Interesting question.Available P ,normally we estimate by extracting with an extract ant like 0.5 M NaHCO3(Olsen's reagent )suitable for neutral, alkaline and calcareous soils.We estimate only inorganic P in the extract and organic P is not estimated.The estimation of organic P in Olsen extract can give an idea of labile P in soil.During crop growth both inorganic and organic P sources supply P to crop plants.The contribution of organic P can be estimated by mineralization study but is not normally done because it requires more time for incubation and results can not be obtained quickly.One alternative is to group the soils according to organic carbon status ,say  

In Pakistan, we are working on P deficiency, How to increase P availability in soil and P interactions with other plant nutrients. For this purpose we are hereby running a project based on use of acidic P fertilizers to improve the response of added fertilizers. Interestingly we observed that increasing soil organic c by adding organic amendments, response of added P fertilizer regardless of the source (DAP, SSP, and phosphoric acid).

Adane your question is intelligently flagged .If i  have correctly got your question , do we expect the soil C:P ratio to behave the  same way soil C:N ratio upon application of  P-source or N-source , whatever the case is ? The response provided by Nadeem proves it explicitly , that yes , narrowing of C:P ratio with the application P-fertilizers will trigger the mineralization and  consequently add to the labile P-pool of the soil , easily accessible to the plants.

There is a positive correlation between organic carbon and phosphorus. By addition of organic carbon, availability of phosphorus increases due to chelating of polyvalent cations by organic acids and other decay products.

There is a relationship between the carbon and phosphorus in the soil. The terms of this relationship are complex. In general, the critical value of C / P ratio in the soil is less than 100

In unfertilized, undisturbed soils, even if vegetated by say forests or grasslands  there will be obviously be a relation.  Higher organic matter will mean higher organic P. But in agricultural soils under high P fertilized intensive agriculture, organic matter may be low but available P can be high. The only constant will be the C:N, C:P, C:S ratios of soil organic matter in a given soil type  

Interesting discussion, and valuable answers!

I would also go for the positive correlation between soil organic carbon and available phosphorus by direct action (as mentioned by Suresh) and indirectly by action of microorganisms such as Pseudomonas spp. and others phosphorus solubilizing microbes.

Please fin attached a review paper on phosphorus solubilizing microorganisms, and methods for phosphorus analysis both organic and inorganic

Dear colleagues,as mentioned by Dr.D.Rao, we should remember that  the C:N,C:P and C:S  ratios are always computed from total nutrients(C,N,P in soil organic matter),not from available plant nutrients in soil.Second point is mineralization of P from soil organic P( present in organic matter in soil) is different from added organic matter.Mr Adane referred to soil organic carbon,not added organic carbon.Dr.Malhotra referred to added organic matter.Dr. Sarwar also referred to added organic matter.Added organic matter on decomposition and mineralization releases P from the material it self.It produces organic acids  which by chelation/complexation release soil P,The proton generated and phosphatases  produced by the action of P solubilzing organisms also mobilize  soil P.In the absence of fresh organic matter, the mineralization occurs in soil from soil organic P.In the study referred to by me earlier from PunJab ,India, soil organic carbon up to 0.2% did not influence crop yield,between 0.2-0.6%,it influenced yield and reduced the fertilizer requirement( bothOlsen P and soil organic P provided P) .At organic carbon>0.6% there was no response to added P indicating that Olsen P and soil organic P provided sufficient P to crop.So with appropriate regression and calibration one can use Olsen P and soil organic carbon as measures of plant  available P in soil and compute fertilizer P requirement of a crop.

Mineralization and immobilization are related to C : P ratio.  If C: P ratio is less than 200 :1  mineralization and more than 300 : 1 immobilization happened.

Mr. Imran,You summarized  very well  what I stated earlier.I compliment you.

When dealing with Phosphorus it is found both in 1) available and 2) inavailable pools. In general terms the role of soil organic matter or Carbon is well correlated with higher levels of both total and available Phosphorus levels.  For optimizing most crops you may not need to worry much if your soil P 1 the olsen Phosphorus is over 30. The P 1 is probably most correlated with crop response. High C/P ratio can lead to immobilization and narrowing it can lead to mineralization which is the soluble form PO4.

