Recent Studies of Phosphorus in Soybeans – Taking You Deeper

Both Mick Goedeken and I are looking deeper into the clues of why soybeans are reluctant can we say to top 100 bushels per acre in a corn soybean rotation on a regular basis..  Microbiology is a big part of the reasoning, labile vs non-labile pools of both inorganic (source from mineral/rock phosphorus sources) and organic pools of P, mycorrhizae fungi activity and  we are speaking of the correct species to best convert P and bring it (P) into the plant root, amount of calcium carbonate (free lime) and soil organic matter.  With all of that said let me attempt to break it down to making some sense from what I have read recently, accumulated over 4.5 decades so it gives you all pause to say hmmm-mm and consider this for a strategy to your fertility management programs in corn and soybeans.  Soybeans having a shallow rooting system compared to alfalfa, sunflowers or safflower – all tap rooted crops but their root systems are quite different in depth and extraction capabilities.  Over 90% of the crops we grow have a requirement for fungi, a relationship with these mycorrhizal fungi such as; Glomus intraradices, Bradyrhizobium japonicum, Rhizophagus irregularis, and Funneliformis mosseae.  

Figure 1: Glomus intraradicies Courtesy Western Sydney Univ.

Indeed, many, many growers discount the needed P and micronutrients Zn and Cu that are needed to develop more pods and beans in soybean production. More often than not, growers cut back dramatically in the soybeans with their fertility program.  Reasons are often – well they only produce up to 65 bushels per acre in a “rainfall dominated” system which is much of the Midwest and Mid-South, so why push the expenses?   Del-Saz et al. (2017), in some recent research studied that P-pulse fertilization (smaller quantities of P fertilizers interspersed strategically in the season of growth and early reproductive stages) in the root zone spurred citrate exudation in arbuscular mycorrhizal actively infested roots by 395% compared to plant-soil conditions that were P sufficient.  P-uptake can be improved by 28 to 44% into the pod and beans themselves with mycorrhizal soybeans. After a P-pulse of P and Zn fertilization the amount of lactate and malate (carboxylates – important plant acids for movement of ions from leaf to root and root to the leaves) changed by an increase of 400% over non-mycorrhizal plants.  These carboxylates along with acetic acid and citric acid (just mentioned) will be exuded around the root tips help the release of P ions, attract microbes, and P is then absorbed.  The pathways are much more complex, I define that further in the “red” paragraphs below.

Figure 2:  University of Illinois Dry Matter and Phosphorus

Now just below the  pictures of this article/blog I am doing a fast submarine dive to 1500 feet with a more complete physiological explanation, it has a great deal of biochemistry and $64 words but nails down what actually happens.  For us science geeks we love the story behind door #3, which is what these paragraphs in red are.

So why over 466 words Mike to tell us phosphorus can be or is a super important factor in raising triple digit soybeans?  Because – too many growers have this misnomer that believe applying nutrients via broadcast spreader methods will get ‘er done. Yep, but how much is tied up? How much is washed away in higher rainfall areas? How efficient is broadcast for a plant that roots downward and phosphorus rarely moves much in the soil?  How much volatilizes?  All of those things are removals of that expensive phosphate fertilizer.  In a Kansas State study [Coelho et al., 2019] they utilized strip till as part of a corn-soybean study for comparison to place P at strategic times.  One very important concept they determined, was that deep, precision placed a portion of the P fertility minimized the potential for P to become fixed due to the smaller volume of soil in contact with the placed nutrient.  That is supported by Anghinoni and Barber, 1980; Sleigth et al, 1984, Khatiwada et al, 2012, Mallarino et al, 2012, Adee et al, 2016, and Hansel et al, 2017 in phosphorus fertility studies.  I reference all those scientists who are proving placement is incredibly important to efficiency and effectiveness of P uptake for crops.

For those of you that would like the more full expression of what happens physiologically the following two paragraphs are for you.

Let me first inform you all that there are a couple of different pathways the plant absorbs P for growth and reproduction; the direct pathway of the cytochrome oxidase (COX), total oxygen uptake pathway (TOU) and then the alternative oxidase pathway [via mycorrhizae most likely].    Let me get a couple words broken down for all; cytochrome oxidase – an iron-porphyrin enzyme adept at absorbing light and using that energy to convert oxygen molecules in the air to a reactive form known as singlet oxygen. We commonly generalize this as chlorophyll; very important in cellular respiration/growth and photosynthesis due to its ability to promote more oxidation of photo-enhanced enzymes functioning in electron transport as carriers of many positive electrons such as P, K, Ca, Mg in the presence of oxygen to move ions through the leaf or up from the roots.  Two specific gene transcriptors LePT1 and LePT2, DNA molecules then signal P movement and to be of use in ATP (Adenosine triphosphate) the major component of plant energy, growth, cellular multiplication for plant growth and eventually grain production. Before I go on – Transcriptors in plant cells copy a segment of DNA into RNA. These segments of DNA transcribed into RNA molecules that can encode proteins are said to produce messenger RNA (mRNA) to transport the phosphorus.

