MASTER GARDENERS: Safe amendments for the soil in your garden

While there are lots of things that can be added to a garden with beneficial results, there are just as many that have no benefit at all or serious consequences.

Master Gardeners web art

Those of you of a certain age will remember Pa Kettle enlisting the aid of Geoduck and Crowbar to help plant corn by placing a fish in each hill. While there is a benefit to using fish as fertilizer, bears, skunks and raccoons will also find the smorgasbord irresistible. It always seems to work in the movies!

A common question gardeners do have is “can I add that to my garden?” While there are lots of things that can be added to a garden with beneficial results, there are just as many that have no benefit at all or serious consequences.

Coffee usage has soared during the pandemic and you can imagine all those mounds of coffee grounds finding their way to the garbage. Coffee grounds are great in the garden! In the compost bin they add nitrogen (green material) to the pile. Used straight in the garden they add a small amount of nitrogen so are not a measurable substitute for regular fertilization. What they do add is organic material that helps aeration and loosens the soil to aid in drainage.

If you have a restaurant or coffee bar that supplies you with lots of grounds, they can also be used as mulch, imparting a pleasant aroma while keeping slugs at bay. Use them liberally around acid-loving plants like blueberries or hydrangeas but not around tomatoes; their allelopathic properties inhibit tomato plant growth.

People frequently question the use of wood ashes in the garden. My advice is this: proceed with care! Without a soil test to check pH, you are shooting in the dark. Soil that is neutral or slightly alkaline can become too alkaline to grow vegetables well. For example, potatoes need slightly acid soil and will become scabby if the pH is messed up. Even small amounts can affect pH and that can last for many years.


Wood ashes are an excellent source of lime and potassium, especially hardwood ashes and can be added to the compost pile, thereby lessening the chance of damage. In all cases keep them away from acid-loving plants like blueberries, azaleas or hydrangeas.

A common household product, Epsom salt, is often recommended for garden use to add magnesium sulfate. Tomatoes, peppers and roses sometimes benefit from adding it to the soil, but again, without a soil test, you don’t know if it is lacking. How much do I use? Is it necessary? Again, repeated use will accumulate in the soil and may cause problems.

Eggshells are often overlooked as a benefit to the garden. One of the common deficiencies in the soil is adequate calcium. The deficiency shows up in tomatoes and peppers as blossoms end rot when the fruit is beginning to mature. We add crushed-up eggshells to the hole when we plant tomatoes and peppers, thereby using up some of the eggshells (we raise a few chickens) and preventing this cultural problem when watering is uneven, temperatures are extreme, or there is root damage.

Another great use for eggshells is as a slug and cutworm deterrent. Sprinkled around the stem of the seedlings, they reduce the damage to plants from these pests. Pine needles used as mulch also reduce slug damage. It was often thought that pine needles make the soil too acidic but their use only affects the very top layer.

These local garden articles will reach you each week throughout the gardening season, but gardening information can be found year-round by clicking on "Yard and Garden” at the University of Minnesota Extension website, , or by visiting our Facebook page at .

Local Master Gardeners will respond to questions via voicemail. Call (218) 444-7916 , and leave your name, number and question.

What To Read Next
Get Local