Allophanic materials oxides and weathered clays can work to fix Phosphorus and make it unable and constitute a big constraint high crop yield and quality in many environments.  A soil may have good total P quanitity by low P 1 remember that is a measure of available pool.

Certain plants such as pigeon pea in the tropics and buck wheat have notable ability to chelate deficient Phosphorus in soil and make it available. This

About 80% of the Phosphorus is mobilized through the mycorrhizal fungi which are unseen to naked eye. Although the crucifers notable do not accept mycorrhizal 80% of all species mobilize the Phosphorus resource through them. The area explored by a mycorrhizal root can be hundreds to thousands of times more giving added capacity for Carbon sequestration and drought avoidance and tolerance.

When soils have very high available Phosphorus they turn off susceptibility to mycorrhizal fungi conserving the photosynthetic carbon for their sole use.

Excessive tillage and lack of active plant covers can drastically reduce the a activity and number of mycorrhizal fungi. Cover cropping and no till can be used as solution as well as including pastures into the crop systems.

Factory farming of animals has put excessive P in the fore front of nutrients deteriorating fresh and aquatic habitats.

The Glomalin protein secreted by the mycorrhizal fungal is hydrophobic and seems to bind soil particules giving aggregation in soil necessary of optimized drainage and aeration.  With good activity much less available Phosphorus is needed in the soil to optimize crop productivity.

some authorities are getting concerned about the sustainability of Phosphorus production and lack of distribution to areas of most need.

On behalf of all participants I thank  Dr.Hepperly for the detailed information provided on the role of mycorrhizae and the conditions under which one can get good benefits of the organism.Although some institutes/companies came up with mycorrhizal cultures for use in crops ,farmers have yet  to get benefit of mycorrizae .In tropical conditions like India the summer temperatures and soil moisture regimes may not help to establish the cover crops.As mentioned earlier ,in soils with high P status also the befit of mycorrhizae  may not be exploited.One important question is how to exploit he benefit of mycorrhizae independent of or relative to P solubilizing bacteria ,fungi and actinomycetes.Are the soil pH or other conditions differ for these organisms?That means can we use them under different conditions?

As a suggestion of a cover crop for tropical plantations try perennial peanut also known as ornamental peanut.

It takes some time in initial cover but has some wonderful ability to be a permanent cover has some shade tolerance is a good ability to fix Nitrogen and can be mowed without being lost.

This type of cover can make it easier to walk around the field. This permanent cover allows the mycorrhizae not to be lost from breaking of the roots in tillage.

It also is good for animal forage and pretty to look at.

Hi, Adane

On the figure the results of our researches on the content available phosphorus in the soil of field crop rotation (end of the second rotation), estimated by Kirsanov’s method (0,2 mol•dm-3 HCl), and content active organic matter (AOM), estimated by method alkaline hydrolyses of soil nitrogen compounds (8 M NaOH during 144 hours) are presented. Spearman's correlation coefficient – 0,707.

I think that this information will be useful to you.

Dear Zenon, Nice work on organic C availability and the mobilizing of Phosphorus.

Is the take home that the available C is driving the Phosphorus solubility in sequence?  What is your take home message?

Paul

Dr.Hepperly and Dr.Zenon,I find some sort of negative relationship between the active carbon and available P in soil.The plots which received manure recorded more active carbon but low available P. Where as the fertilizer received plots have less active carbon and more available P.Also the available P values in general are higher in the experiment .So we need more details of the experiment to discuss the results.

One of Interactions with Phosphorus in acid soils  is that soluble Silicon will stimulate its solubility and absorption resulting in stimulated crop development. It can be conditioned by its ability to reduce Manganese toxicity that is found in some acid oxisols.  Weathered tropical soils are also notoriously for Phosphorus retention or fixation.

If you have an acid soil the Manganese liberated can be toxic especially in high water conditions where chemical reduction leads to excessive solubility and contents that are toxic are generated. 

In an neutral and alkaline soil the reaction is  quite different. First we would more likely  encounter Manganese deficiency than toxicity and in neutral soils Phosphorus is much less likely to be severely limiting.

It may be that Phosphorus solubilization is attuned organic matter under the neutral pH condition. However the organic matter  could be negative in this regard in the acid environment because of toxicity issue in soil as Manganese would lead to issues.

