We are trying to develop different maps for P, K, EC and other related parameters affecting the productivity of soil.
However, the first few questions arise
1. What type of grids/grid size for sampling?
2. Most precise way to take coordinates for the sampling point?
3. How many samples per area?
4. What about depth of sample? Does it vary from crop to crop and Nutrient to nutrient?
Please share your experiences.
Most important is the optimum grid size which is hugely dependent on soil spatial variability both at surface and subsurface with respect to different parameters. ..
Dear Anoop Kumar,
So, in this case we will first take many samples to test the variability and then depending on variability we will adjust our grid size for future sampling?
Dear Colleague,
1) There are two types of precision soil sampling methods:
a) Grid sampling
b) Zone sampling
Please see: www.aces.edu/anr/precisionag/documents/Precision_Soil_Sampling.pdf
The size of the grid is usually one sample per acre (0.4 ha). But it depends of the size and the spatial variabality of the field. If the variability is not much, we can increase the grid size. Zone sampling is better due to its advantages. To create the management zones, we can use EC map, yield map, etc.
2) Since the soil attributes are not usually very much variable in a short distance (several meters), it is possible to use a low cost GNSS receiver to obtain the geographic coordinates of the sampling point.
3) The answer is above.
4) Depth of the sample depends on the crop. For field crops, it is usually 0-30 cm while for tree crops it goes up to 0-30 cm, 30-60 cm, and 60-90 cm.
I hope these answers help you.
Best regards,
Muharrem Keskin
In addition to what our colleagues rose, level of detail information required affects size of grid. For instance, 1km x 1 km, 2 km x 2 km, etc. As grid size increases, the ability of the sample to represent the area decreases. Therefore, there is no fixed grid size for sampling. It depends on the required precision, budget and type of study.
Birhanu , your comments are well taken , but i am tossing up a question , what should be the basis of deciding the size of grid -based soil sampling under such conditions where clues on variation in soil properties are so limited. ...
Dr. Anoop, the question is more clear now. The major factors affecting grid size include but not limited to: field or soil variability, the soil property to be tested, crop types to be grown, and cost associated with sampling and testing. Again to repeat there is no robust grid size for soil sampling. It varies with conditions. High within-field variability justifies dense grid sampling (i.e small grid size) so that the regular sampling points come fairly closer to each other. Meanwhile, researchers indicate that grid sampling at coarser than 1 sample for every 2.5 acres is not recommended, if the goal is to develop nutrient map; like the one you propsed K.
Irfan, any sampling procedure (Sampling distance) which provides minimum coefficient of variation should be adopted as optimum grid size for soil sampling..
Birhanu, you are right , I agree with you..
Regarding type of grid sampling:
There are two types of grid sampling.
(a) Grid point
(b) Grid cell
The best way to improve precision or representativeness is to take as many as sub-samples around the grid to form one composite sample for each grid cecell.
It is possible to GPS and GPS fed tablet mobiles to navigate sampling points. That is done following preparation of base map for the area where each grid points are clearly identified.
You need GIS-based variogram using geo-coded soil sampling sites and interpreting the variation based on coefficient of variation...
Concerning "number of samples per area" raised by Irfan, I would like to say again it depends on field variability, precision required, cost of sampling and testing, etc. For variable fields, we may need more samples to improve accuracy. Some researchers suggest at least 1 sample per acre which also varies with soil parameter to be tested.If possible, it is advisable to collect many sub-samples to make a composite sample to improve your accuracy.
Please read the following article.
Nani et al. (2011). Optimum size in grid soil sampling for variability rate application in site-specific management. Sci. Agr. Vol. 68 (3): 386-392
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