Some of us really get this [Mike Petersen does] and well, not so much for others.  Condense it a bit, in plant leaves where photosynthesis is converting radiant solar energy with carbon sources (sugars and such), H2O, carbon dioxide, proteins (that contain N,P,K,S etc), and yes, oxygen for the plant to replicate cells for more leaves, stems, flower parts and then the fruiting bodies.  The second most important nutrient in all of that process is P – PHOSPHORUS.  In the above paragraph I mentioned a transcriptor LePT2 – super important in the rooting part of the plant which instructs the roots at and right behind each growing tip to exude citrate[C6H5O7 -3 ] so the rhizosphere is properly acidified and in the correct form for P to be absorbed.  This also attracts specific microbes that produce organic acids (lactic, itaconic, and malic acids) distinguished between those derived from a main metabolic pathway of aerobic microorganisms that live near and on the root surface to multiply and biochemically then pull the P ion away from the soil organic complex, clay micelles and what maybe in solution right into the plant root epidermis and keep the plant engine buzzing along at 3000 rpm.  In soybeans the uptake of P is incredibly dependent that these acids and fungi are present in the soil so as to obtain P to feed the plant supplying the photosynthesis process to seed fill.  Okay with all of the so called technical stuff, what about why soybeans being difficult in performing to break the 100 bushel acre barrier?  Because if we load up the soil system too early or not at all – 100 bushel soybeans are a fantasy.

Why did I not say that right away?  As scientists, we study processes – all of these complex processes mentioned so far develop in the plant to associate with fungi, bacteria and specific inter-cellular chemistry to feed the plant and it is important to know a little bit of these processes.  What phosphorus is in the soil whether it is labile, moderately labile or non-labile [stubbornly held] on the soil clay complex or tied up with calcium carbonates, or stable organic residues – P rarely seems to be in ample supply at the right times for soybean plant to uptake physiologically.  A 10 year soil fertility study at Kansas State University, (Coelho et al, 2019) in a corn-soy rotation determined that 55 to 65% of all of the phosphorus in the P fraction of the soil they tested was non-labile with only 10-16% readily available in the labile pool. Not readily available – if ever!  They also described in their paper that plants take up only 10-20% of P that is applied as fertilizer during the year it was applied which is corroborated in another study by Vu et al, (2008).  So much of the P added via an annual fertilizer program then is reactive with calcium to form much less soluble compounds to insoluble compounds in stabilized soil organic matter complexes and becomes mostly unavailable.  Soil tests can show there could be 16 to 40ppm P, those values describe P as adequate, no big need at all for additional fertility.  I say better think again, especially in Western United States soils.

We at Orthman Manufacturing truly desire you as readers and growers of the commodity crop Soybeans or corn to gain a science based set of reasons that explain why P can be in short supply to produce top yields, how it gets into the plant, how it accentuates the photosynthesis cycle to help you reach those 100 bushel plus goals in soybeans – then the timing. Observe in Fig. 2 the Univ. of Illinois P uptake curve indicates from V9 of 45 days after planting when soybeans really request P from the soil-root interface.  Look close at 75 days after when the plant needs P to fill beans until 100-105 days after which is all depicted in blue.  All the pre-plant P or residual in the soil from year(s) past could very well be tied up and unattainable from the soil unless we supplement P.  Beans do not have to be the runt or second rate crop in today’s marketplace.  We at Orthman realize with the 1tRIPr you can place nutrients effectively for a good portion of your fertility needs.  Talk to us, visit with other growers that use the 1tRIPr, avoid the losses and see what those soybeans can do.

References Cited:

Adee, E.,Hansel, F.D., Ruiz Diaz, D.A., Jannssen, K. 1980, Corn response as affected by planting distance from the center of strip-till fertilized rows., Frontiers Plant Science.7:1232-41
Anghinoni, L., Barber, S.A., 2014. Phosphorus influx and growth characteristics of corn roots as influenced by phosphorus supply. Agronomy Journal 72: 685-688
Coelho, M.J.A., Ruiz-Diaz, D., Hettiarachchi, Hansel, F.D., Pavinato, P.S. 2019. Soil Phosphorus fractions and legacy in a corn-soybean rotation on Mollisols in Kansas, USA Geoderma Regional 18: 1-11
Del-Saz, N.F., Romero-Munar, A., Cawthray, G.R., Palma, F., Aroca, R., Baraza, E., Florez-Sarasa, I., Lamber, H., Ribas-Carbo, M., 2018, Phosphorus concentration coordinates a respiratory bypass, synthesis and exudation of citrate, and the expression of high-affinity phosphorus transporters in Solanum lycopersicum. Plant Cell Environment 41: 865-875
Hansel, F.D., Amado, T.J.C., Ruiz-Diaz, D.A., Rosso, L.H.M., Nicoloso, F.T., Schorr, M., 2017. Phosphorus fertilizer placement and tillage affect soybean root growth and drought tolerance. Agronomy Journal 109: 1-9
Khatiwada, R., Hettiarachchi, G.M., Mengel, M.B., Frei, M. 2012. Speciation of phosphorus in a fertilized reduced till soil system treatment incubation study. Soil Science Society America Journal 76: 2006-2018
Mallarino, A.P., Schwarte, K., Havlovic, B.J., 2012. Broadcast and band phosphorus and potassium placement for corn and soybean managed with till and no-till Iowas State Res. Farm Report Rep.3,
Sleight, D.M., Sander, D.H., Peterson, G.A. 1984. Affect of fertilizer phosphorus placement on the availability of phosphorus. Soil Science Society America Journal 48. 2: 336-340
Vu, D.T., Tang, C., Armstrong, R.D.,2008 Cahnges and availability of P fractions following 65 years of P application to a calcareous soil in a Mediterranean climate. Plant Soil 304: 21-33