.t / AGRICULTURAL RESEARCH CENTER
M IFAS, University of Florida
/P C4. Inmmokalee, Florida
Immokalee ARC Research Report SF73-2 September, 1973
FACTORS AFFECTING SOLUBLE SALT INJURY TO .
TOMATOES GROWN WITH FULL-BED MULCH CULTURE
P. H. Everett
With seep-irrigation, high concentrations of soluble fertilizer salts can ,
accumulate at the soil surface in the planting holes of mulched beds. This 'can-baur e
a serious problem in obtaining a full and uniform plant stand. Seedling survival of
both transplanted and field-seeded tomatoes can be reduced by excessive salt. .How-
ever, field-seeded tomatoes are more affected than transplants because germinating
seeds and very young seedlings are more susceptible to salt injury than are 4-6 week
old transplants. Full-bed mulch, while not creating the problem, does accentuate
it to the degree that during hot and dry weather plant stand can be reduced by 40-
50% for direct-seeded and 20-25% for transplanted tomatoes; compared to 0-5% for
either planting method in non-mulched beds.
There are two primary reasons why seedling damage from high soluble salts may
be more severe with full-bed mulch culture than with non-mulched beds. Applying
all of the fertilizer in a single application (mulched beds) rather than by split
application (non-mulched beds) creates a greater potential for high concentrations
of salt in the bed. Secondly, the only area for water to evaporate from mulched
beds is through the planting hole cut in the mulch. As water evaporates, soluble
salts accumulate at or near the exposed soil surface of the planting hole. The
danger from excessive salts at the soil surface is greatest during hot and dry
weather when evaporation is rapid and when water is being supplied by seep-irrigation.
Both act to move soluble salts to the bed surface and into the planting hole near
the seeds or seedlings. Rainfall or overhead irrigation reverses this effect by
moving the salts down and away from the seeds or seedlings, and at the same time
creates a better moisture condition in the soil surface. For mulched beds with seep-
irrigation, a constant water table should be maintained approximately 15 inches
below the bed surface. A fluctuating water table results in conditions that are
either too wet or too dry. Dry soil under full-bed mulch is difficult to fewet and
will contribute to the salt problem.
Symptoms of Salt Injury
Salt injury is usually thought of as a root and/or a leaf burn. This is commonly
referred to as "fertilizer-burn" and results from high salt concentration in and
around the plant roots. During dry weather, with full-bed mulch and seep-irrigation,
the zone of highest salt concentration is in the first 1/4-1/2 inch below the soil
surface. Most of the tomato roots are below this zone of salt accumulation, there-
fore "fertilizer-burn" in the more normal sense is seldom observed on established
plants in mulched beds. However, seeds and stems can be in close contact with this
zone of high salt and are susceptible to salt injury.
Field-seeded. Crops that are seeded directly into mulched beds, with or without
* plug-mix, are very susceptible to salt injury. Plug-mix gives some protection, but
under adverse conditions seedling loss can be severe. If excessive salts are present
at seeding, germination can be inhibited, or the seedlings can be killed before they
emerge. If excessive salt accumulates after emergence, but while the seedlings are
still in the cotyledon or first true leaf stage, the seedlings may dehydrate, turn
brown and die. It is in the latter case that salt injury is most easily confused
with fungal damping-off. Seedlings with salt injury, if examined before they
completely dehydrate, frequently show a blanched area around the stem at or just
* below the soil surface. In contrast, the stems of seedlings with damp-off often
-ppear water-soaked, darker in color and somewhat decomposed.
Transplants. Salt injury to tomato transplants and older seedlings usually
appears as a girdling of the stem at or just below the soil surface. This corre-
sponds very closely to the zone of highest salt concentration. Water movement from
the roots to the tops is restricted by the girdled area, and the seedlings wilt and
die. Plants that are less severely affected may survive, but generally develop an
enRar'ement of the stem just above the soil surface. These plants have a poorly
developed root system, are less productive and tend to break-over in windy weather
or during harvest.
Several experiments have been conducted to develop practices that can be used
with full-bed mulch culture, which will reduce the danger of plant injury from
soluble salts in the planting hole cut through the mulch.
