Phosphorus is immobile nutrient in soil. Mostly only mobile nutrient are absorbed with mass flow. So phosphorus is taken by diffusion method

My understanding has always been that if nutrient diffusion is taking place, their must also be some amount of mass flow. According to the NRCS, plant uptake of phosphorus is sourced from mass flow, although not significantly, and they do not give a thorough explanation of the mechanics of why.

Root extension (Purdue) is also a noteworthy mechanism that plants have that can transport* [thanks, Chandrika] conservative amounts (6-10% of plant needs) of phosphorus to the plant. I think their numbers are a bit assumptious but perhaps not farflung.  It looks like its a good article and goes on to discuss your question with more detail but I am not at my university internet and do not have access to the rest of the article beyond the cover page. The discussion of mass-flow begins at the bottom of page 1. 

pubs.acs.org/doi/pdf/10.1021/jf60127a017

www.nrcs.usda.gov/wps/portal/nrcs/detail/ia/soils/?cid=nrcs142p2_008201

Nutrients such as phosphorus and potassium are absorbed strongly by soils and are only present in small quantities in the soil solution. These nutrients move to the root by diffusion. As uptake of these nutrients occurs at the root, the concentration in the soil solution in close proximity to the root decreases. This creates a gradient for the nutrient to diffuse through the soil solution from a zone of high concentration to the depleted solution adjacent to the root. Diffusion is responsible for the majority of the P, K and Zn moving to the root for uptake. Table 3 gives the relative importance of each mechanism in positioning nutrients adjacent to plant roots for uptake.

Becaouse phosphorus solubility is very low in soil solution, the elements such as phosphorus and potassium take from the soil with diffusion. On the other hand N, Ca and Mg take by diffusion due to high solubility in soil solution.

Your question is a case of where a general principle is taken as a rule, which does not apply to all situations, Cameron's comment above is correct "there will always be some mass flow" The determining factor on how much is in mass flow depends on the Anion Storage Capacity of the soil (ASC) If the soil is dominated by coarse grained Quartz sand, then the ASC is low and mass flow will be significant. Conversely soils with high Calcium, Aluminium and/or Iron where the ASC is high then mass flow will be negligible. And in the case of Allophanic Volcanic ash soils mass flow is almost non-existent.   

For me the question is a little confusing. P is transported to roots mainly through diffusion as it is stated by a number people here, due to low solubility and slow release. But plant uptake is a different matter. To cross the membranes and get into the cell it needs an energy driven transport process mediated by phosphate transporters. These are two different things.

Due to very low mobility of phosphorus plant absorb it from soil by diffusion. That is why phosphorus is generally recommend to apply in soil in full dose during final land preparation or before as it takes more time to be released and very slowly becomes available to the plants through the process of diffusion. 

Answers to your question wrote by otners colleagues seem summarize very well the dynamic of phosphates in soil soultion and the dynamic in the soil-plant system.

Phosphate ions are bound to soil constituents, the strength of binding depending on the nature of the soil, and so do not exchange freely with the solution. The concentration of P in the solution is therefore generally very small. Plants may obtain the bound ions by an ion-exchange process, involving protons secreted by the plant. This takes place at the root/soil particle interfaces, so it may not even involve a true `diffusion' process.  The importance of the mechanisms is shown with, for example, subterranean clover which takes up P tightly bound in soil and acidifies the soil, to the detriment of agriculture. 

Infact , P is such a nutrient which i is intervened maximum  by root interception mainly  through  diffusion , because of processes like adsorption , fixation and release , more stringently than any other nutrient . The extreme immobility of P also encourages to be intercepted by roots through diffusion , but there is some  P-uptake through mass flow as well depending upon the soil moisture level. 

The role of mycorrhizal fungi in root-symbiotic protein-mediated P delivery should not be ignored in this conversation. Ecological / biological phenomena are demonstrating significant contributions in recent research. While chemical mechanisms are classical, conventional, and thoroughly discussed, biological mechanisms are quickly gaining traction. Simple chemistry reveals that diffusion simply does not make sense when dealing with concentration gradients with orders of magnitude differences. The mechanisms involved must frequently involve the use of energy. Please let me know if you would like me to show you some research, however I understand there to be  plenty out there.

As Cameron writes "the role of mycorrhizal fungi in root-symbiotic proton-[ NB not protein] -mediated P delivery" is of major ecological significance and cannot be ignored.  Obtaining P from many soils  is extremely difficult but the biological systems have found a way.

Yes, role of AMs cannot be ignored in P-delivery to plants . soil chemistry and soil ecology have to go in perfect harmony withe each other.

How plants uptake heavy metals?? Mass flow is the principal mechanism or other mechanism also involved in this?? and why the plants adopt a specific phenomena to absorb heavy metals??

The root plants exudates to stabilize, demobilize and bind the contaminants in the soil matrix, thereby reducing their bioavailability. These all are called as phytostabilization process. Certain plant species have used to immobilize contaminants in the soil and ground water through absorption and accumulation by roots, adsorption onto roots, or precipitation within the root zone. This process is for organics and metals contaminants in soils, sediments, and sludges medium . Plant roots take up metal contaminants and/or excess nutrients from growth substrates through rhizofiltration (=root) process, the adsorption, or, precipitation onto plant roots or absorption into the roots of contaminants that are in solution surrounding the root zone. This process is for metals, excess nutrients, and radionuclide contaminants in groundwater, surface water, and wastewater medium .

Contaminant uptake by plants and its mechanisms have been being explored by several researchers. It could be used to optimize the factors to improve the performance of plant uptake. The plants act both as “accumulators” and “excluders”. Accumulators survive despite concentrating contaminants in their aerial tissues. They biodegrade or biotransform the contaminants into inert forms in their tissues. The excluders restrict contaminant uptake into their biomass. The range of known transport mechanisms or specialized proteins embedded in the plant cell plasma membrane involved in ion uptake and translocation include :i proton pumps (″-ATPases that consume energy and generate electrochemical gradients),ii co- and antitransporters (proteins that use the electrochemical gradients generated by ″-ATPases to drive the active uptake of ions), and iii. channels (proteins that facilitate the transport of ions into the cell). Each transport mechanism is likely to take up a range of ions. A basic problem is the interaction of ionic species during uptake of various heavy metal contaminants. After uptake by roots, translocation into shoots is desirable because the harvest of root biomass is generally not feasible. Little is known regarding the forms in which metal ions are transported from the roots to the shoot .

Heavy meta is taken up by plant through mechanisms called phytoextraction, phytostabilisation, rhizofiltration, and phytovolatilization ...

Anoop Kumar Srivastava Sir many thanks for your detailed explanation. In short, we can conclude that plants uptake heavy metals by specific mechanisms i.e phytoextraction, phytostabilisation, rhizofiltration, and phytovolatilization. So there is no role of mass flow, diffusion or root interception ?