Most important is to know the feeder root zone of any crop , and this zone will not vary much , despite other favorable soil conditions,since it is more of genetically controlled than any other mechanical factors. Very often , there is strong effect of seasonality on feeder root distribution pattern . So , fertilizer application needs to be tuned according to seasonality of feeder root distribution pattern . Therefore , we will confine to such soil depth only , rather than worrying for other depths , where there are lesser proportion  of feeder roots , unnecessary adding to the fertilizer doses, with much reduced fertilizer -use-efficiency.

Hi Jose,

Thank you for your answer. This answer I have expected. The recommendation in Germany is also to do the soil sampling in the topsoil, e.g. 0- 30 or plow depth.

But what about the subsoil, e.g. the depth of 30 - 60 cm? Do we need not consider this?!

What do you think, how high is the nutrient content in the depth of 30 to 60 cm.

Is it still relevant to the nutrient supply to the plant?

Thank you for a personal assessment? :)

Hello Sefan,

in my opinion it must be clear for which reason do you want to measure the amount of these nutrients. Then you also know if you should sample topsoil and/or subsoil.

I guess you want do the analysis because of the soil quality regarding the plant growth. Therefore you should sample at least the rooting depth of your plant species. Because the plant takes the nutrients from this soil layer.

In Switzerland mixed samples based on up to 20 samples per field are used for the nutrient analysis (see link or on the homepage of Agroscope). So you will get a value [nutrient mg l-1] which is representative for the field and the sampled soil depth. In a next step you can calculate how much K is stored in one hectare [t ha]. For this you can check the reference below.

Matthias

Reference:

Forschungsanstalten ART, ACW (1996): Schweizerische Referenzmethoden der Forschungsanstalten Agroscope. Band 1 Boden- und Substratuntersuchungen zur Düngeberatung (Acker-, Futter-, Obst-, Wein- und Gartenbau).

https://www.fibl.org/fileadmin/documents/shop/1158-bodenuntersuchung.pdf

Moin Matthias,

thank for your answer and the reference.

Your are right, with the comment sample the rooting depth, but we don't do it. We do the sampling just in the topsoil on farming fields, don't we?!

Best greeting to Basel, Stefan

I used to work with the traditional top soil only to find out later that working with root depth gave me sometimes shocking results. In some areas of southern part of Puerto Rico, top soil 7.2 pH, sub soil reading 4.5 pH with high and almost toxic levels of Al, Mn and Mg. I only work with root depth.

Hi Roberto,

thank you for your answers.

I have the same experience, for example, in the topsoil (0-30 cm) pH values around 6.5 or phosphorus values (P) of about 10 mg / 100 g soil and in the subsoil (30 - 60cm) significantly lower pH values of 4.5 - 5 and P values of

The normal depth is only 15 cm (Plough layer). If you are specifically interested you can look for deeper depths also depending on your academic interest.

In my experience, the highest root length density (RLD) is found within the top 30 cm of the soil profile. This is therefore the soil depth with the highest root activity. The plant is most likely to benefit from nutrients contained within this depth. Sampling wthin a depth of 0-30 cm would therefore be more meaningful.

Dear Stefan,

    For top soil nutrient analysis you can go for 15 cm (plough layer)..

The soil chemistry, physics and biology are dynamic. The plant root mass is also dynamic altering and adjusting its needs throughout its phenological stages. The soil analysis is none but a referential variable to work with. If you farm by dumping on the ground most of the nutrients at pre-planting then you may be limiting the genetic expression potential of your crop to say the least. During the rainy season, heavy metals like aluminum will not affect roots in oil palm, but once the rain recedes, in some areas found throughout Colombia, aluminum will be available to the plants at phytotoxic levels. So when do you best take the soil samples for analysis and at what depth to better manage your crop? Alfalfa is another crop whose root mass will continuously grow deeper with the years in the Sonoran Desert of Arizona. It is not unusual to find alfalfa tap root beyond 2 meters in length with active nitrogen fixing nodules at that depth! That is why I prescribe to conducting soil analysis to the different root levels. 

Hi Stephan

I have exactly the same problem with sampling depth, one of my objectives for  my Msc research is to measure nutrient movement  through out the profile in increments of 0-15,15-30,30-45,45-60 cm respectively. what I have observed is that the cations increase significantly through out the depth, and in the case of P, the opposite is true. I have samples that measure 50 mg/kgP (Bray 1) on average on the 0-15cm depth, and the below depths measure less than 10 mg/kgP. So my opinion is that 30-60cm soil measurements are important too especially for noting potential compaction problems that could be caused by high Mg as well as salinity problems. If I may ask, what is the objective of your research?

Portia:

Thank you for sharing this data. Very valuable and coincides with my field work results.

Dear Portia,

thank you for your answer and data.

The observation that the cations are washout into deeper depths, I have also recognized, in my research. Cations are subject to leaching especially on sandy soils and in regions with a positive water balance. Phosphor other hand, is not subject to leaching and remains in the topsoil. So I would explain your observation that the cation contents increase with depth and  the phosphorus contents will not increase with the depth.

