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Watering - the Ideal Soil Water Relationship

 Watering—The Ideal Soil/Water Relationship  

Water is an essential ingredient in all plant growth, for almost every botanical process takes place in its presence.  Water is necessary in plants’ food-manufacturing process (photosynthesis), is the main constituent of living cells, and abounds in young plant tissues.  It keeps stems and leaves stiff.  And water is the main ingredient in most of the vegetables we eat, comprising, for example, 91 percent of asparagus, 87 percent of beets, 95 percent of cucumbers, and 94 percent of tomatoes.

You might think that all you have to do in order to grow sumptuous vegetables is pour on the water.  Unfortunately, Mother Nature doesn’t work that way.  Too much water is just as harmful as too little.

Water in the soil carries dissolved nutrients that are absorbed by plants’ roots.  Soil air provides a constant supply of oxygen while carrying off carbon dioxide.  An ideal soil for plant growth generally contains 50 percent solid matter and 50 percent pore space (the space that allows absorption of liquid).  Moisture should occupy about half of the pore space. 

When you swamp the pore space (complete fullness is called field capacity), you cut off the oxygen supply and stop root growth.  The longer the air is blocked, the greater the damage.  Once the roots are damaged, organisms that cause rot enter; root rot frequently sets in.

When you water so much and/or so often that you keep more than 50 percent of the pore space filled yet don’t keep the soil quite saturated enough to cut off the plant’s oxygen, you generally create lush leaf growth at the expense of fruit development.  This happens to tomatoes, eggplant, and peppers that are only slightly overwater.  The symptoms are lots of green leaves but little fruit.

On the other hand when water occupies less than 50 percent of the pore space, the plant must work harder to grow.  Water stress affects vegetable plants in different ways.  A cucumber stops growing stops growing; a tomato ripens all its fruit; muskmelons lose their sweetness during the ripening period.  Once a vegetable experiences water stress, it can’t be salvaged.  The first symptom is wilting leaves.

Yet another cause of problems is an uneven water supply.  When the soil dries out completely; is watered, dries out again, and is watered, the plants suffer.  In fact, damage can occur if this happens only once.  Carrots can crack, cabbage heads can split.

Critical Watering Periods 

Vegetable                                          Critical Period 

Asparagus                                           spear development

Beans                                                  pollination/pod development

Broccoli                                               head development

Cabbage                                             head development

Carrots                                                root enlargement

Cauliflower                                          head development

Corn                                                    tassel/ear development

Cucumbers                                          flower/fruit development

Eggplant                                              flower/fruit development

Lettuce                                                head development

Melons                                                flower/fruit development

Onions                                                 bulb development

Pumpkins                                            flower/fruit development

Squash                                                flower/fruit development

Tomatoes                                            flower/fruit development


How Much Water? 

To keep your vegetables growing at their fastest rate, water until your particular soil type is filled to field capacity in the main root zone (about 1 foot deep for most vegetables, about 2 feet deep for corn, tomatoes, and a few other large vegetables).  Don’t water again until most of the available water has been used.

The amount of water your plants use depends on temperature, wind, rainfall, and individual needs.  Sandy soil holds less water than clay, yet clay holds water so tightly that some of it is unavailable for plant use.  Loam makes adequate amounts of water readily available to your vegetables.

If you know what kind of soil you have (See Blog Do You Know Your Garden Soil), you can estimate roughly when your soil is saturated, or has reached field capacity. 

Soil Water-Holding Capacity 

(Inches of water needed per foot of soil depth) 


Sandy                                      8

Loam                                       1.0

Clay                                         1.5


Water Use Guide    

Season                            Warm Weather       Cool Weather 

                                        (80° F and above)  (Below 80° F) 

Summer                                   .25-.35                         .15-.2

Spring/Fall                               .1-.2                             .1-.15

A fail-safe watering method that maintains the correct degree of soil moisture is to water deeply for two to four hours, and then wait until the soil dries out to a depth of 4-8 inches before watering again.  Check this with a trowel.

Mulching Means Moisture 

Gardeners can also use mulch to ensure adequate soil moisture for vegetable plants.  A mulch is any material spread on the garden bed that shades the ground completely, reduces soil evaporation, and helps retain moisture.  Many organic materials will make a good mulch; you can try wood chips, sawdust, grass clippings, horse manure, or compost.

Place organic mulches around your plants’ stems, and cover the ground completely.  Since organic material cools the soil, mulch in late spring after the soil temperature has warmed to at least 65° F.  The best time to spread organic mulch is immediately after a rain, as it keeps the moisture in the soil for an extended period of time.

Water has such importance in the garden that it is worth repeating the fail-safe approach mention above:  Water deeply for two to four hours, and then don’t water again until the soil dries to a depth of 4-8 inches.  If you follow this rule and garden in well-drained soil, you will probably be able to deal with conditions ranging from drought to several days of rain.

© Copyright by Karen Newcomb