Topic 8




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


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.

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

Mining and Production Video

a. 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 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.

F. Reclamation and Restoration of Land After Phosphate Mining

G. Phosphorus in the environment.

H. Cadmium and phosphate

I. Radiation and phosphate rock

J. Uranium From Phosphates



Back to the SSC 051 Website