When we look at the liberation of Nitrate in soils they some a peak release related to peak daylight also timed with peak microbial activity in the soil is seasonal. The soil economy basically comes from the soil organic matter and is dependent of environmental constraints for its mineralization. Mineralization needs to peak with peak vegetative growth to get peak crop development and yield. When soil reserve is low the dwindling supply will limit late season crop development.

Phosphorus fixation and retention is a big issue particularly in tropical weathered acid soils but also in some volcanic soils.

Best approaches for Phosphorus particular in the tropics and volcanic and organic soils need a lot more attention I suppose.

In a acid soil infertility situation the situation is complex because we need to simultaneous deal with multiple toxicities and deficiencies. It is also the great frontier because that is an area with abundant sunlight and temperature for rapid crop development.

Some food for thoughts.

The figure (in file) shows the role of pH in the formation of active organic matter (AOM) and availability of phosphorus in acid soils. Pay attention to difference soil pH of 0-20 and 21-40 cm (next file).

Interesting Zenon . In  both the figures , it is almost the same point of intersection  for pH and available P with treatment NPK+ siderat compared to control. And same is the scenario with respect to soil pH with the same treatment  at both the depths at 0-20 cm and 21-40 cm . While in control , it is altogether different soil pH  at both the depths. how do you conclude , few words of opinions,  you can add to let the discussion go further please.

The trend I could observe is that the farm manure buffered the soil pH quite well  and maintained it at higher level.It also maintained available P at higher level.But the straw could maintain pH at higher level but the available P is at lower level, indicating some sort of immobilization.

Much of the origiinal soil work was all focused on Nitrogen and its application. The flaw of this is the high fossil fuel cost and the long term degradative effects of ammoniated fertilizer on soils. especially those which are acid or acid prone.

This is why we need to focus on legume based farming systems as proven out in rhe Rodale Farming Systems long term field trials. If we have healthy legume components in our farming system and through them we can supply ample Nitrogen for top yields without the high energy cost, economic input and soil degradation of ammoniated inputs. This is the result of biological systems approach. 

What makes this discussion even more important is the Phosphorus limitations in the tropical world in particular and the need to address this through the beneficially use of manures. The integration of animal and plant systems show the best promise for this need.  The Phosphorus component is particularly useful along with micronutrients in assuring biological Nitrogen fixation is optimized.  I see Phosphorus as both problematic and an opportunity in relation to manure and its beneficial use.

For discussion it can be useful for looking at individual nutrients and their cycles but for benefits to occur our academic insights need to be applied into farming systems that are proven through Long Term Farming Systems Trials. This will avoid the soil degradation issues the world now faces if this can be done intelligently into the future.  Our research tends to short term but our need nutritionally and ecologically are long term issues especially if we look at the neglected role of the food system for reversing greenhouse gases. 

When we talk about reversing climate change through reducing greenhouse gas our discussion will inevitable go to long term monitoring of the content and accural or disappearance of soil Carbon. The soil carbon is critical to water first and foremost which is agricultural principle limiting component. But also the nutrients for crop production are all transferred to the plant from a mineral in solution even when we talk about mycorrhizal transfer. 

Even when we look at the physical, chemical and other constraints we must never lose track that the agricultural system is a biological system and the biology works through physical and chemical and geological materials. All of this points to a new for long term perspectives and team work across many interest groups. 

I think that the the increase of Corg in the soil was systematically associated with a decrease of P available under alkaline alkaline soil. In the contrast , in acid soil , the increase of SOC can improve P soil availability through orthophosphate stabilizing.

While  The soil carbon is critical to author factor as water, organic matter...........so P availability was almost varies with these factors

When we talk about organic carbon we have distinctly different Carbon pools represented with different roles and reactions involved.

First we have hardened Carbon which is not soluble and shows low degradation. Second we have on the other hand the available soluble dissolved Carbon has very different quality and capacity. This active carbon is chelator mobilizer and readily food source for plants and associated microbes.

The dissolved organic carbon would be readily labile and its ability to stimulate microbial activity would like increase the available P through microbial solubilization favoring plant rooting establishment and growth.

On the other hand the presence of humin type of organic Carbon would react very different and may well have more effect as a promoter of fixed Phosphorus if it complexes with the humin material rather than being mobilized by the soluble organic Carbon. C;P ratio and acidity alkalinity both are known to influence the relative solubility.

