Why do we need to put phosphate fertilizers along with rhizobium?

Question

Isn't phosphorous a macro nutrient? So it should be present in the soil in sufficient quantities... but still we add phosphate. Why?

Answer 1

Phosphorus is indeed a macronutrient!!! If you're wondering why it is insufficient in natural soils then read on. Phosphorus is, in fact, the most limited of the three primary macronutrients in most natural soils (being nitrogen, phosphorus and potassium). This is because it is released very slowly from insoluble phosphates (on rock surfaces) and rapidly becomes “fixed” after being released, making it difficult for plants to obtain their phosphate ions (intake form of phosphorus). Few unfertilized soils release phosphates fast enough to support the high growth rates of many plant species. Also, plants tend to only obtain their phosphate ions from the little “polyprotic phosphoric acid” in the soil, they are often very deficient in phosphorus and for this reason phosphate ions are used in high quantities than nitrogen and potassium in NPK fertilisers (N:P:K = 1:2:1).

P is found in natural compounds in soil (however this is very limited); it is more often found in NPK fertilisers in the forms of: Orthophosphates ( H2PO4- or HPO42- ) and Polyprotic phosphoric acid ( H3PO4 ).

The following reasons are why phosphorus is a primary macromolecule and why it is VERY essential to plant nutrition;

• To build ATP (the energy currency of plants) from respiration ;

• To build DNA and RNA (genetic information) ;

• To build the cell membrane ;

• To aid the growth of a plant in its early stages (seedlings and root development) ;

• To perform phosphorylation ( the modification of the activity of various enzymes – this is responsible for cell signalling (which is the ability of cells to perceive and correctly respond to their microenvironments – aka tissue homoeostasis))

Answer 2

Phosphorus (P) is a macronutrient and, after nitrogen (N), typically the limiting factor for plant growth. Potassium (K) is also often added in fertilizers.

P is an important plant macronutrient, making up about 0.2% of a plant's dry weight. It is a component of important compounds, including nucleic acids, phospholipids and ATP. Consequently, plants cannot grow without a supply of this nutrient. P is also involved in controlling key enzyme reactions and in the regulation of metabolic pathways (Schachtman et al., 1998).

Certain plant species actively increase root densities around phosphorus-rich patches (Snapp, 1995), because it is critical to optimize it's uptake.

<sub>References
- Schachtman et al., Plant Physiol (1998); 116(2): 447-45).
- Snapp, Plant and Soil (1995); 177(2); 211-21</sub>

Answer 3

I'll try to give a wide answer, not only to why we have to add chemical fertilizers, but also to why this is not our best choice.

One thing is the isolated chemical compounds we add to the soil (like N, P and K), other is its natural fertility (which include a good part of the periodic table, and also many micro- and meso-organisms, what improves productivity and quality of products).

Sir Albert Howard, in his "An Agricultural Testament", reminded us about the danger of the specialization in the isolated chemicals, and warned us of the futile reasons why they came to be so widespread and dominant:

"The artificial fertilizers are being widely used. The characteristic of Western agriculture is the use of artificial fertilizers. The industries engaged in fixing atmospheric nitrogen for the production of explosives during World War I, had to find other markets; the use of nitrogen fertilizers in agriculture increased, and today most Western farmers based their fertilization programs in the cheapest forms of nitrogen (N), phosphorus (P) and potassium (K) available. What we might call the NPK mentality dominates the farms as well as research stations of agricultural research. Industrial vested interests in times of national emergency settled firmly and could no longer be dislodged."

(Sorry, it's been re-translated from a Portuguese translation.)

He also added that the only way to preserve the soil's original (and healthier) fertility is to keep animals along with the plants, like Indians and Chinese did for millennia and still do (whose farmers he used to call "my teachers"). The animals complete the natural cycle of the nutrients, and the high faunal diversity -- that exists when sufficient patches of native biomes are preserved -- helps fight against plagues. Both India and China are still countries of so-called "Megadiversity", even with more than a billion people each and such long history. We, Westerners, on the other hand, are destroying our fertile land in an alarming rate, as you can see with deforestation/desavannization in Brazil and many other countries in the Americas, places with far less people, and far less time of "civilization".

But, of course, all of this is only feasible in a world with a substantial peasantry, and not one who follows the machine-guided + urbanization philosophy that rules in Western world today.

Finally, some patches of land will naturally have more nutrients than others. That's why there has ever been "good lands" and "poor lands". And since we want to use them all to survive, we may end up using some chemical help here and there. But that should be seen as an exception, not as rule.