Deep-planting. Since the zone of highest salt concentration is in the upper
1/4-1/2 inch of the bed surface, one method to reduce soluble salt injury is to
deep-plant. Deep-planting refers to placing seeds or the roots of transplants in
the bottom of a 1 1/2-2 inch deep hole having a 2" diameter (Fig. 1). Care should
be taken to insure that the hole is no deeper than two inches. If the hole is too
deep, young seedlings of field-seeded crops can become spindly due to reduced light
in the hole, and transplants may be damaged by having their roots placed in soil
* that is too close to the water table. The hole is left open and only enough soil is
used to cover the seeds or roots. If plug-mix seeding is used, deep-planting can
be accomplished by placing the mix in the bottom of the 1 1/2-2 inch hole so that
the top of the mix is 1-1 1/2 inches below the bed surface. With this method, salts
acc-Umulate in the soil around the upper edge of the hole rather than around the
seeds or stems of the seedlings, as is the case with shallow-planting, when the soil
near the seeds or seedlings is level with the bed surface. There has been no serious
problem, even after heavy rains, from soil washing into the hole and covering the
emerging seedlings of field-seeded tomatoes. Figure 2 shows the rate of salt
accumulation in soil near deep or shallow-planted seeds during the fall season of
1971 and spring seasons of 1972 and 1973. Soil samples were taken daily or every
other day for 16 to 20 days after seeding. The volume of soil sampled was a 1/2 x
2 inch core in which the seeds had been planted. In 1971, salt accumulation was
fairly constant and at approximately the same concentration for both deep and
shallow-planting during the first eight or nine days, after which there was a sharp
increase with shallow-planting. Salt accumulation with shallow-planting continued
rt a higher concentration than with deep-planting until the test was ended on the
sixteenth day after seeding. Shallow and deep-planted hills showed 27% and 5% salt
injury, respectively. A similar pattern was true in 1972 except a 0.12" rain occurred
on the twelfth day after seeding. This reduced the salt to about the same concen-
tration in both the shallow and deep-planting. After a couple of days salt concen-
tration began to increase for both planting methods, but at a faster rate for the
shallow-planting. By the twentieth and final day of sampling, salt concentration
was 19,000 ppm for the shallow-planting and 8,000 ppm for the deep-planting. In
this test, 19% of the shallow-planted and 3% of the deep-planted hills showed
* symptoms of salt injury. In both 1971 and 1972, two light rains, which occurred
d-jring the first week after seeding, contributed to the slow rate of salt acctaula-
tio:i during this period. In 1973, no rain occurred until the seventh day after
seeding and salts accumulated rapidly in the shallow-planted hills, but not in the
dace-planted hills. A 2.70" rain on the seventh day reduced the salt concentration
Figure 1. Cross section of plant beds sIowing
shallow and deep-planting methods.
I I"., _
/ \ rain
I I I I I
, I i i
Days after planting
Figure 2. Effect of deep vs shallow-planting
on ppm soluble salt in the planting hole
as determined by the saturated paste nrthod.
_ I Ii .
I ~~ ~ I
to about the same level for both shallow and deep-planting. Two additional rains
held the salt concentration at a low level during the remainder of the sampling
* period. Only 2% of the shallow-planted hills and none of the deep-planted hills
showed salt injury.
Top-watering. A recent study with full-bed mulch and plug-mix seeding indicates
that top-watering is necessary to reduce salt injury to seeds and young seedlings
in shallow-planted plug-mix, but is not as essential when plug-mix is deep-planted.
If the mix is dry when planted, at least one top-watering is necessary to moisten
the mix sufficiently for seed germination, regardless of whether it is planted deep
or shallow. However, after the initial watering, the percent salt injury for seed-
lings in deep-planted plug-mix was approximately the same with or without top-
watering. Salt injury in shallow-planted plug-mix was reduced from 26% without
top-watering to zero when top-watered every three days (Fig. 3). There was only
0.10" of rain during the first 30 days of this test and maximum temperatures ranged
from the low to middle 80's. Under these conditions, 90 to 95% of the hills had
emerged by the fourth day in all treatments except the shallow-planted non-watered
(Table 1). In the latter treatment only 32% had emerged by the 4th day, 52% by
the 6th day and 85% by the 8th day, while 15% of the hills failed to emerge.
Fertilizer placement. Improper fertilizer placement will contribute substan-
tially to salt injury of tomato seedlings. The following placements under full-bed
mulch have been tested.
Starter fertilizer spread in a 30"-36" band on a false bed, then
bedded-over to a 3" depth. The remainder of the fertilizer was
banded in a narrow band on bed surface 9" to each side of seed
Starter fertilizer broadcast and incorporated prior to forming
plant beds. Remainder of fertilizer placed as in Placement A.
Starter fertilizer placed in a narrow band, 3" below and 3" to
each side of seed row. Remainder of fertilizer placed as in
Starter fertilizer placed in a 10" wide band, 3" directly below
seed row. Remainder of fertilizer as in Placement A.
Starter fertilizer placed in a narrow band on bed surface 4" to
each side of seed row. Remainder of fertilizer as in Placement A.
All fertilizer broadcast and incorporated prior to forming plant
Starter fertilizer spread in 18"-20" band on bed surface over
seed row. Remainder of fertilizer placed as in Placement A.
The starter fertilizer in these tests was either 5-8-8 at 500 lb/A or 18-0-25
at 140 Ib/A. The nitrogen and potash in this amount of starter fertilizer is
sufficient to supply the needs of the plant during early growth, which is the most
critical period for salt injury. If soil test indicated a need for phosphorus, it
was supplied prior to bedding by broadcasting and disking in superphosphate.
