Where to test: choosing sample points
Since you cannot test ALL of the soil at all of your project sites, you will need to select a few places to test. This is called ‘sampling‘.
The exact locations where you will do the tests are called the ‘sample points‘. A key principle is that each time you test, you will do the tests at the same set of sample points, so that you can track changes to that soil over time.
If your project is working with multiple sites (e.g. 15 smallholder farmers, or 20 schools), and if your resources and capacity allow, we recommend that you aim to test soil at every site. You could train project participants (e.g. farmers, school teachers, children) to do the tests for themselves, or your team could visit the sites to do the tests.
If it is not feasible to test at all sites, you will need to select a subset of the sites. Keep in mind that the more sites you test, the better. However many or few sites you test, try to make the subset as representative as possible. For example if many of your project sites are located on hillsides, ensure that you include a good proportion of hilly sites in your test group.
At each site, you will need to choose at least two sampling points so that you can compare soil health in different situations. But first, decide which sites you will test at and who will do the tests.
Action Learning 3.3a: Decide test sites and testers
Make a list of all sites where your project is (or its participants are) working. This might be individual farms, schools, community demonstration gardens or other places. Given available resources and capacity, is it practical and realistic to test the soil at each of these sites?
If not, choose a subset of the sites where you will test. Remember to try to make the subset as representative as possible.
Decide who will do the testing – will it be your project team, or will your team teach project participants to do the tests for themselves? Think about project capacity and accessibility of sites, and the need to repeat these tests in future.
Where to dig?
For each site in your test group, you will need to decide two or more sample points. These are the exact positions where the test samples will be dug, and will be determined by your objectives at each site, as well as the characteristics of the site and the soil.
Deciding on soil sampling points in community projects involves several key considerations to ensure that the data collected is representative and useful.
1) Start by identifying the specific objectives of the project, such as assessing soil health, contamination levels, or agricultural potential.
2) Next, consider the landscape features, land use patterns, and potential sources of contamination in the area. It’s also important to involve community members in the decision-making process to ensure that local knowledge and concerns are addressed.
3) Finally, ensure that the sampling points are accessible and that the sampling methods comply with any relevant regulations or guidelines. This collaborative and methodical approach can help yield valuable insights for the community’s needs.
The method for choosing your sample points is called the ‘sampling pattern‘. To get meaningful data from your soil sampling, it is important to choose a sampling pattern and depth that fit your objectives. There are three basic pattern strategies: random, grid or zone sampling. We recommend using the Zone sampling strategy, which is good for projects where we want to monitor our efforts over time.
Zone Sampling Strategy
If you already know parts of the site that behave differently (due to soil type, past management, terrain, or yield history), divide the site into a few “management zones.” Then, sample each of these zones separately by taking a soil sample composite from it. Zone sampling uses existing knowledge – like soil maps or yield data – to draw boundaries.
Action Learning 3.3b: Identify the different zones in the land or sites that your project covers.
You could draw a sketch map of the site(s), or use an existing base map, and mark the zones. Seek input from people that know the ground well – project participants or field staff perhaps.
Guiding questions to help you decide where to do your soil sampling:
1. Define Purpose:
Have we clearly stated why we are sampling the soil?
Are we testing for contamination, fertility, soil health, or land-use impacts?
2. Understand the Site:
Have we mapped the project area?
Are different land-use zones identified (garden beds, edges, runoff areas)?
Are there known or suspected contamination sources (roads, old structures, industrial activity)?
Have we noted variation in soil type, vegetation, slope, or drainage?
3. Incorporate Community Input (if you work in community projects)
Have community members shared their questions and concerns?
Have we identified areas they prioritize for testing?
How do I choose where to sample?
Sampling points should be representative of the different zones that the project covers, and be accessible over time.
Our advice is to focus on as many sampling points (per zone) as you can realistically keep track of. In other words, the number of sampling points selected depends on your capacity for monitoring.
Generally, more sampling points means more significant results, but it also means more effort (time and resources) are required. We advise starting with a few zones (and sampling points) first, and gradually expanding so that the whole project area is represented.
If there are 10 zones, select the 5 most important ones, and define at least 2 sampling points in each. In subsequent years you can do the same for the other zones, or increase the number of sampling points per monitoring area.
Action Learning 3.3c:
Decide with your team/community for each site, how many sampling points you will do (and where). Remember to choose a minimum of 2 sampling points to be able to compare and observe the variation of your soil.
Also the soil sampling has to be done in an area where you (or for example kids from the schools) or machines (e.g. tractors) will not interfere so that you can really observe the soil characteristics with no disturbing elements that could be responsible for soil compaction for example.
How to record your chosen sampling points (when in the field)
1) Mark the location: GPS or name it and mark it (to be able to come back)
When defining sampling points, mark these physically (using sticks and labels) and digitally by recording their geocoordinates and depicting them in the digital polygons/map of the project site.
2) Take a photo at the same location, same direction (a ‘point photo’). It will also allow us to see the vegetation and its evolution.
Action Learning 3.3d: Create a Soil File or Folder
Prepare a shared file with all the site information you (and partners) already have (maps, previous photos, historical information for land uses).
You will be able to store and add the information every time you do more soil sampling so that you can share with the project funders and the community all the results that will support taking field decisions and report the work done and measure the impact in your projects.