While immediate response is favor by labile Carbon the humin would more likely govern water retention and soil structure. In reality we need relatively high levels of both for maximum plant production environments.

For maximum Phosphorus availability we need a sufficiently low C:P ration not to immobilize and a soluble available Carbon content to be a ready microbial food source can help mobilize Phosphorus when the C:P is not exceeding high.  To reiterate the alklanity and acidity and presence of metals will also interact.

When we look at the mineral study the mycorrhizal plant can have 7 times the ability to get Phosphorus from the soil than a non mycorrhizal counter part. All of these interactions are related to complex equilibrium reactions influence by the physical, geochemical, chemical and biology factors involved.

What is so wonderful about mycorrhizae is that they work to engage in the low Phosphorus environment by the plant stimulating them showing a type of biological  intelligence by their ability to turn on and off as needed. In a purely chemical approach we may lose the biological system reactive nature.

In tropical countries, though additions are organic manures is recommended, the P requirements of crops may not be met adequately at the recommended doses of organic manures, thus making it necessary to apply P fertilizers. Unfortunately, this has not been the case generally!

When we look at the deficiencies of Phosphates for plant, animal and human nutrition, we must remind ourselves like all nutrition we are dealing with a goldilocks situation we must be able to balance the quantity and availability within a zone of not too much and not too little.

On the economic stage of the major elements it is most expensive and geologically the materials for its manufacture are largely concentrated in but a hand full of countries the use trend suggests the global cache may be exhausted within a century or so if use patterns continue. Because of this situation the use of manures will need to better exploited into the future especially when peak Phosphorus lifts its ugly head.

 Phosphorus is notably a lesser mobile mineral than some of the other nutrients but if soil erosion is unabated it is lost into the water system causing the loss of fresh and salt water resources through eutrophication. In relation to Phosphorus the concern needs to first focus and reducing soil erosion loss taking the Universal soil loss and being able through our agricultural forest and grassland management to assure we are not bleeding away our future.

The use of raw manure directly does not represent that case. When raw manures are composted and used strategically this makes are regenerative use as the benefits of Phosphorus are less likely to be lost in the water system.  We need to continue to have our farming systems improved so they are recycling the major nutrients into soils for long term society benefit rather than continuing to enlarge aquatic dead zone from open cycle nutrient cycles which are promoted when erosion is not conclusively controlled. 

United States Geological Survey Report 1128 shows the impairment of the Chesapeake Bay a major North American estuary system which is endangered receives 59% of the excess Nitrogen from synthetic ammoniated fertilizer and 32% from raw manure use our studies at Rodale Institute Farming Systems showed that using a legume based farming system would provide all the requirement for a maize and soybean crop system and would require any addition of synthetic fertilizer in addition when the animal system is incorporated with the plant production system Phosphate is retained and increased by strategic use of manure composted and applied. The use of cover and no till are also important as they assure the benefits to the soil do not get washed away. 

During this study the soil organic matter has returned to over 5% which was case of the virgin landscapes. Many of the world productive soils have 2% or less soil organic matter for generation of open cycle land management which has left our precious earth depleted. We need to utilize our modern science to reverse the deleterious issues related to our nutrient use. 

A soil fertility program needs to be anchored on soil conservation where the soil erosion issues are conclusively conquered. 

Dr.Hepperly,I appreciate you for summarizing and putting at one place  all major points pertaining to P fertility management (keeping the environmental aspects also in focus) .If some of these points  are well taken  care ,both sustainability and environmental issues will automatically be  addressed.

In addition to all previously very interesting quoted, I underline the very significant correlation obtained at much experiments, between soil organic matter biologically decomposed ,and the liberation of available amounts of Phosphorus into soils.

Yes Mr Nikolas, Last our results show a significant correlation (r2=0.79**) between C-BM (microbial biomass) and P available in the soil under P deficient conditions

It is the formation of the absorbing complex.

 For Mr. Anoop Kumar Srivastava, In our last results under low P conditions, The major finding of our result show that C/N ratio for Micoial biomass was almost significantly decreased compared to +P. These results were observed during 2 growing seasons and under calcareous soil. I think that this question should be more explained. But i think we must focussed on C/N or C/P ratio for microbial biomass to better understing the changes.

