Usingtomatoplants to assessthe release of potassium
Ifromfertilizers
2Allen V. Barker
ABSTRACT
MATERIALS AND METHODS
A laboratory experimentis describedto showhow
the relative release rates of K fromfertilizers can be
determinedin a short period of time. Whengrownon
a high level of ammonium
nutrition underK-deficient
conditions, tomato(LycopersiconesculentumMill.)
plants form brownishstemlesions. Theselesions do
not formif adequate
K is supplied.Variousfertilizers
can be rankedfor K release on the basis of their effectivenessfor alleviation of lesion formation.Soluble K fertilizers and woodashes,if suppliedin sufficient quantities, stop lesion development
whereas
rock or mineral K materails prove too insoluble to
amelioratelesion formation.
Any commercial variety of tomato may be used, because
none is known to be completely resistant to lesion formation (3). The medium in which the plants are to be grown
may be soil or a potting mix based on soil. The K fertilizers
should be mixed dry into the soil. The amounts of soluble
K fertilizers
to be used may need to be established experimentally depending on the medium to be used as soils with
differing K supplying powers or K fixation capacities may
give different results. Usually soils with low K supplying
power or high K fixing capacity will allow rapid lesion development after ammonium-Nis supplied to the plants. The
insoluble K fertilizers or other materials should be added in
amounts to supply K equivalent to that given by the soluble
fertilizers. Usually a range of K treatments is desirable.
The tomato plants should be uniform and sturdy. Plants
about 15 cm tall are ideal. These can be produced in about
6 weeks after seeding with one transplanting 2 weeks after
seeding. The plants should be placed singly in 12 or 15-cm
plastic or ceramic flower pots filled with the mixed medium
and fertilizer.
It is not necessary to remove the soil from
the roots of the tomato, and transplanting a small ball of
soil with the plants usually diminishes setback and has little
influence on the total soil mass in which the plants are
grown during the experiment. Allow about 1 week between
planting and the initiation of the ammoniumtreatments to
overcome transplanring shock.
The concentration of (NH4)2SO4 used may vary between
0.01N to 0.08N. The more concentrated the solution, the
quicker the lesions appear. However, very concentrated
solutions may cause one to fail to distinguish between K
fertilizers and maycause all plants to develop lesions. Also,
highly concentrated NH
4 solutions may cause considerable
foliar damagewhich closely resembles K deficiency but with
considerable necrosis along the leaf midrib. Stems also beA solution of 100 ml of 0.04N (NH4)2SO
4come brittle.
applied daily normally causes lesion appearance in 5 to 7
days. Treatment may be continued with the daily applications of (NH4)2SO
4 for 1 to 3 weeks. Only very good, fastrelease K fertilizers
will prevent lesion development under
these circumstances, and further elongation of the treatment time is unnecessary to enable distinguishing between
the value of the fertilizers.
In circumstances promoting
rapid plant growth and quick utilization of N, lesion development may be delayed, but the treatment period after
lesion development need not be extended past 1 to 3 weeks.
Once the lesions begin to form they rapidly develop with
continued application
of ammonium-N. Also, once the
lesions develop they do not heal unless K fertilizer is given.
Severe lesion development is a relatively permanent blemish
so that if the (NH4)2SO
4 applications are stopped, the extent of lesion development can be assayed at some reasonable date following cessation of treatment.
Additional index words: Ammonium
toxicity, Soil
fertility, Teaching
experiment,Soils education.
lesions
S TEM
esculentum
form
on tomato
(Lycopersicon
Mill.)
plants grown on ammoniumN nutrition
in soils-unfertilized
with K or in Kdeficient
nutrient
solutions.
However, providing
the plants with ample K nutrition
eliminates
lesion
formation
even though the level
of ammonium-N
nutrition
may be far in excess of plant needs (1, 3).
Because of this
phenomenon,
tomato plants
may
be used in a bioassay to evaluate fertilizers
or soils
for their
K-supplying
power for plant
growth.
Vegetative
materials,
wood ashes,
and chemical
fertilizers
release their K quickly to a growing plant.
Mineral materials,
such as granite dust and greensandwhich contain 5 to 11% total K20, are very insoluble,
slow release
agents considered
equivalent
to the highly soluble chemical fertilizers
by some
people (5, 6) but generally
considered
inadequate
to support plant growth by others (2, 4, 7).
This paper describes
a procedure
which may be
used as a laboratory
or greenhouse experiment
to
show the relative
release of K by various fertilizer
materials.
1paper No. 2058, Massachusetts Agric. Exp. Stn., Univ.
of Massachusetts, Amherst.
2Associate professor, Plant and Soil Sciences, Univ. of
Massachusetts, Amherst.
63
JOURNAL OF AGRONOMIC EDUCATION
64
Table I-Ammonium injury rating of tomato stems
RESULTS
The results which follow except for the plant
analyses, done by the author, were obtained by a
six-section class of 165 students. Each section completed the experiment and served as a repIication of
the treatments. The experiment ran by the class
compared potassium chloride (50% available K),
wood ash (5% K soluble in N HCl), greensand (5%
total K, cf 5), and Georgia granite dust (5% total K,
Hybro-Tite Corp., Lithonia, Ga.). The range of K
concentration used was 0, 0.5 (omitted for granite
dust and greensand), 1.0, 2.0 and 4.0 g K/pot. The
pots were 1 5 cm,holding about 1,500 g of a potting
mix of 7 parts sandy loam, 3 parts peat moss, and
2 parts sand by volume. The concentration of the
(NH4),S04 was 0.04 N , and the solution was applied 100 ml/day for 15 days.’
