Why do we see Phosphorus a limited nutrient?

by:  Michael Petersen, Orthman Agronomist


We are well aware for the most of us in the Farming world that phosphorus (P) is a limiting nutrient for top yields, stalk health, reproductive health right around flowering, early vigorous growth in corn especially and in western United States calcareous soils gets tied up.

On this site I will be looking into articulating what those limitations are.  Sure we see Universities talk at great lengths about N and some about P and its importance – but with the phosphorus mines across this nation being mined heavily, rapidly diminishing rock phosphate sources hold concerns about what the future brings.

Microbiologically the number of species that work directly on organic sources and the P fertilizers we apply are not that high in number.  They are predominantly aerobic (requiring oxygen to respire/live) creatures. Penicillium, Psuedomonas, and  I want to focus on Bacillus subtilis, member of the Firmicutes phylum.  This strain of bacteria are one of the most common that work on phosphate we fertilize with.

Bacillus subtilis is known to have a symbiotic relationship with the Azotobacter (a nitrogen-fixer affiliated bacteria) only six species of this cyst forming aerobic bacteria exist. These two work together on insoluble phosphorus within soil normally that is material like rock phosphate in dry fertilizers. The phosphorus gets trapped with the clay particles and begins to freeze ionic speaking, because the phosphate ion tightly bonds to the positive cations (calcium, iron, magnesium, silicon, and aluminum) found in the soil. B. subtilis coordinates with Azotobacter vinelandia by helping to release the phosphate bonds and release the phosphate (PO4) throughout the upper 6 to 10 inches of the soil profile. Without the addition of B. subtilis, the phosphorus can’t do its job effectively and further hinders agriculture. The phosphorus can’t move around to the plants and help maintain prosperous growth. Soil microbiologists consider B. subtilis and arbuscular mycorrhizae are  both a good alternative to insoluble phosphate fertilizers.

Scientists out of the microbiology world have discovered that B. subtilis and Azotobacter can be an aid to seed germination which is a big deal in certain seed crops like sorghum, canola, and small seeded vegetables. B. subtilis is able to take up DNA from its environment and creat antibiotics for itself and the host plant root it lives on to protect it from pathogens.  Quite the organism to aid a plant due to the net it makes to help its host.  See the figure below – electron micrograph image of the Bacillus subtillis colony and the net it produces around the colony. The strands you see are actually millions of these microbes swarming and releasing a slime layer which is what you see to the outside of the image, making the colony mobile to move about on root surfaces or on the soil liquid interface.  Probably more than you wanted to know, but think about the lack of mobility of P in the soil; here the microbes distribute what they use and secrete away.

Color electron micrograph of a colony of Bacillus subtilis on a media plate

Because B. subtilis is mobile with flagella (short string-like tails act as whips to scuttle the bacteria cells around on the root surface and in the soil solution) this bacteria can redistribute PO4 in the upper portions of the soil profile (0-9 inches) and feed roots in that section.

Another biological phenomena in the soil surface horizons is the arbuscular mycorrhizae that can infect roots to live inside the cortex of the roots symbiotically and bring N, P, S and Zn back to its host.  These ultra thin strands or hyphae that extend out of the infected root cells grows outward to access soil organics, humic acids, peptides, polysaccharides (complex sugars), a host of cellulosic materials – all to feed its host which requires simple sugars from the plant.  The plant gets the much better side of this relationship.

Microbes are extremely important to the breakdown of P in soils, can dislodge the tightly bound PO4 ions and make them able to interact with the roots.  Cyanobacteria, specific species of Glomus sp. mycorrhizae are included in the list of phosphorus solubilizing microbes which has Aspergillus sp., Penicillium sp., Trichoderma sp., and Actinomycetes a very robust group of bacteria in cropland soils.  Scientists have determined that the Actinomycetes are able to withstand dramatic temperature fluctuations from hot to cold and remain viable and energetic to solubilize phosphorus in the organic fraction and added phosphate fertilizers.  For those who are small grain farmers in with their row crops; the dryland farmers who have wheat and corn in rotation – you have an added advantage that wheat residues and old wheat roots are occupied by several genus of bacteria that remain to be phosphate solubilizers for the next crop.  For those of you that consider cover crops or companion crops, wheat has this relationship to continue a so-called home for these bacteria.

All great bits of information to provide you with tools and knowledge that the soils can be managed to aid with releasing P naturally and not use so much added phosphate.


Sources for this post:
Schaechter, Ingraham, and Neidhardt in Microbe. ASM Press 2006
Todar, K. “Todars OnLine Textbook of Bacteriology
Morikawa, M. Journal of Bioscience and Bioengineering, 2006 Vol 101, #1, 1-8
Sharma et.al., Phosphate solubilizing microbes sustainable approach for managing phosphorus deficiency in agricultural soils. Springer Plus 2013, 2:587