When it comes to growing tomatoes,
the single most important factor that affects yield and fruit quality
is irrigation. Growers may have a certain margin for error when applying
pesticides or fertilizers, but in improperly irrigated fields, yield
losses can reach up to 25% of the entire crop, said Tim Hartz, an
extension specialist with the University of California at Davis. “Improper
irrigation is theNo. 1 cause of crop yield losses in tomatoes,”
Hartz said. Every year, tomato growers perform a difficult tightrope
walk act by trying to balance their watering regiments so that their
plants get enough, but not too much water, Hartz said. Growers don’t
want to over water their plants because of the risk of exposing them
to phytophthora root rot. But, if the plants don’t get enough
water, the plants will become stressed and under produce. While this
may sound simple, it’s not always easy to tell how much water
plants are getting, Hartz said. “Growers assume that when they
put water on the fields, that the water is going to go down a couple
of feet. But if they go and probe their fields, sometimes they’ll
find that the water didn’t get very far down because of the
soil structure. And even though the grower’s irrigations schedule
is correct, the plants aren’t getting enough water,” Hartz
said. When the growers irrigate, they need to take a number of factors
into account as well as the plants’ basic water requirements,
Hartz said. The most important factors are soil type and how well
different soil types absorb water.
Growers tend to assume that water will penetrate sandier soils more
easily that clay soils. But what determines water penetration has
more to do with how much organic matter the soil has and the amount
of magnesium and calcium in the soil. Soils low in organic matter,
or those that have more magnesium than calcium in the top layers of
soil, can have a dificult time with water intake. Water tends to well
up at the surface and create a seal, making it difficult for further
water applications to penetrate the soil.
In order to make sure that his soil gets good water penetration, Jeff
Dolan, field manager for the DiMare Co., Newman, Calif., spoon feeds
his tomato plants with calium every time he irrigates. The trick to
good irrigation practices, aside from making sure there’s enough
calcium in the top layers of the soil, Dolan said, is to check the
fields regularly by hand to see if they need water.
“We have machines that monitor soil moisture. But the problem
with them is that they’re just monitoring one spot in the field.
You need to check different areas of the field, including the head
lands, the drain ends, and the middle,” Dolan said.
Dolan checks most of the fields by hand each day to monitor soil moisture.
By keeping tabs on the fields’ irrigation needs, it’s
easier to avoid one of the most common pitfalls: waiting too long
between irrigations, Dolan said.
“When they wait too long between irrigations, they also tend
to over irrigate,” Dolan said.
Too much water at once can smother the plants by cutting off the roots’
oxygen supply, Dolan said. Although plants that sustain damage to
their primary roots earlier in the season may grow secondary roots
to compensate for their inital losses, the plants never quite recover
fully enough to be able to produce optimum yields, Dolan said. While
growers need to be careful to avoid over irrigating their plants in
the mid- to late season, it’s crucial for them to remember to
give their plants plenty of water in the early season to avoid stressing
the plants when they are setting fruit.
Although furrow irrigation is a common practice among some growers
in Fresno County, Dolan uses sprinkler irrigation in the beginning
of the season.
When the transplants are small and the water has to move 30 inches
across the furrows to reach the plants the plants aren’t getting
enough water fast enough, Dolan said.
“We can get the same amount of water to the plants with sprinkler
irrigation in six to 12 hours as we can in 48 hours with furrow irrigation,”
Dolan said.
Once the plants have taken off and are actively growing, it becomes
more diffcult to irrigate sufficiently without over irrigating, Hartz
said.
