Discover the science behind how phosphorus-based Agrochemicals and pesticides impact our environment. Explore eutrophication, water quality issues, and solutions for sustainable agriculture.
Author: Syeda Naqshe Zuhra
Importance of phosphorus
I. Important component of cell, take part in cell division e.g. H3PO4
II. Imp role in inheritance
III. Not taken as elemental form, taken up as protein, carbohydrates and fats
IV. It is essential nutrient for plant growth
V. Plant take it from native source and fertilizers
Source of Phosphorus
- Chloro-apatite
- Fluoro-apatite
Chloro-apatite is safer than fluoro-apatite. If these minerals are concentrate. Concentrate means to treat the minerals with H2SO4, HNO3. Impurities dissolve and precipitate with acid and minerals purified.
Fate of Phosphorous
- Every source of fertilizer Rock phosphate, MAP, DAP convert into ionic species (phosphoric acid) then available.
- Ionic species phosphoric acid converted into their species called speciation
- Availability of Speciation of ionic species depends on soil properties
I. Redox potential
II. PH
III. Calcareousness of Soil
This figure showed that redox potential increase with oxidized condition and lessen with reduced condition.
Dominant element in acidic soil at pH 6.5= Al, H, Fe
Dominant element at pH 8.5 = Ca
Dominant element at pH 10 = Na
This graph showed that at pH 6.5 dominant element are Fe, Al, H. Phosphorous precipitated with these dominant elements and become less-available.
Maximum available phosphorous at pH 7.5
At pH 8.5 P precipitated with Ca and become Calcium phosphate crystals and become unavailable
At pH 10 P precipitated with Na and form Na phosphate which is soluble so, availability of phosphorous is done. But max. availability is just at pH 7.5
iii. Calcareousness of Soil
More calcareous soil more Ca conc. Which adsorbs the phosphorus and form crystals of Ca-phosphate which make phosphorous unavailable to plant.
Q= What is the dynamic of Fe under wheat and rice condition?
In soils with higher pH, Fe is readily oxidized, and is predominately in the form of insoluble ferric oxides. At lower pH, the ferric Fe is freed from the oxide and become ferrous oxide, and becomes more available for uptake by roots. Rice field is always in submerged condition so, due to standing water in the field oxygen is not pass out so, it showed that it is reduced condition. In the reduced condition redox potential is low and pH is low and more availability of Fe+2and become available to plants. In wheat there is no submerged condition. It is always in oxidized condition. So, iron is in ferric form which is insoluble and not available to plants.
Environmental implications of phosphorus?
- Impurities
- Eutrophication
- Interactive effect with other nutrients
- Excessive intake effect human health
1. Impurities
Impurities associated with P are essential and non-essential elements
Essential= Fe, Zn, S (increase plant growth up-to certain limit)
Non-essential= Heavy metals (Cd) Small amount cause toxicity
This graphs showed that growth of plant increased with increase of essential elements such as Fe, S, Zn but at certain limit after excessive amount of these essential element reduced the plant growth. Non-essential element (heavy metals) not take part in growth of plant. But their increase in concentration reduced the growth after certain concentration of these heavy metals. Means non-essential (heavy metals) are toxic.
Some elements accumulate and some assimilates.
Accumulation: Metals conc. Increase in plants but have –ve impacts bcz they don’t participate in metabolic activity. (Accumulation sites in plant= Vacuole, mitochondria)
Assimilation: Metals conc. Increase in plants but have –ve impacts bcz they don’t participate in metabolic activity
Plant bodies have transporter, Cd store in vacuole, bark and leaves Cd moves with Zn and Ca. So, it moves in membrane and reduce the photosynthesis.
There are two type of fates in soil. If its conc. Of metals is more then it precipitates and if conc. Is less then it absorbed. So, Cd has less conc. So, it adsorbed.
As an impurity what is the role of Phosphorous? = Cd
Phosphorous is a source of Cd. So, if we apply more P-fertilizer its mean we add more Cd. In our farm soil Cd conc. Is 0.03 to 0.058.
Plant bodies have transporter, Cd store in vacuole, bark and leaves Cd moves with Zn and Ca. So, it moves in membrane and reduce the photosynthesis.
2. Eutrophication
Phosphorous fertilizer applies to agricultural soil. Eutrophication: excessive richness of nutrients (P and N) in a lake or other body of water, frequently due to run-off from the land, which causes a dense growth of plant life called blue-green algae.
Agricultural run-off: flush out soluble P which is readily available. Then it accumulates into ponds and small channels. Then blue green algae proliferate in ponds and small channels.
Irrigation through flooding: flush-out soluble P and enter into lakes and ponds
Effects of Eutrophication
I. Light penetration restricted
II. Temperature differences
III. Surface water occupied with algal blooms
IV. Marine animal come out the surface and do breathing because under the water O2 is less
V. Root of algae dead with passage of time. So, bacteria decompose these dead algae
VI. Marine life endangered
Bioavailability
Total fraction:
a) Available
i. Soluble
ii. Exchangeable (Physio sorption)
b) Un-available
i. CO3, HCO3
ii. Organically bound (Chemisorption)
iii. Fe, Mn oxides and hydrides
Bioavailability: it is the small part of total available fraction
Phyto-availability: Metal content just available to plant
Added metal in soil: 20 ppm
Bioavailable fraction: 19.9 ppm
Phyto-available fraction: 2ppm. This 2ppm conc. Is also very toxic.
How metals are bio-available to plants?
Availability of metals depends on following:
a. Ionic form
Cd in salt form is neutral e.g. CdCl2
Fe in salt form is neutral e.g. FeCl3
b. Ionic species type
Cd= Cd+2 = less available
CdCl+= more available (Cd more available in Cl affected soil)
Fe= Fe+2 = reduced form= more available
Fe+3= oxidized form= less available
c. Complexation
Metals are in complex form. With root exudates metals split into their ions and plants
d. Chelation
Fe-Chelates (Fe-EDTA) common application
Soil Properties
What is the role of mineralogy in availability of metals?
Soil properties also helps to metals to available to plant. There are different types of soils
1:1 type: Kaolinite: pH dependent charge = hold metal element in temporary base and easily available
OH-= increase pH
With increasing pH mineral structure change, O- become free which combine with any metal and make it available
- 2:1 type: Vermiculate: Isomorphic Substitution (Permanent charge) metals bound strongly via H- and covalent bonding
When metals structure isomorph means having similar structure with minerals then it replaces the mineral place. Al < Si < Mg
Al replace with Si and Si replaced with Mg
Also read: Implementation of Sustainable Agricultural Practices in Different Parts of the World
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