Of the seven placements tested, A and B have
D, E, F, and G have been the most unsatisfactory.
traction in the planting hole with Placements A, D
been the most satisfactory, and
Figure 4 shows the salt concen-
and F. Incorporating or mixing
X Shallow, no top-water (26% salt injury)
S- Shallow, top-water (no injury)
c Deep, no top-water (1% salt injury)
Deep, top-water (no injury)
0 10,000 /
I I I I I ,
0 4 8 12 16 13
Days after planting
Figure 3. Effect of top-watering on soluble salt
concentration in deep and shallow-planted piug-mix.
A Placement A
D Placement D
F Placement F
(see text for description of placements)
S I e I _
4 8 12 16 13
Days after planting
Figure 4. Effect of fertilizer placement on
soluble salt concentration in planting hole.
all of the fertilizer in the beds (Placement F) has been consistently one of the
poorest placements with respect to salt injury. However, if only the starter
* fertilizer (10-15% of the total) is mixed in the bed with the remaining 85-90%
placed on the bed surface in a narrow band 9" to each side of seed row (Placement B),
very little salt injury has occurred.
Placing the starter fertilizer on the bed surface over the seed row (Placement
G) can cause severe salt injury to shallow-planted tomatoes but has been one of the
better placements for deep-planted tomatoes (Fig. 5). No plug-mix or top-watering
was used in this test. Tomato seed were planted at the bottom of a 2" deep open
hole (deep-planted) or 1/2" below the bed surface (shallow-planted). Seedlings in
90% of the deep-planted hills and in only 8% of the shallow-planted hills had emerged
by the 6th day after planting. However, after the salt concentration was reduced
by a 2.70" rain on the 7th day, seedlings began to emerge from the shallow-planted
hills and reached 88% emergence by the 12th day after seeding. Rains occurred
periodically during the remainder of this test and salt injury to seedlings was mini-
mal. Therefore, the principle adverse effect of this fertilizer placement when
used with the shallow-planted method was to delay seed germination and emergence.
However, in a similar test, using fertilizer placement G, when only 0.10" rain fell
during the first three weeks after planting, 52% of the shallow-planted hills either
failed to emerge or were lost to salt injury after emergence. With the same fer-
tilizer placement but deep-planted, only 7% of the hills failed to produce healthy
seedlings (Fig. 5).
Fertilizer rates. The higher the fertilizer rate the greater the potential
for salt injury. For this reason, application of fertilizer in excess of that
required for acceptable yields is not only uneconomical because of the extra cost
* of tunee-'fmaterial, but can also result in lower yields due to salt injury.
Fertilizer rates for tomatoes should be adjusted according to the expected
length of the harvest season and/or the expected number of pickings. The following
rates, in terms of nitrogen, are suggested for tomatoes grown with full-bed mulch
on south Florida's sandy soils.
a. Ground (unstaked) culture 1 or 2 pickings as mature-green
120-150 lb N/acre
b. Stake culture 4 to 5 pickings as mature-green
200-250 lb N/acre
c. Stake culture 15 or more pickings as vine-ripe
300-350 lb N/acre
Potassium (K20), in most cases, can be applied at 1.5 to 2 times the amount of
nitrogen. The rate of phosphorus should be determined from soil test and previous
Grower's should be very careful not to apply starter fertilizer at too high a
rate, because it is this fertilizer that contributes most to the salt problem during
the early growing period.
Bed shape. Soluble salts under full-bed mulch move to and accumulate at the
* highest point of the bed. When tomatoes are shallow-planted on a crowned bed, the
risk of salt injury is greater than when shallow-planted on a flat bed. Therefore,
if the shallow-plenting method is used, the surface of the finished bed should be
as flat and level as possible. In theory, a bed with a slightly depressed center
would be best. This bed shape has been tested and is very effective in reducing
X--X Shallow (wet season)
--_- Deep (wet season)
X--..X Shallow (dry season)
S....... Deep (dry season)
..-I ........ I .....I. /- f ..}
1 I I I I ____
0 4 8 12 16 18
Days after planting
Figure 5. Effect of fertilizer placement G on soluble
salt concentration in the planting hole for a wet
or dry season, with deep or shallow-planting.
salt accumulation near seeds or seedlings. However, after rains, water will accumn-
late in the depression on top of the mulch. This creates a condition for high
kbumidity which would enhance disease development. The deep-planting method, when
used with either a flat or crowned bed, accomplishes the same purpose as a bed with
a depressed center, but without creating an area for water to accumulate.
Table 1. Effect on tomato seedling emergence of deep and
shallow-planted plug-mix with and without top-watering.
Planting Top- Total hills emerged on Failed
depth water 4th day 6th d&yv 8th day to emerge
Deep yes 95 98 98 2
no 92 96 97 3
Shallow yes 90 97 97 3
no 32 52 05 15