Soil Sampling

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- [Rachel] Soil testing is an essential tool used to determine soil fertility.

The information we gain is the basis for nutrient recommendations that help maximize profitable crop reduction in addition to protecting water quality.

How we sample our soils is going to be crucial.

Soil samples that you send to the lab should be representative of your field.

A very small amount of that sample is analyzed to determine soil test results and recommendations.

Inaccurate results are most commonly caused by errors in sampling: garbage in, garbage out.

If the soil samples are not taken correctly, or are not representative of your field, the results and recommendations are not going to be useful.

Soils vary due to underlying geology, topography and prior field management such as crops grown and crop rotation, fertilizer or manure application pattern, and tillage or no-till history.

So when you look at a landscape, soils are going to vary from farm to farm, field to field, and within the same field.

This can all make sampling a challenge.

By following basic soil sampling practices, you can minimize the effects of soil variability and get recommendations based on the average conditions in your field.

Sample uniform areas.

Don't send a sample that represents 10 acres of 10 different crops.

Alternatively, don't send a sample representing multiple fields that have different management histories and soil types.

You wanna sample a field with the same management.

Take a composite sample by taking a large number of subsamples and mixing them together to form one representative sample of a field.

At a minimum, you want to collect 15 to 20 subsamples from an area that is 10 acres or less, and taking more subsamples can further reduce variation.

One method of sampling is the grid method.

Using flags or geological features, you can break a field into grids and take a sample within each cell.

The number of cells within each field will depend on how intense you want to sample.

But again, we recommend a minimum of 15 to 20 subsamples.

Another method is to walk through the field in a zigzag pattern and take random subsamples.

When sampling, avoid atypical areas such as wet spots.

If those areas are large enough, you can sample them separately.

And lastly, sample between rows.

To take samples, a soil probe is ideal, because it takes an equal amount of soil from all depths.

If you are using something like a spade shovel, don't use all of the soil collected from the shovel.

Because of the shape, you're taking more soil from shallow depths, and the lower depths will be underrepresented.

Instead, take a small portion or a slice of the soil from the shovel that will represent all depths equally.

While taking subsamples in a field, a clean plastic bucket can also be handy.

Do not use galvanized metal buckets, as metal may react with soils.

We often get questions about soil sampling depths.

The key here is to follow instructions from the lab that you will send your samples to, because soil depth is important.

This table represents soil tests taken from the same field but at different depths.

As you can see from the shallowest sample depth and the deepest depth, results and recommendations will differ based on the depth that you sample to.

The Penn State Agricultural Analytical Services Lab does offer a soil fertility test.

You can go to the website by visiting the link above and find soil sampling instructions, which include soil depth guidelines.

For cultivated crop fields, including tilled and no-till fields, sample to a plow depth of six to seven inches.

For pastured fields, follow the same general soil sampling guidelines, but sample to a depth of three to four inches.

Once you've finished collecting subsamples in a field, mix subsamples in the bucket, and allow soil to dry.

Best practice is to lay the soil on paper by a window or in a well-ventilated room.

Do not place the soil in an oven to dry.

Once dry, package the sample as instructed by the lab, and wait for results.

Where to send your samples, lab testing for the majority of Pennsylvania soils include the Penn State Agricultural Analytical Services Lab, as well as others listed below.

These labs all use the recommended tests for Pennsylvania soils.

If you have any additional questions, contact your local extension educator, or visit us online.