If you want accurate fertilizer and lime recommendations for your pasture, the number of soil cores you collect, and how many acres each composite sample represents, matters just as much as the lab test itself. Collecting too few cores or sampling too large an area with one test kit is one of the most common reasons pastures may not respond as expected to soil test fertilizer recommendations. Let’s go over how to take a good composite so that you get the growth you should across the entire pasture.
What is a composite soil sample?
When you pull multiple soil cores across a pasture and mix them together in a clean bucket before sending a subsample to the lab, you are creating a composite soil sample. This composite represents the “average” fertility of that specific area. The goal is to dilute small-scale variability (like an old manure pile or a bare spot) so the results reflect the overall field, not a single lucky or unlucky core.
For pasture owners, thinking in terms of “management zones” is helpful. A management zone is an area that has the same soil type, slope, cropping/grazing history, and fertilizer or manure history. Each management zone should get its own composite sample.Starting a brand-new pasture? Aerial images and comparing aerial images across time with a tool such as Google earth can help you see how plants have grown on that piece of land over time and can give you a sense of what the different management zones will naturally be.
Image 1. Historical imagery showing consistent differences in forage production in a potential future pasture. Note how the natural management zones can be seen as consistent differences over more than a decade of imagery.
How many soil cores should I collect per sample?
MySoil recommends taking somewhere between 10 and 20 individual cores for each composite sample in a uniform pasture. For small, relatively uniform paddocks, 10–15 cores is usually adequate. For larger areas, or where you know there is more variability, aim for the upper end of the range (15–20 cores).
Key guidelines:
- Never base a soil test on a single core or shovel-full of soil.
- Treat 10 cores as the absolute minimum, and 15–20 cores as a best practice for most pastures.
- Use a zig-zag walking pattern across the sampling area, taking a core at regular intervals.
- Stay consistent with depth: for grazed pasture, MySoil recommends a six-inch sampling depth. If unable to obtain six-inch samples, a four-inch sampling depth can be used.
How many acres can one composite sample represent?
The number of acres a single composite can represent depends on how uniform the area is, not just its size. A flat, uniform 10-acre pasture with the same soil type and management history can often be represented by one composite sample. In contrast, a 5-acre field with steep slopes, wet spots, and obvious differences in forage growth may require two or more separate composites.
Practical rules of thumb you can use:
- For most uniform pastures:
- 1 composite sample per 5–10 acres works well.
- For pastures with unknown history or more variability:
- 1 composite sample per 3–5 acres is safer.
- For highly variable fields or where you want precision:
- Break the pasture into smaller management zones and sample each zone separately, even if that means multiple samples within a single 5-acre paddock.
This approach strikes a balance between cost and accuracy. On a small farm or homestead, it often makes sense to sample more intensively (more samples per acre) because total acreage is limited and each paddock is exceptionally important.
When should I split a field into more than one sample?
Instead of asking, “How many acres can one test kit cover?” a better question is, “Does this whole area behave the same?” Split a pasture into multiple composite samples when:
- Parts of the field look or perform differently (thin forage vs dense growth).
- There are obvious slope changes, wet spots, or soil texture differences (sandy vs clayey areas).
- Some sections regularly receive extra manure (around gates, waterers, shade, or feeding areas).
- A portion was recently reseeded, renovated, or managed differently than the rest.
In these cases, taking separate composite samples allows you to tailor lime and fertilizer to what each zone needs, instead of over-applying to already-rich areas and under-applying to poorer ones.
Step-by-step example: sampling a 10-acre pasture
Here is a simple checklist to follow:
1. Divide the pasture into logical zones
- Walk the field and note any obvious differences in soil type, slope, wetness, or forage growth.
- If everything looks fairly uniform, treat the whole 10 acres as one zone.
- If one end is wetter and always grows more grass, make that a separate zone.
2. Decide how many composite samples you need
- Uniform 10-acre field: 1–2 composite samples.
- Variable 10-acre field: 2–3 composite samples (for example, dry hill, mid-slope, wet bottom).
3. Collect the right number of cores
- For each composite, pull 15–20 cores at the recommended pasture depth of six inches using a zig-zag pattern.
- Avoid gates, mineral feeders, manure piles, and obvious odd spots in your “average” sample.
- Mix cores thoroughly in a clean bucket, then fill the lab bag or box with the mixed soil and follow all directions provided by the soil testing lab.
4. Label each sample clearly
- Use names your customer will remember, like “Future Pasture – Shallow Soil 1 acre” or “North Pasture– 9 acres.”
- Keep a simple map or note on your phone so you can match future samples to the same zones or map in your soil testing app if that is an available feature.
Why the number of cores and acres per sample matters
The more representative the sample, the more valuable the soil test. When you use enough cores and keep each composite to a reasonable, uniform area:
- Fertilizer and lime recommendations line up better with real-world performance.
- You avoid over-liming or over-fertilizing “good” spots just because they are mixed with poor areas.
- You can see trends over time for each paddock or management zone and adjust your program with confidence.
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