soakers or flood types of irrigation prevent impact
damage to the surface of bare soil such as in gardens;
but a high percentage of the water enters the soil near
the equipment and an even distribution is difficult
to obtain. Remember, too, that there is no lateral movement
of water in the soil-only vertical. Flood types of equipment
also require very level land to acquire distribution.
Flooding can be used when watering one tree or shrub.
or rotating sprinklers are handy and fairly efficient.
There may be some impact damage on bare soil but this
can be prevented with a light mulch. There is no impact
damage on lawns since the turf forms a protective carpet.
Distribution with these systems leaves something to
be desired, as do all irrigating systems. The spray
is definitely affected by wind.
types of hoses have about the same 'advantages and disadvantages.
They too may be affected by air currents, but they do
form a very fine spray which causes little or no damage
to the soil.
of these sprinkling methods are portable and can be
placed in different positions each time they are used,
which helps to some degree to compensate for uneven
distribution. Stationary sprinkler heads do not have
this advantage. They do offer a very nice spray, however,
and are extremely convenient-in fact almost too convenient
since there is a tendency to irrigate too often.
areas where water shortages or restrictions are invoked
you must decide what to water. Grass will run short
of water first but bluegrass that turns brown from dry
weather is only dormant and not dead. It looks bad but
will recover with fall rains. Annual vegetable and flower
gardens can be a complete loss without water but all
in all they are relatively cheap crops and will be replaced
the following spring. Trees and shrubs are expensive
perennials and in some cases cannot be replaced. If
they have good root systems they can withstand severe
dry spells without injury. If they have poor root systems
they will need help.
following is probably the most logical watering preference
in an area of water shortage: (1) trees and shrubs with
poor root systems; (2) gardens; (3) lawns; (4) trees
and shrubs with good root systems.
irrigating, use ample water to recharge the soil, distribute
the water as evenly as possible and be more conscientious
during the critical periods of the various plants. Also,
don't hesitate to irrigate during the heat of the day,
and be sure to consider the condition of the root systems.