During my research, I noticed that in multiple regression analysis, the nutrient content in the subsoil (30 - 60 cm depth) are a determining factor in relation to the yield level.

For this reason, my question which sampling depth is recommended by other colleagues.

In the below-mentioned publication, I describe my observation with a short note.

https://www.researchgate.net/publication/260502756_Part_Field_Management_Comparison_of_EC-value_soil_texture_nutrient_content_and_biomass_in_two_selected_fields

Conference Paper Part Field Management: Comparison of EC-value, soil texture,...

@Roberto what are you working on if I may? 

@Stefan, I have followed the link you attached on you previous response and I find the research that you do very interesting and its in line with what I am currently studying, thank you for sharing your publications.

In soil testing we normally recommend 6-8 inches or 15-20cm depth soil sampling, irrespective of nutrient. But in case of nirate N,profile soil sampling  up to30-60 cm is recommended as  nitrate can leach down the profile .For  growing arable or field crops there no advantage to sample beyond 30 cm as most of the inputs like manure and fertilizers are applied to surface layer and most of roots and stubles are also recycled into surface layer. Surface soil may better reflect the nutrient status of soil .For crops in horticulture specially in orchards sampling up to 60cm is recommended as roots go deeper and also  presence of some impervious layers may restrict the root growth. Sampling to deeper layers in horticulture may help to take up some remedial measures before the orchard crop is established.Also the soil conditions  can also be improved by the application of amendment like gypsum,lime and farmyard manure.

Most important is to know the feeder root zone of any crop , and this zone will not vary much , despite other favorable soil conditions,since it is more of genetically controlled than any other mechanical factors. Very often , there is strong effect of seasonality on feeder root distribution pattern . So , fertilizer application needs to be tuned according to seasonality of feeder root distribution pattern . Therefore , we will confine to such soil depth only , rather than worrying for other depths , where there are lesser proportion  of feeder roots , unnecessary adding to the fertilizer doses, with much reduced fertilizer -use-efficiency.

Let em put another response . Best course would be to study the rhizopshere zone by studying the root distribution pattern , that will predominantly decide the depth of soil sampling . Never forget to have indepth look at the seasonal variation in root distribution pattern , accordingly the depth of soil sampling , especially in perennial crops will vary..

The results from a study I conducted in coastal Kenya on a maize-legume intercrop showed that the zone of moisture extraction extended from the soil surface to a depth of  40 to 90 cm, depending on whether the two crops were planted at the same time or staggered. This implied that the effective rooting depth (ERD) was 40 to 90 cm. Effectively, substantial nutrient uptake will take place up to these depths. For crops with ERD similar to that observed in this study, sampling to any depth between 40 and 90 cm would give a better indication of the soil's potential to supply nutrients to the crop.

Under such conditions , we need to look at the relative distribution of roots vis-a-vis major proportion moisture extracted from different soil zones. And , we need to concentrate on that soil zones , where from , majority of nutrient supply comes . This is how , we need to define effective rooting zone ...ERD will  vary according to soil depth as well besides texture to be in a position to meet out both moisture and nutrient demand both..

 Drs Stefan and Hemedi,I support your observations.Plant roots respond to moisture availability and take up nutrients from the zone where they are located.In a column study,maize roots penetrated up to 60cm with greater percentage of total root length between 20-40cm in well watered situation.Withholding water greatly restricted root proliferation in  the upper part but resulted in deeper penetration and higher soil water depletion rates at depth,compared with well watered columns.The deep roots of the unwatered plants exhbited very high soil water depletion rates per unit length.Many times the tools and techniques available with us and our resources may not permit us for sampling to depths beyond 30-40 cm depth especially in field crops.

Root growth and water uptake by maize plants in drying soil. by Sharp,R.E. and Davies,W.J. J.Exp.Bot .(1985) 36(9):1441-1456. http://jxb.oxfordjournals.org/content/36/9/1441.short

Dear Stefan,

I recommend collection of both surface and subsurface soils for analysis of nutrients availability. In my research, I found out that (in open field condition) rice crop, popularly believed to be surface soil feeder, draws more than 40% of its total N from soil below top 15-cm layer (about 27% from 15-30 cm and 23% from 30-45 cm soil layers).

Hope you find this input useful.

It will always be good and make sense , no doubt in collecting soil samples from both depths, 0-30 cm and 30-60 cm . And , contribution from both depths will depend upon relative distribution of roots and soil moisture , without any generalisation . You can also study the root distribution pattern , how roots are distributed along different soil depths to arrive at database conclusion..

Dear Stefan and my other RG colleagues,

In my answer given earlier, there is one typographical error. Kindly note that rice crop draws about 13% (not 23% as typed earlier) from 30-45 cm soil layer. Thanks for noting the correction.

Concerning P, K, Mg nutrients, having information from 30-60 subsoil makes only cost without any possibility to control it. So it is useless information.