characteristics of w a t e r
soluble nitrogen fertilizers
By Roger Funk, Vice President of Research and Development, Davey Tree, Kent, Ohio
Nitrogen is the k e y s t o n e of a lawn f e r t i l i z a t i o n program, a n d both s o l u b l e a n d slowly s o l u b l e s o u r c e s
a r e a v a i l a b l e to the turfgrass industry. S o l u b l e nitrogen f e r t i l i z e r s a r e less e x p e n s i v e than the slowly
s o l u b l e s o u r c e s but, in g e n e r a l , h a v e a h i g h e r b u m
potential a n d a r e m o r e likely to b e lost through
l e a c h i n g a n d volatilization. T h e s e risks c a n b e
m i n i m i z e d , h o w e v e r , if the contributing f a c t o r s a r e
understood.
through a c e l l m e m b r a n e from the side that h a s the
lowest o s m o t i c p r e s s u r e to the side that h a s the
highest p r e s s u r e . S i n c e root c e l l s a c t i v e l y a b s o r b
n u t r i e n t salts, the o s m o t i c p r e s s u r e of the cell s a p is
n o r m a l l y h i g h e r than that of the s u r r o u n d i n g soil
solution — a n d w a t e r is a b s o r b e d into the root
tissue. T h i s p r o c e s s , in fact, is h o w plants a b s o r b
w a t e r . H o w e v e r , if e x c e s s f e r t i l i z e r salts in the soil
solution i n c r e a s e the osmotic p r e s s u r e a b o v e that
of t h e c e l l sap, w a t e r is d r a w n out of t h e roots, a n d
the r e s u l t a n t i n j u r y is t e r m e d " f e r t i l i z e r b u r n . "
S y m p t o m s of f e r t i l i z e r b u r n r e s e m b l e those of
drought i n j u r y s i n c e , in both c a s e s , t h e i m m e d i a t e
p r o b l e m is lack of w a t e r in the plant.
T h e r e l a t i v e t e n d e n c y of a f e r t i l i z e r to r e l e a s e
salts a n d i n c r e a s e the osmotic p r e s s u r e of the soil
solution is m e a s u r e d b y the salt i n d e x . T h e h i g h e r
the salt i n d e x v a l u e , the g r e a t e r t h e t e n d e n c y of a
f e r t i l i z e r to i n c r e a s e the osmotic p r e s s u r e a n d the
g r e a t e r the b u r n potential. T h e salt i n d e x e s of common s o l u b l e nitrogen f e r t i l i z e r s a r e listed in T a b l e
1, w h i c h also c o m p a r e s the a d j u s t e d salt i n d e x e s ,
b a s e d upon the total n u t r i e n t c o n t e n t .
E n v i r o n m e n t a l f a c t o r s such a s t e m p e r a t u r e ,
humidity, a n d soil m o i s t u r e also a f f e c t the b u r n
potential of a f e r t i l i z e r . A s the a i r t e m p e r a t u r e inc r e a s e s a n d a s the h u m i d i t y d e c r e a s e s , the w a t e r
r e q u i r e m e n t of plants i n c r e a s e s . B e c a u s e of the inc r e a s e d w a t e r r e q u i r e m e n t s f o r plants, the l e v e l of
s o l u b l e salts in soil solution that is " s a f e " during
Fertilizer burn
F e r t i l i z e r s contain salts that a r e s i m i l a r to t a b l e
salt (sodium c h l o r i d e ) e x c e p t that f e r t i l i z e r salts
c o n t a i n the e l e m e n t s e s s e n t i a l f o r plant growth.
W h e n salts dissolve in w a t e r , they d i s s o c i a t e into
positively a n d negatively c h a r g e d ions, a n d it is in
this f o r m that nutrients a r e a b s o r b e d b y plant
roots. S o l u b l e f e r t i l i z e r s a r e in the salt form w h e n
a p p l i e d to turfgrass, w h i c h a c c o u n t s f o r their i m mediate availability for absorption. Slowly soluble
f e r t i l i z e r s m a y contain s o m e s o l u b l e salts, but most
of the nutrient salts a r e r e l e a s e d over a p e r i o d of
time as the slowly s o l u b l e f e r t i l i z e r is h y d r o l y z e d
or d e c o m p o s e d in the soil. T h u s , a m a j o r d i f f e r e n c e
b e t w e e n s o l u b l e and slowly s o l u b l e f e r t i l i z e r is the
r e l e a s e rate of the nutrient salts.
S a l t s dissolved in soil solution i n c r e a s e the osm o t i c p r e s s u r e that governs the flow of w a t e r
a c r o s s a root cell m e m b r a n e . W a t e r a l w a y s m o v e s
Table 1. Solubility and Salt Indexes of Soluble Nitrogen Fertilizers
Primary analysis
(percent)
Total
plant
food"
Salt
index b
Adjusted
salt
index'
Solubility'1
Fertilizer
Formula
ammonia
urea
ammonium nitrate
ammonium sulfate
sodium nitrate
calcium nitrate
potassium nitrate
monoammonium phosphate
diammonium phosphate
ammonium polyphosphate
(liquid form)
NHa
82.0
82.0
47.1
57.4
H2NCONH2
46.0
—
—
46.0
75.4
163.9
67
NH4NO3
(NhUhSCh
NaNOa
Ca(NOa)2
KNOa
NH4H2P04
(NH4)2HP04
NH4PO3
33.5
—
—
33.5
104.7
312.5
118
21.0
—
—
21.0
69.0
328.6
71
16.0
—
—
16.0
100.0
625.0
73
15.0
—
—
15.0
65.0
433.3
134
N
"Percent N + percent P-Or, + percent KjO (primary analysis).
b
Rader, L. F., Jr., et al. 1943. Soil Science 55(3):201-18.
