Should we collect the bulk soil away from the base of the tree or the soil which falls off from the roots after vigorous shaking should be considered as the bulk soil?
Thats a very intelligently placed question . While working on perennial crops, most challenging job is to delineate the rhizosphere zone of the trees, either vertically or horizontally . Vertically , its 0-20cm representing more than 80% of feeder roots, by and large, however there is a definite contribution from sub-surface as well . Horizontally , we always define as below the perimeter of trees (often called skirt belt sampling ) . This is all about rhizosphere zone . While bulk soil must be between the two trees, avoiding the feeder roots , called non-rhizosphere soil. But , as the perennnial trees grow in size in terms of plant canopy , roots of the two trees start overlapping with each other , so the tree canopy. Under such situation , sampling soils from rhizosphere and bulk soil become more complex in nature. But , most conspicuous difference between rhizosphere soil and bulk soil , visually would be the density of feeder roots .
Rhizosphere soil sample can be taken from 0-2mm distance from roots while Bulk soil sample can be taken behind the mentioned distance from the root.
It is important to mark the feeder roots of the tree from which rhizosphere soil should be collected. Then remove the litters from the surface soil. Collect soil from the surroundings of the marked root digging up to 15 cm depth. This is the surface soil. 15-30 cm depth is the sub-surface soil. Be sure that the soil should be the near surrounding the roots.
Think on what data you want to achieve: the interpretation and discussion of your data can only be done with data derived ´with the same sampling strategy. Insofar there is no general answer to your question: if you will discuss it with data from literature: follow exactly the design under which this data were obtained. If in your own environment: keep sampling exactly the same for each treatment.
Thats a very intelligently placed question . While working on perennial crops, most challenging job is to delineate the rhizosphere zone of the trees, either vertically or horizontally . Vertically , its 0-20cm representing more than 80% of feeder roots, by and large, however there is a definite contribution from sub-surface as well . Horizontally , we always define as below the perimeter of trees (often called skirt belt sampling ) . This is all about rhizosphere zone . While bulk soil must be between the two trees, avoiding the feeder roots , called non-rhizosphere soil. But , as the perennnial trees grow in size in terms of plant canopy , roots of the two trees start overlapping with each other , so the tree canopy. Under such situation , sampling soils from rhizosphere and bulk soil become more complex in nature. But , most conspicuous difference between rhizosphere soil and bulk soil , visually would be the density of feeder roots .
The volume and distribution of roots in perennial fruit crops is thought to reflect that of canopy area, and it is often considered as a circular area in soil parallel to the canopy area with the trunk at the centre. Soil within this circular area could be considered as rhizhosphere zone.
See one of my publication for soil sampling procedure in fruit trees.
Cheers!!!!
Article Assessment of microbial biomass and enzyme activities in soi...
Our experiences say that , as the perennial fruit crops grow in age , their canopy ( when orchard age is in its peak orchard efficiency) is 1.2-1.5 times shorter in perimeter than diameter of rhizosphere zone. Or , rhizosphere diameter is 1.2-1.5 times the diameter of the canopy area. this complicates the issue of defining the rhizosphere soil versus bulk soil...?. And , therefore , we need to define the rhizosphere zone in terms of root density , like at what horizontal depth from tree trunk , there is a maximum feeder root density..and vice-versa for bulk soil.
As a thumb rule, soil adhering to the root surface is designated as rhizosphere soil and the res t of the soil is termed as bulk or non-rhizosphere soil. Generally rhizosphere effect was found up to 3 mm distance from the root surface.
Please find enclosed Handbook of Methods Used in Rhizosphere Research byJörg Luster, and Roger Finlay , an excellent publication for your reference and get a clearer picture about rhizosphere soil and bulk soil.
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