Suppose a plant requires 10 units of P in an ideal soil. When switching to sandy highly calcareous soil >90% and pH of 8.4, what would you suggest to make up for the fixation please?
How much more will be necessary?
Will it make a difference to give the whole dose at once or rather divide it over a period of time?
@ Ben, to determine exactly the amount of P fixation in your sandy calcareous soil, please determine the P sorption isotherm of your soil. Normally, in calcareous soil phosphorus retention and immobilization take place due to precipitation and adsorption. Since soil pH is considered a major soil variable affecting the P sorption, an acidic P fertilizer could result in low P adsorption compared to alkaline one. In general, P fixation is highest in Alfisol.
This is an interesting question. But I think there's a misunderstanding underlying the question itself.
When we do soil testing and produce the resultant recommendations, they are based on the use of a specific soil test. In theory we could extract all the P from a soil in a soil test, but we never actually do that when assessing soil fertility. The amount of P we are considering available in the soil is relative to the soil test. Likewise, the phrase "an ideal soil" soil doesn't mean anything. All fertility assessments and recommendations are relative to the soil test used.
In the US, we use several different soil extractants for soil fertility assessment. And each is used for specific types of soils. Attached is an image from a 1994 publication of different extractants used in the US. Extractants may be considered useful for some soil type and not others. And the results from one soil test for a given sample will not be the same as the results from another soil test.
So, one soil test may report a P value of 100 kg/ha and another may report 50 kg/ha. Either could be "correct", because these values aren't absolute. They are just relative to the test used. And the crop recommendations are based on these relative values.
The upshot is that in your case, you would have conducted a soil fertility test on the "idea soil", and used the crop recommendations for that soil test. On the other soil, you would want to conduct a separate soil test, using the appropriate extractant, and crop recommendations based on that type of soil test.
These crop recommendations are essentially empirical. Based on a specific soil test value, adding a specific amount of fertilizer elicits a specific crop response. The numbers change based on the underlying soil test extractant and method used.
Image source: Chapter Current Approaches to Soil Testing Methods: Problems and Solutions
To add to Salvatore's comments, one should also conduct calibration studies an a particular soil test. This would include characterizing the P status of a number of soils in a greenhouse study to evaluate plant growth without add P and also with adding known rates of P to each of the soils. From this one can obtain preliminary calibration response curves for each soil and a generalized curve for the group of soils. Since field conditions are usually uncontrolled environments when compared to the greenhouse, one should then conduct field trials using the same techniques (soil without P and with known rates of P). Both of these are labor and time consuming. However, in this way, one can realistically determine the amount of P needed for a given soil based on a given soil test. These results may vary from region to region based on soils, crops and climatic variables (rainfall, temperature). There is no "magic" number that fits all situations and soils.
International Journal of Agricultural and Statistical Sciences
Availability of P in soil is highly determined by soil pH and it is hindered at pH of 8.4. The question is highly scientific and useful. I recommend following the steps as outlined @Larry Cihacek to determine the requirements. Best regards
Phosphorus is highly available at pH 7.22, in case of calcareous soil, applied P fertilizer get fixed as calcium phosphate, more the applied P more will be the P fixation and build up of Ca-P, hence in order to reduce P fixation go for application of organic manures and P solubalisers like PSB along with P fertilizer, generally phosphorus use efficiency may go up to 40 percent only hence applied P gets fixed and results in build up of P. first you test the available P content in soil, if its already higher than you reduce the amount of recommended P fertilizer and apply P solubalizers and also organic manures, this will help to reduce P application rate and also reduces P build up and improves P use efficiency. in care available P is lower then apply recommended dose of P or 125% recommended dose along with organic manures. application of organic manures reduces the P fixation and P build up and improves P use efficiency. Do not add more P fertilizer unless the initial available P in soil is low.
Besides the pH value, the are more factors influencing P availibility which you might consider.
The preceding post already mentioned the application of organic manures which not only contribute to P solubility induced by increased microbila activity, but contain a more or less big amount of soluble P themselves. In case you use mineral P fertilizers, take care that they are water soluble.
One of the best means to protect P from fast fixation in calcareous soils is band or even spot application into the soil - the concentrated fertilizer increases the time span until fixation takes place. Timing is also important - as near as possible to the main uptake period in the growing season.
If P availibility is a severe problem in your situation, the choice of crops could be limited to those which are strong in acquiring soil P. Here I agree with Salvatore that an ideal soil does not exist and that the specific soil, climatic, and farming situation have to be taken into account.
A really reliable answer to your question would require field or at least pot tests.
Check out a International Journal of Agricultural and Statistical Sciences
A very good question! To get equivalent yields of a crop from the soil with high P fixation compared to under normal soil (10 unit P), first, you should determine the critical levels of p requirement under that soil and that environment. But we are not sure about the level of available P under that situation. However, in general, higher rates of P are needed under high P-fixing soils than normal soils!
Experience in Australia is that in highly calcareous soils availability is (temporarily) greater when the P is applied in solution
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