Fertilizers and beyond

Rob Zimmer
What is the best fertilizer to use for flowering shrubs, such as roses? That all depends on your soil test results. Find out more in this week's column.

Many of the questions I receive from gardeners deal with fertilizers, soils, pest control and other products from the garden shelf. Visiting the garden center can be an intimidating experience for many gardeners, confused by the hundreds or thousands of different packages that line the shelves.

Over the next few weeks, I'll take a look at some of the products found on the shelves at your local garden center and help make some sense of it all.

This week, the topic is fertilizers, soil and soil amendments. These products are often the most overused, misused and misunderstood gardening products. This misuse has an impact, as we see during the summer when many of our lakes and rivers turn bright green with algae bloom.

Step 1: Have a soil test done

What type of fertilizer to add to a garden, lawn or container is probably the top question I receive from gardeners during the beginning of the growing season each year.

For some reason, over the past 20 or 30 years, gardeners have been brainwashed or trained by marketing companies to assume that they "must" add some type of fertilizer whenever they plant.

This misinformation has resulted in excessively over-fertilized lawns and gardens across our state.

A simple rule before applying any fertilizer is to have a soil test done through your local UW extension office or garden center.

A soil test offers an excellent profile of your soil conditions, indicating what, if any amendments or fertilizers are needed in order to grow their crops that you wish to grow.

Soil test kits are available at your local UW extension office, as well as at many garden centers and hardware stores. These are sent out to be tested, with results returned to you that give you priceless information about your soil make up.

Instead of guessing and wondering what types of fertilizers and amendments your soil might need, have an inexpensive soil test done and take the guesswork out.

Don't be intimidated by the rows and shelves filled with fertilizers, soils, amendments and more. Rob Zimmer will help you make sense of it all.

Making sense of fertilizers

When you visit your local garden center or home store, you'll discover dozens of varieties of fertilizers available on the shelves.

Your soil test will help narrow down which choice to make based on the results sheet you are provided.

If you are unsure what your results mean, bring the results sheet to your local garden center or UW extension office for assistance.

Fertilizers are characterized by the NPK ratio. This is the set of three numbers printed on the front of every fertilizer bag, box and container. These numbers stand for the percentage of nitrogen, phosphorus and potassium in that container of fertilizer.

Nitrogen promotes strong green growth, phosphorus promotes healthy bloom and disease resistance and potassium promotes root growth, flowers and plant hardiness.

If your soil test recommends a higher source of nitrogen, for example, you'll choose a fertilizer with a larger first number in the series.

From there, fertilizer choices can be narrowed to organic versus non-organic.

Organic fertilizers provide long-lasting, slow release availability of nutrients from an all natural source. This may be from plant material, animal material or rocks and minerals.

Non-organic fertilizers are created from chemical or synthetic sources and are often quick release, short acting, intense doses of fertilizer for rapid consumption.

While many gardeners frown upon non-organic fertilizers, there is a place for both types in the garden.

In general, organic fertilizers provide a slow release, all natural, long-term solution for improving your soil's health for the specific plants you are trying to grow.

Examples of organic fertilizers may be things such as bonemeal, blood meal, kelp, seaweed, compost, milorganite and manure.

Non-organic fertilizers include all purpose, balanced fertilizers such as the many varieties of bottled or boxed selections you'll see on the shelf. Many of these are marketed as plant specific, such as tomato fertilizer, rose fertilizer, etc. Some of these work well for their intended use, but research also shows that they may provide no extra benefit than a good, balanced, all-purpose fertilizer.

I will have even more details on selecting fertilizers coming in a future column.

Organic soil amendments are used for a number of purposes in the garden.

Soil amendments

Your soil test results will also indicate what, if any, amendments may be needed for your soil to achieve ideal structure and composition.

Your soil make up will be clearly identified on your soil test results. Depending upon what you indicate you would like to grow, the results will make recommendations on all natural soil amendments to create an ideal structure.

Soil amendments include items such as lime, compost, peat moss, sulfur, sand, vermiculite, perlite, coconut coir, gypsum and others.

Bags and bags of soil

Soil itself can be confusing to gardeners who aren't quite sure what they are looking for. Many garden centers and retail outlets offer bags upon bags stacked on pallets of dozens of varieties of soil.

Potting soil, potting mix, topsoil, garden soil, compost. What does it all mean?

Here's a quick rundown:

  • Potting mix is generally a light, soilless mixture of planting material used mostly in containers, seed starting and other uses.
  • Potting soil is a light planting medium containing actual soil, that can be used on its own in containers.
  • Topsoil is generally used as an amendment or addition when mixing garden soil or creating a garden. Because it can come from a number of different sources, from dry field to rich, organic compost, quality is highly variable and topsoil is generally not used alone as a planting medium.
  • Garden soil is a pre-blended mixture of soil and amendments, ready to use in the garden. It is generally heavier than potting soil, so not normally used in containers.
  • Compost is a rich, organic combination of decomposed leaves and other material that is used as a soil amendment or fertilizer. Compost is not recommended as a planting medium on its own as it is usually too rich.

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