I need help interpreting my soil test.

Asked July 25, 2019, 3:36 PM EDT

I recently did a soil test for my back yard that I plan on seeding from scratch. I will attach a picture of the test results. My first question is why are my phosphorus levels so high and is there anything that can be done? Will this be a problem when I do the seeding? The directions say to apply phosphate and potash during the preparation of the soil. I am not sure why I would add more phosphorus if the phosphate levels are so high. I planned on tilling in fresh dirt and some compost when doing preparations, given these results should I skip the compost? What sort of dirt should I use to till into the existing soil? In addition do you have any seed recommendations for a shady lawn? Are there any distributors that you recommend? Thank you, Erik

Hennepin County Minnesota

1 Response

Thank you for your question.

Soil test results can be a wealth of important information. However, they can also be very confusing and as you discovered, seemingly contradictory. Chemical nutrients in the soil are generally in two forms: available and non-available. For a plant to utilize these nutrients they must be water soluble since that is the only way they can enter the plant. This is what “available” means. Commercial fertilizers are high in available nutrients though some times they may be “slow release” (i.e., slowly dissolving in water) or “quick release” (i.e., quickly dissolving in water). Non-available means that the nutrients are in the form of chemical complexes that are not easily dissolved in water though they might be over a long period of time. A trite example. Plants need iron. So if one put large amounts of iron filings into the soil, the soil would test high for iron but plants would be unable to utilize these filings for their iron requirements.

With these thoughts in mind, a few comments about your soil test results:

1). Typical of many soils in your county, the texture is coarse which means that it has a high sand content. Among other things this indicates that there is relatively poor water retention. This compounds the challenge of maintaining good turf. Your organic content of 3.9% is on the low side. Good turf should have an organic content of 5% or higher. By all means DO ADD COMPOST when you do your seeding. This should raise the organic content. Also when you start mowing, leave the grass clippings on the yard. In the fall consider mulching leaves into your turf rather than raking them up. Install a mulching blade on your mower. All of this will elevate the organic level.

2). The recommendation to add phosphate as a fertilizer simply means that while the overall phosphate level of your soil is high, it is probably not readily available to your grass. This is consistent with my initial comments. See:

https://www.smart-fertilizer.com/articlesaphosphorus

3). With respect to the recommendation of amounts to be added you will see that this is based on an annual basis. Usually half of the recommended amount would be added in the spring and the other half in the fall. Since you will only be doing a fall application, add just half of these amounts this year after you finish your seeding (0.5 pounds phosphate, 0.5 pounds nitrogen, 1 pound potash).

4). For your dirt, use top soil (NOT black dirt) and apply as you indicated.

5). Do not use bluegrass seed as it does not tolerate shade very well. Instead focus upon one or more varieties of fescue. See:

https://turf.umn.edu/news/finding-right-grass-seed

https://extension.umn.edu/lawn-care/renovating-lawn-quality-and-sustainability#seed

6). Lastly, you may find the following helpful:

https://extension.umn.edu/planting-and-growing-guides/lawn-care-calendar

Please contact us again should you have additional questions

Good Luck!!