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Spring is a great time for soil testing

Author: Anne Sawyer

Believe it or not, spring will come. In spite of the snow that's recently swamped much of the state, the soil will soon be warming and it'll be time to plant before we know it.

Before you put anything in the ground, however, you should consider doing a soil fertility test to provide optimum nutrition for your plants and avoid unnecessary fertilizer applications. The most reliable way to determine how best to fertilize is to do a soil test.

Soil testing: Why, when, and how often?

A soil test provides information specific to your crops and soil. It also incorporates past cropping history, giving you nitrogen 'credit' for leguminous crops incorporated into your rotation. You can save time, money, and angst by receiving lime and fertilizer recommendations rather than guessing what might be best. By applying only what's needed, you will also minimize any adverse environmental impacts from fertilizer overuse.

Excess soil nutrients contribute to problems such as algal growth in lakes (too much phosphorus) or nitrates in groundwater (too much nitrogen). And while you can buy home soil test kits, they are not calibrated to the soil characteristics we have in Minnesota, nor do they provide recommendations specific to your situation.

Sampling before or after planting is generally most convenient, such as in early spring or late fall. With diverse crop rotations in vegetables, you may prefer other times of year. However, to compare results over several years, sampling at a consistent time is best. If you suspect a severe nutrient deficiency during the season, you can also sample then, but it may be too late to completely 'fix' a deficiency once crops have shown symptoms. You may still be able to improve overall growth, though, and be better prepared for the following year.

In general, you should consider a basic soil test once every two to three years for annual crops, depending on the intensity of production and fertilizer/lime application needs in previous years. Testing once every three to five years may be sufficient for most berries, orchards, and grapes.

What you can learn from the U of MN soil testing lab

Kale and cabbage seedlings, some showing nutrient 
Photo: Anne Sawyer

While there are several private soil testing labs in the region, the University of Minnesota Soil Testing Laboratory also offers soil tests specific to commercial fruit and vegetable growers at highly competitive prices. The 'regular series' soil test ($15) will give you estimated soil texture, organic matter, phosphorus, potassium, pH and lime requirement. Additional tests are also available, such as calcium and magnesium ($7); zinc, iron, copper and manganese ($12); boron ($7). Nitrate to 24" ($8) and sulfur ($7) are also offered, but are only recommended under specific circumstances, as described in the instructions on the soil test submission form.

The analysis will also come with an interpretation of results and recommendations for fertilizing based on what you want to grow and your anticipated yields. For more information about yield estimation and fruit and vegetable nutrient requirements, refer to the Nutrient Management for Commercial Fruit & Vegetable Crops in Minnesota bulletin. You can also test for lead ($16) and/or soluble salts ($7). While most Minnesota soils are generally considered nonsaline, soluble salts can be problematic in high tunnels and should be monitored, particularly in soils that are not well-drained.

How can I know if I'm getting a good sample?

Remember that the soil test is only as good as the sample! When collecting your soil sample, you want it to truly represent the growing area of interest. You may need to submit multiple samples for large and/or very diverse growing areas. Regardless of your sample area, you'll want to collect multiple subsamples within your 'representative area' and mix them all together before sending approximately two cups of mixed soil to the lab

Where to sample in fields
In fields, a representative sample should be from an area with fairly uniform soil color and texture as well as similar cropping history, including fertilizer, lime, and manure applications. Samples should represent no more than 20-25 acres, and no more than 5 acres if the area is hilly, has highly variable drainage, or other obvious inconsistencies.

Collect 15-30 subsamples in either a random (A, below photo) or zig-zag (B) pattern, across the entire area. You'll need fewer subsamples in more uniform areas and more in highly variable areas. In outdoor raised beds, collect samples from the beds themselves rather than from areas between beds (C).

Soil sampling in fields. Adapted photos/graphics from Dr. Paulo Pagliari

Where to sample in high tunnels
If sampling in a high tunnel, collect subsamples within growing beds. If sampling the entire tunnel area, collect at least 4-5 subsamples per bed. If beds are performing differently or you have varying fertility needs in each, you may want to consider analyzing soil from each bed individually.

Example subsample placement in a high tunnel. 
Photo/graphics: Dr. Paulo Pagliari

Where to sample in orchards and vineyards
When gathering soil samples in an orchard or vineyard, collect samples within rows, between the plants (see photo, below). Take 15-30 subsamples within a representative area - usually 5 acres or less, depending on variability in topography as well as soil and/or plant growth characteristics.

Disperse the subsamples throughout the planting, either in a random pattern or evenly spaced along rows. If sampling evenly along rows, the number of subsamples per row will depend on the number and length of the rows. For example, in a small orchard with 15 rows, collect 2 subsamples per row.

For perennial fruits, soil nutrient analysis used in conjunction with tissue nutrient analysis can be particularly useful for managing fertility as well as for diagnosing nutrient issues throughout the growing season. For more information regarding proper procedures and timing for plant tissue sampling, refer to the Nutrient Management for Commercial Fruit & Vegetable Crops in Minnesota bulletin.

Example subsample placement in an apple orchard.
Photo/graphics: Annie Klodd

How to collect samples

You'll need a clean (no rust) shovel, trowel, auger, or soil probe to obtain soil; a clean plastic pail to mix subsamples; and a bag in which to put your final sample for the lab. You can use a bag supplied by the lab, or you can put your sample into a plastic bag or small, disposable plastic container.

For each subsample, you'll first want to scrape the soil surface clear of plant residue and litter. Then, collect a sample from the top 6-8" of soil for annual crops and 10-12" for perennial crops. If using a shovel or trowel, first dig a hole to the required depth. Then, obtain a 'slice' of soil from the side of the hole to get a uniform sample throughout the profile and avoid getting more soil from the top and less from the bottom.

Collect a 'slice' of soil for a good subsample.

Once you have collected all of your subsamples in your bucket, mix them together and fill the sample bag or container with approximately two cups of soil. Ensure that it's tightly sealed, print out the horticultural sample submission form, and mail or deliver your sample to the lab.

What about my results?

The Soil Testing Lab's website is updated to reflect current soil testing turnaround time, so you know when you can expect your results. The report you'll receive will include not only the analytical results, but also any recommendations for lime and fertilizer necessary for your crops. There are sample reports available from the Soil Testing Lab as well as other resources to help you interpret your results.

And as always, if you have questions, contact either the Soil Testing Lab or your local Extension office for assistance.

Anne Sawyer, Extension Educator - On-Farm Food Safety/Horticultural Systems

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