in the soil
P moves very little in the soil; leaching is not a problem.
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.
Fertilizer Conversion Factors % P x 2.29 = % P2O5, % P2O5
x 0.44 =%P
WHY IS PHOSPHORUS EXPRESSED AS P2O5 AND POTASSIUM EXPRESSED AS K20 IN THE SOIL TEST REPORT RECOMMENDATIONS AND BY THE FERTILIZER COMPANIES?
In earlier times, before current lab instrumentation was developed, chemists used a gravimetric (weighing) method after ignition to determined the phosphorus and potassium content of fertilizers in the form of phosphorus oxide (P2O5) and potassium oxid e (K2O). By convention, the amounts (or analysis grade) of phosphorus and potassium in fertilizers are still expressed in this oxide form. The Association of American Plant Food Control officials have developed a uniform state fertilizer bill which says that available P2O5 and soluble K2O must be guaranteed by the manufacturer and so the guaranteed analysis must still be expressed in the oxide form. Nitrogen content has always been expressed as simply N.
According to the conventional fertilizer standards, a 100 pound bag of 10-10-10 contains 10% or 10 pounds of nitrogen, 10% or 10 pounds of P2O5 and 10% or 10 pounds of K2O. Since P2O5 is really only 44% actual elemental phosphorus and K2O is only 83% actual elemental potassium, a 100 pound bag of 10-10-10 contains 10% or 10 pounds of nitrogen, 4.4% or 4.4 pounds of elemental phosphorus and 8.3% or 8.3 pounds of elemental potassium.
Perhaps the reluctance of the fertilizer industry to convert to expressing the nutrients in the elemental forms is due to the perception that less fertilizer is being purchased for the same amount of money. A 100 pound bag of 10-10-10 containing N, P2 O5 and K2O would be equivalent to a 100 pound bag of 10-4.4-8.3 containing N, P and K.
Once the fertilizer is added to the soil, the oxide forms, P2O5 and K2O, are no longer used when discussing these two nutrients. The amount of these nutrients analyzed in the soil is expressed as the pounds per acre of P and K. The plant content is expressed as the percentage of P and K.
Forms of Soil Phosphorus
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.
of P present in the organic form in three soil series differing
in O.M. content.
of P in Organic Form
Soil Minerals - apatite [ Ca3(PO4)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".
Calcium Phosphates are formed in soils with pH greater than
Phosphorus in soil solution
Concentration is low 0.05 to 0.2 ppm
The solution concentration is affected by solubility and
amount in the solid phase.
Phosphorus fixation - The reduction of solubility
of fertilizer P that is added to the soil. Most frequently a
problem on acid soils.
Fertilizer P reacts with:
Al and Fe in soil solution
Exchangeable Al and Fe
Hydrous oxides of Fe and Al
Factors affecting phosphorus fixing capacity
amount and type of clay - Kaolinite clays usually have associated
with them a high Fe and Al oxide content.
Soil Acidity - at pH's below 5 there is a large amount of
Phosphorus level of the soil. If it has been built up over
the years, the P fixing capacity is satisfied.
Method of Application or Placement
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
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.
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.
Top Dressing - Very little reaches roots because of limited
movement in soil. Use only when necessary (turf, pasture)
Types of crops and conditions when crops are responsive to P.
Crops that have a short season (ex. snapbeans)
restricted root systems (potatoes and cotton)
growing in cool seasons (potatoes, cabbage, etc. )
Ability to use Fertilizer P
||% Fertilizer Recovered First Year
are amounts needed for maintenance application
Water soluble P - fertilizer compounds dissolve
Citrate soluble P - compounds that dissolve
in ammonium citrate
Citrate insoluble P - the portion that is
insoluble in both water and ammonium citrate
Available P - water and citrate soluble
Mining and Production Video
Rock Phosphate - Source of all P fertilizers;
deposits are found world wide. N.C. deposits have
been mined by Texasgulf Inc. (now
PotashCorp Phosphate) since the mid sixties.
Florida and North
Carolina produce about 85% of the United States
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.
Acid treated phophate materials
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
Superphosphoric Acid - (72% P2O5)
- Made by concentrating the 54-56% through evaporation used
in liquid fertilizers.
Ordinary Superphosphate - React sulfuric acid with
20 % available P2O5
(85-90 % water soluble)
contains 8-10% sulfur as gypsum (CaSO4)
used in mixing and blending fertilizer and is applied
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
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
Others - Nitrophosphates - Nitric acid
and rock phosphate. Ammonium polyphosphates
- Made by neutralizing superphosphoric acid with anhydrous
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.