Salt index per unit of plant food = (2) x 100/( 1 ).
d
Parts in 100 parts pure water at 32° F.
e
TVA.
c
P i Os
KJO
90
13.0
—
46.0
59.0
73.6
124.7
13
11.0
48.0
—
59.0
29.9
50.7
43
18.0
46.0
—
64.0
34.2
53.4
10.0
34.0
—
44.0
29.9e
67.9
25
—
ALGAE OUT
of Your Course & Park
PONDS...LAGOONS
LAKES...any water area
with
cool, h u m i d w e a t h e r m a y c a u s e b u r n i n j u r y during
p e r i o d s of w a r m w e a t h e r or low h u m i d i t y or both.
Soil m o i s t u r e is a m a j o r f a c t o r in d e t e r m i n i n g the
f e r t i l i z e r ' s potential to b u r n . If the soil is r e l a t i v e l y
dry, a f e r t i l i z e r will h a v e a g r e a t e r e f f e c t on inc r e a s i n g the osmotic p r e s s u r e of the soil solution.
C o n v e r s e l y , if the soil is s a t u r a t e d , the f e r t i l i z e r
salts will d i s p e r s e and the o s m o t i c p r e s s u r e will
not i n c r e a s e g r e a t l y . In a d d i t i o n ,
the
é v a p o t r a n s p i r a t i o n of w a t e r will h e l p cool the
plant and r a i s e the h u m i d i t y n e a r the soil s u r f a c e ,
e f f e c t i v e l y r e d u c i n g the p l a n t ' s w a t e r r e q u i r e m e n t .
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Leaching
Nutrient l e a c h i n g is the r e m o v a l of s o l u b l e fertilizers from the root zone by the d o w n w a r d p e r c o l a tion of w a t e r . Most of the s o l u b l e f e r t i l i z e r nitrogen
will b e p r e s e n t in o n e or m o r e of t h r e e f o r m s : amm o n i u m (NH.» + ), n i t r a t e s (NCb—), and u r e a (CO
(NHI)I).
A m m o n i u m is w a t e r s o l u b l e , but the strong
attraction b e t w e e n the positively
charged
a m m o n i u m ion and the negative sites on c l a y
m i n e r a l s and soil o r g a n i c m a t t e r p r e v e n t s l e a c h i n g .
A m m o n i u m , h o w e v e r , is r a p i d l y o x i d i z e d to
n i t r a t e s w h e n the soil t e m p e r a t u r e is a b o v e 5 0 ° F .
N i t r a t e is a n e g a t i v e l y c h a r g e d ion and, as such,
is r e a d i l y l e a c h e d b e c a u s e it d o e s not bind to soil
p a r t i c l e s . L e a c h i n g of n i t r a t e from the rooting z o n e
is a much g r e a t e r p r o b l e m in c o a r s e - t e x t u r e d soils.
R e s e a r c h h a s shown that nitrate m a y b e l e a c h e d
about 1 inch of r a i n f a l l for e a c h inch of r a i n f a l l in
c l a y loam soils to 2.5 i n c h e s for e a c h inch of r a i n f a l l
in s a n d y loam soils.
U r e a f e r t i l i z e r is r e a d i l y s o l u b l e w h e n it is first
a p p l i e d to the soil, but w h e n it c h a n g e s to ammonium it is h e l d by c l a y and h u m u s is a form that
is r e a d i l y a v a i l a b l e to plants. U n d e r f a v o r a b l e temp e r a t u r e and m o i s t u r e conditions, u r e a h y d r o l y z e s
to a m m o n i u m c a r b o n a t e and then to n i t r a t e within
less than a w e e k .
Volatilization
V o l a t i l i z a t i o n involves the c o n v e r s i o n of nitrogen to a m m o n i a gas, w h i c h is lost to the atm o s p h e r e . T h i s p r o c e s s is f a v o r e d by a l k a l i n e soils,
dry soils, soils with a low e x c h a n g e c a p a c i t y , and
warm temperatures. When conditions favor
volatilization, 25 p e r c e n t or m o r e of the a p p l i e d nitrogen may b e lost to the a t m o s p h e r e .
W h e n a m m o n i c f e r t i l i z e r s and u r e a a r e p l a c e d in
the soil, the a m m o n i a gas that they r e l e a s e is h e l d
by the soil p a r t i c l e s . H o w e v e r , w h e n u r e a or amm o n i c f e r t i l i z e r s a r e p l a c e d on top of the soil, the
r e l e a s e d a m m o n i a d o e s not h a v e the c l a y or moisture to hold it from b e i n g partly volatilized.
All of the c h a r a c t e r i s t i c s of nitrogen f e r t i l i z e r s
should b e c o n s i d e r e d w h e n a turf f e r t i l i z a t i o n program is p l a n n e d . If the m a t e r i a l s a r e a p p l i e d prope r l y for the existing soil and e n v i r o n m e n t a l conditions, s o l u b l e nitrogen f e r t i l i z e r can b e just as eff e c t i v e as slowly s o l u b l e s o u r c e s in providing the
turfgrass plant with the nitrogen it r e q u i r e s .
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