Don’t guess on garden fertilizer

COLUMBIA, Mo.– Don’t add fertilizer to your garden unless you know what the soil needs.

The best way to make sure your plants get the right nutrients is to have the soil tested, says Manjula Nathan, director of University of Missouri Extension’s Soil and Plant Testing Laboratory (SoilPlantLab.missouri.edu) and MU associate professor of plant sciences.

“We frequently get questions from customers like, ‘I apply fertilizer every year. How come my plants aren’t doing well?’ Most of the time the problem is they never have done a soil test and have been guessing on fertilizer requirements.”

Without soil testing, you may end up over- or underfertilizing, resulting in an imbalance of soil nutrients, she says.

Soil tests for lawns, gardens and farm fields are available through MU Extension for a nominal fee. A basic soil test report includes information about soil pH, organic matter content and key nutrients.

Nathan says lab tests are more accurate than an over-the-counter test kit, and they provide specific suggestions on fertilizer and other amendments based on what you want to plant, whether it’s flowers, vegetables, turf or trees.

A 2010 summary of lawn and garden soil tests revealed that about 55-75 percent of the soils tested had high or very high phosphorus levels, and about 65-80 percent had high or very high levels of potassium, she said.

Too much fertilizer isn’t just a waste of money. Overfertilizing can lead to brown or withered plants, and promote leaf growth over blossoms and fruit. Excess nutrients in runoff can find their way into streams, lakes and rivers.

Taking soil samples

For soil sampling, use a garden spade to remove a slice of soil, going about 6 inches deep. Break off the soil on either side of the slice to leave a strip about an inch wide and 6 inches long. Take about six samples from random spots in the garden. Avoid depressions and berms, which don’t give good soil samples. 

Mix the samples thoroughly and collect about 1 pint of the resulting composite. You can get free sample boxes and submission forms from your county MU Extension center. You can also download and print the form at www.extension.missouri.edu/MP555. Fill out of the form completely to receive fertilizer and lime recommendations for the plants you want to grow.

You can take samples to your county MU Extension center or, if you live in the Columbia area, bring them directly to the lab at 23 Mumford Hall on the MU campus. (There is free short-term customer parking directly behind the building.) A basic soil test is $10 per sample if you submit directly to the lab. There is a small shipping charge if you take samples to an MU Extension center.

Soil test results

A soil test report will include recommendations for adding nutrients, in pounds per 1,000 square feet (or pounds per acre for farms, commercial fruit and vegetable operations, and turf).

Another key part of the report is the pH, a measure of how acid (low pH) or alkaline (high pH) the soil is. Most fruits and vegetables grow best in soil with a pH of about 6-7. Soils tend to become more acidic over time, so you might need to increase pH by adding lime—an acid-neutralizing material typically made from crushed or ground limestone. If you’re growing plants that like acidic soil, such as azaleas and blueberries, you might need to add sulfur to lower the pH.

Specialized tests

Compost and manure can improve soil health, increase nutrient- and water-holding capacity, reduce erosion, and aid movement of air and water in the soil. They also contain nutrients and nourish microorganisms that release nutrients. This can reduce the amount of fertilizer your soil needs, so you might want to test your compost or manure if you use a lot of either.

If you are starting an urban garden, there may be dangerous levels of lead in the soil. While a soil test for heavy metals is expensive—typically $45-$75, depending on the test—you only need it once if results show safe levels.

The lab also offers tests for water, greenhouse media and plant tissue. Go to soilplantlab.missouri.edu/soil/testfees.aspx for a complete list of tests and fees. If you have questions, contact the lab at 573-882-0623 or [email protected].

For more information from MU Extension on lawn and garden topics, including free publications, articles and online resources, go to www.extension.missouri.edu/LawnGarden.