If we look at Liebig's model for limited nutrition the idea is that the most limiting nutrient will limit the overall system of plant growth. For nutrition Nitrogen can be limited from the soil but not more legumes. After all about 80% of the air is gas Nitrogen.

Phosphorus has no gas state can be truly limited in the soil and cannot be taken out of an atmospheric supply. Moreover with sufficient Phosphate legumes do optimize the biological Nitrogen fixation. Even with Phosphorus the total amount may not really be limited just the proportion that is solubilized. About 80% of all plants can get about 80% of their Phosphorus requirement through mycorrhizal fungi which are not considered by a strict soil chemist.

Chemistry and biology ideally are used together and support each other. The efficiency of limited Phosphorus will be favored when soils are kept covered with active roots this prevents the loss of soil the biggest issue limiting our world crop system. The most available Phosphorus source in tropical low infrastructure areas will be manures and their long term ability to retain and recycle will come when properly composted. All of these issues are not going the help unless we are conserving the soil through conclusive reductions of erosion to below the Universal Soil Loss Equation.

considering the predicted unsustainable nature of current Phosphate use we will need to optimize the use of animal and other wastes beneficially into the land resource and target the increase and maintenance of mycorrhizal fungi and root Phosphate soluble bacteria along with renewed incorporation of legumes into our cropping systems to achieve better Phosphorus utilization and when done well the soil carbon levels will aid in all aspects of the use of water which is key limiting factor in our global crop production system.

Although 75% of all Phosphate is used in Industrialized temperate economies 75% of the real need is in areas of the tropics such as India African and South America.

This is quite a concern about their allocation of scarce resources to provide a lifting of all ships and promote a social equity. I am not sure how or even if this could ever be done.

Dr.Hepperly and Dr.Latati,very interesting discussion continuing on the topic.I agree that the world is disproportionately placed with respect to P mineral reserves and also the distribution of manufactured P fertilizers.But it is not easy to help the disadvantaged nations.The countries should become economically strong(to purchase fertilizers) and also use available resources efficiently.To utilize the biological systems efficiently and to utilize the P present in soil organic matter we have to understand the nature of soil organic P.Organic P in soil may range from 20-80 percent  with an average of 50% .Of this 50%,only 30-50% is well characterized(I can be corrected).Phytate (penta and hexa phosphate esters of inosital) is the most abundant identified Po compound in soil comprising 50%of the total Po.The other P compounds present in small amounts are phospholipids and nucleic acids .Phytate is  strongly bound to soil P adsorption sites and is probably more strongly held to solid phase than the inorganic P.Most recent studies using 31P nuclear magnetic resonance spectroscopy helped to identify two species of organic P in soil-the high molecular weight fraction and low molecular fraction.The high molecular fraction(>10kDa) is P bound by phosphomonoester linkages of supra/macro-molecular structures accounting for nearly 60-70 percent of soil organic P.This may be the P bound to soil humus.So major portion of soil organic P may not be easily available for ready mineralization .One more point to note is that mineralization is more easily facilitated in soils low in available P.Also substrate  availability in the form of fresh organic matter like a legume straw  is needed.Apart from C:P ratio in straw ,certain  minimum amount of P in straw is important,say 0.2-0.3%for easy mineralization.So,the amount of  free phytate in the soil,P sorption capacity,available P in soil and amount and nature of organic material added(P content) decide the mineralization of P in soil.Of course the extracellular enzymes(in rhizosphere) phytases and phosphatases of plant origin,the similar enzymes produced by microbes for mineralization are important.C:N has also role in decomposition and mineraliation.Microbial biomass mineralization is not a big problem compared the other factors and processes mentioned above.Now we need to do a lot of basic work on P mineralization in soil.

Earlier, I gave an example of changing the content of mobile Phosphorus, the active phase of organic matter (AOM) and pH. Now, I add a change of mobile forms of Potassium in the soil profile (in File). Moreover, Phosphorus and Potassium were determined in the same extract 0.2 M HCl (Kirsanov´s method). I think that these data will be useful to continue the discussion.

Dear Dr Latati , I am so sorry for responding to you so late . Let us always consider the much better sensitivity of  soil microbial biomass nutrients ( C, N, P etc ) than soil microbial biomass or the available index of different nutrients .  While debating on this issue , it will  be an interesting exercise , how does microbial biomass nitrogen , microbial biomass carbon and microbial biomass phosphorous undergo changes in response to changes in soil  C:N ratio , C:P ratio. It will be still interesting to test , how C:N ratio or C:P ratio buffer under either low N or low P  fertility soils .

