Hi Solomon,

Could you clarify what you mean by 'critical'? Are you asking about soil test phosphorus levels that would require P amendments or about soil P saturation where we might expect increased leaching losses above some threshold?

Steve

Hi Stephen,

Critical phosphorus concentration is soil test level of phosphorus below which the probability of a response to added fertilizer is high and above which the probability of such a response is low.

For development of critical P concentration in soil, it requires 1. correlation phase and 2. calibration phase.

First find out the  correlation of the amount of P taken up by a test plant with the amount of P extracted from the soil by different chemical extractants.The extractant having better correlation should be chosen for the soil P test.

Once the extractant (chemical solution) has been selected, it is essential to relate soil test level to relative crop yield so that a critical level can be identified (the soil test

level where P is no longer limiting). Thus, soils can be categorized as deficient or sufficient in plant-available P depending on how the crop response relates to the concentration of P in the soil test extract.  To do this, soil test P level is correlated with relative crop yield (expressed as a percent of maximum yield) to determine the ability of the extractant to relate the nutrient concentration to crop productivity. 

Critical P determination involves application of different levels of P fertilizer in the crop field to create P gradients. The critical levels may be different for different crops, soils, weather condition. Then, taking soil samples from the different plots receiving different levels of P, testing available P using various extracting chemicals and correlate these available P with yield data. Then, select the best extracting chemical that best correlate with yield. Using Cate and Nelson quadrant procedure, you can determine critical P level. The P requirement factor is the amount of P fertilizer needed to increase the soil P by one ppm. This can be determined using the same data set. I encourage you to visit the following link.

http://agris.fao.org/agris-search/search.do?recordID=XF2015047877

Good luck!

Drs. Naik and Mohammad are right on point.. There are a number of P extractants that are widely used by various scientists and Soil Testing Laboratories.  Some of these include Bray #1, Olsen, Mehlich I and Mehlich III for which there is ample calibration  information available in the literature as well as commentary on the procedures.  When you select an appropriate extractant for your soils and  the required procedures, then utilizing Dr. Mohammad's guidance, you should have the ability to make P recommendations based on your soils and soil test values.

I would simply emphasise that the tests per se do not have fixed critical values.  They must be calibrated experimentally as described, and between soils it is not unusual to for critical concentrations to vary by a factor of 3 and sometimes much more.  Lastly, the tests must be performed exactly as described, i.e. the results are operationally defined not well related to specific pools in the soil.

I would simply emphasise that the tests per se do not have fixed critical values.  They must be calibrated experimentally as described, and between soils it is not unusual to for critical concentrations to vary by a factor of 3 and sometimes much more.  Lastly, the tests must be performed exactly as described, i.e. the results are operationally defined not well related to specific pools in the soil.

Solomon, I think they are right on points, however, we need to know  the P concentration you are talking about. Is it soil total P concentration, soil available P concentration, soil water soluble P concentration and so on.  The soil might have high P but it does not mean the it is available to plants. Yes you might calculate the critical P concentration  but this will vary with  the type of plant, type of soil, the form of P, and most importantly time

Dr. Mohammad and Dr. Naik, interesting to note the responses

 Mr.Solomon,if you use chemical extraction procedures  to estimate available P in soil,say Olsen or Bray,the answers of Drs.Naik and Mohammad  will serve your purpose.But Mr.Maru has raised the issue of soil solution P.If one desires to determine the critial soil solution P and P requirement(based on P adsorbed),then one has to use adsorption isotherm to arrive at those parameters.These paramers are important in high P sorbing red and laterite/lateritic soils of the tropics.

Dear Annangi,

Sorry if I used your name wrongly. FYI, We used the Olsen soil P chemical extraction method.

There varieties of different phosphorus avalabiility testing methods in soils. Most common of them are Olsen method which is widely used for neutral and alkaline soils and Bray-Kurtz whgich is used for acid soils. The critical concentrations are therefore very mach related to the method used. For Olsen methods the sufficiency limits are 8-25 mg/kg soil. There are quite much difference between the lower and higher limits. This depends on largely plant species. The plants have very strong root development and realtively smaller P requirement may grow near lower limit whitout any economic yield loss. On the other hand, weaker root development  with high requirement, it is better to target the upper side of the limit in fertilization practice. In short, the P concentration and P requirement are method and plant dependent maters.

Dear Mr.Solomon,Two procedures are used to find out the critical limits of soil test P after determining P extrctable by some important methods   and conducting crop response  study in pot culture or field experiments.Cate and Nelson'scatter plot and Nelson and Anderson's statistical procedure.The following references may be consulted.

Cate,R.B. and Nelson,L.A.Jr.1965.Tech.Bull.Int.Soil.Testing.Ser.,North Carolina agric.Exp.Stn.,1,15.

Nelson,L.A.&Anderson,R.A.1977.In :Soil Testing.Correlating and interpreting the Analytical Results,A.S.A .Spl.Publ.29,19.

If available you may also consult one of our publications on similar aspect.

Soybean responses to applied phosphorus and sulphur on Vertic Ustochrepts in relation to available phosphorus and sulphur.Subba Rao,A. and Ganeshamurthy,A.N. 1993.J.Indian.Soc.Soil Sci.42,606-610.

Hi Solomon, as Dr. Rao suggested "Nelson,L.A.&Anderson,R.A.1977.In :Soil Testing.Correlating and interpreting the Analytical Results,A.S.A .Spl.Publ.29,19." is a very good publication to estimate both critical P concentration and P requirement factor. In addition, the following recent publications may be additional source of information in relation to your questions.

In Iran top-dressing P fertlization at alfa alfa fields is very common

The references attached by Dr.Agegnehu are pertinent to  the theme and the question asked.In the first one critical limits were worked out  for wheat crop by using both scatter plot of Cate and Nelson and also the regression model of Nelson and Anderson.In the second paper the critial  P concentration in soil using Bray 2 method and P requirement factor for barley were determind.I appreciate the good work of Ageghehu especially conducted under  farmer's field conditions.

Dear Dr. Agegnehu, Thank you for the documents.

All is possible by using P adsorption isotherms. and to calculate the internal or external phosphorus of any crop you can use the Boundary Line Technique (Webb 1972). The fertilizer requirements are crop specific and site specific and can be estimated as external and internal P requirements. the external P requirement can be determinated by plotting the solution P levels developed for crop growth against 95% relative yield of crop. The term “internal P requirement” can be defined as the concentration of phosphorus in the diagnostic plant parts associated with near maximum (95%) yield.

Good luck..