Figure 1 shows the lesions on tomato stems.
Lesion formation was rated on a scale from 0 t o 3,
where 0 was no lesion development, 1 was a few
scattered lesions per stem, 2 was widespread lesion
development with individual lesions distinguishable,
and 3 was widespread lesions with coalescence of
lesions. In Table 1, the results of the lesion ratings
are shown. Granite dust and greensand were ineffective in stopping lesion development. Wood
ashes completely prevented lesion development, as
KCI did, also, if enough was applied.
Fresh weights are shown in Table 2. Granite dust
gave no improvement in growth over the control,
and greensand gave statistically better growth only
at the highest level of application. Wood ashes and
KCI both improved growth at lower levels of application then depressed growth as the amount of fertilizer became excessive probably reflecting soluble
Material added
Amount of
K added
Granite
dust
-Visual
g/pot
score
0
2.8 bc*
2.8 be
0.5
1 .o
2.0
4.0
2.0 b
2.7 bc
2.7 bc
3.0 c
2.3 bc
2.7 bc
--t
* Means followed by
Wood
ash
Greensand
KCI
2.8 be
Oa
Oa
Oa
Oa
-
different letters are significantly different (P
Duncan’s multiple range test. (0 no injury; 3 = severe injury).
t Treatments omitted.
2.8 bc
2.3 bc
Oa
0.5 a
0.8 a
< 0.05) by
Table 2-Fresh weights of tomato plants
Material added
Amount of
K added
Granite
dust
Wood
ash
Greensand
KCI
- glplant -
glPot
0
0.5
1.0
2.0
4.0
78 bcd*
--t
64 a
73 bc
68 ab
78 bcd
9 2 def
89 cdef
105 f
78 bcd
115 f
114 f
76 bcd
54 a
78 bcd
100 f
98 et
81 bcde
68 ab
* Means followed
by different letters are significantly different (P GO.05) by
Duncan’s multiple range test.
t Treatments omitted.
Table 3-Percentage of K in tomato stems
Material added
Amount of
K added
Granite
dust
Wood
ash
KCI
-%K,d~wt.-
glpot
0
0.5
1.0
2.0
4.0
Greensand
3.19 a*
--t
3.08 a
2.61 a
2.94 a
3.19 a
3.25 ab
3.50 ab
3.52 ab
3.19 a
4.98~
5.54 c
5.18 c
5.41 c
3.19a
4.02ab
4.70 be
5.73 c
5.81~
* Means followed by
different letters are significantly different (P Q0.05) by
Duncan’s multiple range test.
t Treatments omitted.
salt injury.
Table 3 shows the percentage of K in the tomato
stems. Potassium content is related inversely to
lesion development but not in direct proportions.
Granite dust and greensand did not raise the percentage of K in the stems above that found in the
control. All K concentrations in the plants grown
on wood ashes were above that of the control, and
no lesions developed on any of these plants. With
KCI at its lowest level of application, lesion development was rather severe (Table l),and the K concentration in the stems was not significantly greater
than that of the control. With significant increases
in K in the plants treated with KCl, lesions were
eliminated. The wood ashes contained 20% Ca and
2.5% Mg, both of which have some effect in alleviating lesion formation (1) and which may have accointed the advantage‘of wood ash over KCl.
Fig. 1-Lesions on tomato stem. Left, normal stem; middle,
moderate lesion development; right. severe lesion development with entire stem darkened by lesions.
BARKER:
TOMATO PLANTS IN FERTILIZER EXPERIMENT
DISCUSSION
This experiment is remarkably simple to run and
works infalliably even when a large number of participants are involved in setting up the experiments.
The results are very striking and convince students
that fertilizers have different values related to their
release of nutrients. One may criticize the use of
(NH4)2SO4 as giving a harsh treatment and favoring
the soluble fertilizers over the rock forms. The
amount of (NH4)2SO4 used in a short-term experiment is not in excess of that needed to grow a 100g tomato plant in a 15-cm pot, although if the application of N were extrapolated to a per hectare
rate on the basis of weight the quantity would seem
enormous, but for that matter so would the quantities of granite dust, greensand, or KC1. Also, when
a soil of very low fertility with respect to available
K is used, lesions form after only one or two applications of (NH4)2SO4 which amount to 55 to 110
mg N/pot or about 80 to 160 kg N/ha fully extrapolated. Therefore, criticism of the harshness of
the treatment does not seem valid. Larger amounts
of (NH4)2SO4 are needed only when the K supplying power of the soil is high or when growing conditions are favorable and the NH4-N is rapidly assimilated by the plant. As a point of caution, however,
hot conditions in the summer in the greenhouse
65
may cause a lot of foliar damage to result from the
NH4-N before lesions develop on the stem and may
shorten the life of the experiment or give plant material which is not usable; therefore, the spring and
fall are the best times to run this experiment.
This work has been conducted by the author
several times outside the classroom. The results
were always the same as those found in the classroom.