Characteristics
in the soil
-
P moves very little in the soil; leaching is not a problem.
Exceptions
to this generalization are organic soils and very coarse sands.
-
More inorganic than organic P is present in mineral soils
-
There may be a large total amount of P in a soil with very litle
available to plants.
-
Crops remove P in smaller amounts than N and K.
-
Phosphorus
Fertilizer Conversion Factors % P x 2.29 = % P2O5, % P2O5
x 0.44 =%P
A.
Forms of Soil Phosphorus
1.
Organic P - 30 - 50% of total P in most soils is in the organic
matter. The C:N:P ratio of soil organic matter is about 100
- 10 -1. If 10 lbs. per acre per year of N were mineralized,
about 1 pound of P would be mineralized.
Amount
of P present in the organic form in three soil series differing
in O.M. content.
| Soil
Series |
%
OM |
%
of P in Organic Form |
| Hyde |
20
|
80
|
| Portsmouth |
5
|
50
|
| Norfolk |
0.5
|
15
|
2.
Inorganic P
1.
Soil Minerals - apatite [ Ca3(PO4)2]
* CaF2
2.
Hydrated oxides of Fe and Al - In acid soils, P reacts with
Fe and Al. These compounds are relatively insoluble and not
available to plants. This is called " phosphorus fixation".
3.
Calcium Phosphates are formed in soils with pH greater than
7.
4.
Phosphorus in soil solution
a.
Concentration is low 0.05 to 0.2 ppm
b.
The solution concentration is affected by solubility and
amount in the solid phase.
5.
Phosphorus fixation - The reduction of solubility
of fertilizer P that is added to the soil. Most frequently a
problem on acid soils.
a.
Fertilizer P reacts with:
1.
Al and Fe in soil solution
2.
Exchangeable Al and Fe
3.
Hydrous oxides of Fe and Al
4.
Clays
b.
Factors affecting phosphorus fixing capacity
1.
amount and type of clay - Kaolinite clays usually have associated
with them a high Fe and Al oxide content.
2.
Soil Acidity - at pH's below 5 there is a large amount of
exchangeable Aluminum
3.
Phosphorus level of the soil. If it has been built up over
the years, the P fixing capacity is satisfied.
B.
Method of Application or Placement
1.
Broadcast
-
When applying large amounts of fertilizers
-
When P fixation is not a problem
-
To build up P in the soil
*
Encourages deep rooting and good distribution
2.
Banding - row crops
-
When P fixation is a problem
-
When soil test levels are low
-
When applying a low rate < 50 lbs. / acre
-
There is a need for a starter effect for seedlings
*
Provides maximum availability by decreasing contact with the
soil and places a supply of P in the root zone.
3.
Combination of Broadcast and Band . Use broadcast to apply
large amounts insuring a reserve of P in soil. Band for a readily
available supply to get seedlings off to a fast start.
4.
Top Dressing - Very little reaches roots because of limited
movement in soil. Use only when necessary (turf, pasture)
C.
Crop Characteristics
1.
Types of crops and conditions when crops are responsive to P.
a.
Crops that have a short season (ex. snapbeans)
b.
restricted root systems (potatoes and cotton)
c.
growing in cool seasons (potatoes, cabbage, etc. )
2.
Ability to use Fertilizer P
| Crops |
% Fertilizer Recovered First Year |
| Potatoes |
7.6 |
| Corn |
11.8 |
| Soybeans |
18.2 |
3.
Crop Requirements
| Crop |
Yield |
lbs. P2O5 |
| Corn |
100 bu/A |
60 |
| Wheat |
40 bu/A |
30 |
| Soybeans |
40 bu/A |
25 |
| Tobacco |
2000 lbs/A |
30 |
These
are amounts needed for maintenance application
D.
Phosphorus Fertilizers
1.
Solubility Terminology
a.
Water soluble P - fertilizer compounds dissolve
in water
b.
Citrate soluble P - compounds that dissolve
in ammonium citrate
c.
Citrate insoluble P - the portion that is
insoluble in both water and ammonium citrate
d.
Available P - water and citrate soluble
2.
Fertilizer Materials
a.
Rock Phosphate - Source of all P fertilizers;
deposits are found world wide. N.C. deposits have
been mined by Texasgulf Inc. (now
PCS Phosphate) since the mid sixties.
Florida and North
Carolina produce about 85% of the United States
total.
Rock
phosphate is insoluble; therefore, it is not a good
fertilizer without being processed. Applications of
rock phosphate on very acid soils (i.e. organic in Eastern
NC ) has been shown to become slowly available to plants.
To be made readily available to plants phosphate rock must
be acid or heat processed.
b.
Acid treated phophate materials
1.
Wet process phosphoric acid. Rock phosphate
is treated with a sulfuric acid (H2SO4)
to produce phosphoric acid plus calcium sulphate (gypsum)
and the gypsum is removed. This is sometimes called "green
acid" and is 54-56% P2O5.
This acid is used to make triple superphosphate and liquid
fertilizers.
2.
Superphosphoric Acid - (72% P2O5)
- Made by concentrating the 54-56% through evaporation used
in liquid fertilizers.
3.
Ordinary Superphosphate - React sulfuric acid with
rock phosphate.
-
20 % available P2O5
(85-90 % water soluble)
-
contains 8-10% sulfur as gypsum (CaSO4)
-
used in mixing and blending fertilizer and is applied
directly
4.
Concentrated Superphosphate (triple) - Made by treating
rock phosphate with phophoric acid.
-
46% P2O5 (86-90% water soluble)
-
Contains very little sulfur < 3% S
-
used in mixing and blending and applied directly
5.
Ammonium Phosphates - manufactured by ammoniating
phosphoric acid. Monoammonium phosphate
11-48-0, Diammonium phosphate 18-46-0.
These materials have excellent storage and handling properties,
highly water soluble, high nutrient content, uniform granules,
and are economical. DAP is the most popular and is produced
by PCS phoshate (formerly known as Texasgulf, Inc. ) in
North Carolina.
6.
Others - Nitrophosphates - Nitric acid
and rock phosphate. Ammonium polyphosphates
- Made by neutralizing superphosphoric acid with anhydrous
ammonia.
E.
Thermal - Processed P sources
- More expensive; account for a small portion of fertilizer
produced. Furnance grade phosphoric acid is used in the food
and drug industries.
F. Treatment of Phosphate Rock.