Dr.Zenon,I once again looked into the files attached by you. It appears to me that the active organic carbon increased with farmyard manure application and decreased with the straw addition ,possibly indicating that the FYM  has role in enriching the active or labile carbon pool .FYM has also buffered the soil pHKCl and maintained it at higher level .FYM+NPK has also maintained higher pH compared to NPK alone.pH remained low with straw but higher than NPK alone.I do not know the benefit of siderat .But it further reduced the subsoil pH(lower than surface soil).The available forms of P and K increased with FYM application and also with NPK+FYM. But both available P and K decreased with straw addition.With straw plus NPK the available K content slightly increased.In the well decomposed manure, the amount of P and the nature of carbon pools appear to influence the active carbon in soil,the pH buffering and the release of P and K through mineralization.Possibly the low P content in straw and its slow decomposition appear to not much influence the soil active carbon,pH buffering and nutrient availability through mineralization. Others may add or correct my interpretation.On microbial biomass nutrients please look into the following paper. C:N:P stoichiometry and nutrient limitation of the soil microbial biomass in a grazed grassed site under experimental P limitation and excess.Griffiths et al Ecological Processes 2012,1:6 open access.

Dear Adane ji

Scientists different corners have well discussed on C:P ratio in soil.  In my view, as I understood, it is not correct to correlate C and P.  You can correlate C:N:S but not P. Because all labile P does not come from organic alone.  While N and S mainly comes from organic source. Hence, some times we do not get  +ve correlation if we look into organic C and labile P levels.

Dr.Rajakumar,I agree with your view.As mentioned by me 9 days ago,C:P ratio is not the only criteria for facilitating organic P mineralization. At least four conditions favour mineralization of soil organic P.1.Low available P in soil which works as driving force  for microorganisms to act on organic matter in soil to acquire P.2.Addition of fresh organic matter containing relatively high P like a legume straw helps to enrich the labile pool and facilitate decomposition by microbes.3.The amount of organic P available for mineralization,either fresh P or already existing P in soil.I have already gave some details of quantity and nature of organic P in soil in my earlier reply.4.microbial biomass C:N:P may not be a limitation in mineralization of P as microbial bodies maintain certain constant  but low ratios compared to added organic materials 'C:N:P ratios. Microbial biomass on the death of microbes(when carbon source is a limitation),undergo decomposition and mineralization and supply P to plants.But water ,temperature and aeration should not be a limitation for the process.

Mr.Imran,mycorrhizae can can contribute to mobilizing and acquiring P in soils but they may not meet the requirements of 80% of crop  plants.There should be sufficient native or residual P in soil for mobilization by mycorrhizae.

I agree with Dr Rao view.

If available P content in soil increases, then obviously organic matter production will increase, because, during plant synthesis of organic matter, always some amount of P is accumulated, although lower in amount in comparison to N and C.

Dear colleagues, my personal works concerning the subject, showed a strong positive correlation between increased soil microflora activity and in P-Olsen soil enrichment; the P bioavailable amounts liberated are not too large, but the effect is significant in large scale cultivations. The effect is also very significant by using natural rock phosphate as P basic application; effectively the P solubilized amounts in P are larger in soil with lesser alkalinity but where soil organic matter promote soil microflora activity.

Answering to the question, you have to take into account: I) Fresh Soil Organic Matter promotes soil microflora biological activity, II) Intensity of activity of Soil Miicroflora promotes P solubilisation and III) P-solubilisation increases available- P amounts

Nikolaos Chouliaras I agree with your view, fresh organic matter addition, may be in the form of roots, stubbles or manure/compost form, may increase the available P in soil by increasing the microbial activity and through microbe mediated P mineralization and solubilization processes. P present in the organic input and soil P both may contribute to increased P availability through the above mentioned mineralization and solubilization processes. Hydrogen ions(organic acids) produced during the decomposition process and P hydrolyzing enzymes(phosphatases) may be involved in the processes.During the crop growth the root exudates produced may also help in P solubilization avaiability to crop plants.

I agree very much with Rao; he added very important details concernig available -P, particularly that concernig the originated from soil organic matter and soil microflora.