Fertilizer? Compost? Lime? What Should I Do to Make My Soil Better and My Plants Thrive?

February 20, 2010

Most of the time my answer to most gardening questions is another question: “Have you tested your soil?”. Whether the question is “What should I add to make my soil better?” or “Why are my leaves so yellow?” or “Why aren’t my berries plump?” the answer may lie below the surface. What we so often ignore is that a large part of our plant’s life system is literally rooted in the soil. If the soil is out of whack, the plants are going to be out of whack.  And, if you’re growing edible plants, if your soil is toxic, your food may become toxic as well.

About a year ago, I wrote the following article about testing soil. And, for the most part, I think it is still valid. However, I’ve changed my mind about using home test kits and now prefer to ship my soil off to an accredited soil lab. Sure, doing a home test can be relatively inexpensive and it can be a fun family project. But, there’s also a lot of room for newbie user error. Plus, once you get your DIY results, it may take a lot of digging to come up with the answer to “Now what do I do?”.

Instead, for about the same price, sending samples off to soil labs like Amherst Soil and Plant Tissue Lab or Midwest Soil Labs or any number of other soil labs will get you much more information to work with. In addition to testing for pH, the labs will advise how, when and with how much of what to adjust your pH. They’ll check for heavy metal content and advise you if your soil is not only safe to grow food in but also if it is safe for your family to simply be in. They’ll check for all macro and micro-nutrient levels and provide suggestions for how to adjust for any that are overly or under abundant. And, their recommendations are not simply based on what they find in your soil sample but also on what you tell them you are or plan to be growing in your soil.

In my own garden this fall I sampled nine different beds and sent them into UMass for analysis. Within two weeks of shipping out my materials (and one week of their receiving it), I received an email with detailed analysis of my soil. The short of it: no heavy metals beyond naturally occurring levels, soil that needed liming to adjust pH, a need to add Nitrogen and in some cases relatively high Calcium and Magnesium and twice the level of desired organic material (oh well, it helps with moisture holding capacity in my sandy soil.) . Nothing surprising and a bit of relief. As regular readers know, I grow a lot of food. And, I’ll admit it, shamefully, I had never checked my heavy metal levels in the past. Now I know it’s safe.

So, what’s the next step? Over the past few days in near 60F Seattle sunshine, I pulled weeds, weighed dolomitic lime and worked with Bob to amend my dormant beds that needed it. Once the rain does its work helping the lime convert in the soil, I’ll be adding blood meal to the beds that need it. Next fall, as per the lab recommendations, I’ll use a home kit to check my pH, which may again get a bit too acidic. If it does happen, I’ll add more lime (in smaller ratios as the lab suggested) to keep the pH corrected. And, yes, I’ll be adding composted material to the beds as well. Yet, as I work to neutralize my pH, I’ll also need to reduce the amount of fertilizer (by this I mean things like blood meal and compost) I add. Since my soil is already filled with decent nutrient levels, it will be critical not to over-fertilize. Yep, this can happen. And, not only can it cause Nitrogen burns to my plants, but it can also create Aluminum and other toxicity to my plants along the way.

Sure, it seems complicated, but with 9, $15 lab tests in my file, I feel armed to take on the soil!

Read on for the original post and learn more about DIY home kits here:

(Original Post from February 16, 2009)

It’s that time of year when just about every client is asking  Is it too late to mulch my beds? or  What kind of fertilizer should I put on my plants? or I just read I need to lime my garden to make it sweet; what does that mean? Now that a few winter bloomers are showing their stuff, the sun is making an occasional reappearance, newspapers are running “what to do in the garden this month” pieces, and the Northwest Flower & Garden Show is opening in 3 days, the race to dive into spring is on in Seattle. And, soil care is a good place to focus.

Simple At-Home Soil Test Kit

Simple At-Home Soil Test Kit

When I get these soil care questions — even if the fertilizer question is about a plant, I consider it a soil issue — I try to start by educating clients on how plants interact with soil at different times. That explanation is a little deeper than I’m going to go into here, but let’s start by answering the three questions listed above.

The first answer to all of these questions is to ask whether the client has tested their soil. Without knowing what is already going on with the soil, it’s nearly impossible to generalize about what should be added to it. Pick up a simple at-home soil test kit for under $20 here.

Is it too late to mulch my beds? It isn’t too late to add composted mulch to beds to top dress them. Heck, I find myself out spreading mulch in all seasons. Mulch does more than just add nutrients to the soil. It also encourages soil microbia to establish healthy populations in the garden, and their work builds soil tilth. As well, it protects roots and discourages weeds. So, yes, mulching can be done now. However, mulch alone may not solve every garden issue. As well, too much woody mulch (for instance) can change the soil’s nutrient and pH balance over time. So, again, testing your soil is a good idea.

What kind of fertilizer should I put on my plants? My first response to this question is don’t add a fertilizer now. If you intend to quick feed your plants, right now isn’t the time to do it. When the soil is cold and plants are still dormant for winter, fertilizers just plain don’t work. Plants don’t use the material, so you waste money putting them down. Plus, the fertilizers then leach through out soils and into the water tables with a little help from all our winter rains. So, don’t try to add fertilizers to your soil now. However, now is a good time to take soil samples to send out to a lab or test at home with a small soil testing kit to get an understanding of what your soil macro (and possibly micro) nutrient needs are.

Last weekend, I used my soil core sampler to take several soil cores from my own garden. My biggest curiosity is about beds for edibles. By growing plants that I expect to yield food, I’m asking a lot of the soil. I’m taking a lot of the nutrient value from it over and over, so it’s important that I figure out what I need to give back to it to ensure good harvests and a healthy long term eco-system.

I kept each soil sample for each section of the garden isolated in an open bag. I let the soil dry out for a week, and sifted out much of the organic matter. I then ran three simple tests on the samples from each area of the garden: one for pH, one for Nitrogen (N), one for Phosphorus (P), and one for Potash/Potassium(K). Although I tested from three different areas of the garden, I learned that in general my garden is Nitrogen deficient, has varying deficient/adequate levels of Phosphorus, and has an abundance of Potash.  Knowing this, I’m able to then select what to add to the garden beds later in the season to adjust the Nitrogen and Phosphorus deficiencies.

Because I want to add a slow release material that won’t leach right though the soil, and because I don’t want to add a lot of Potassium, I’ll probably be adding something like a blood & bone meal amendment a little later in the season. (Honestly, I’m still looking over my options and prices, so look for more thoughts on this later. Worm teas and some seed meals may be other nice options.) Since I’ll be adding slower release materials, I will want to add them to these beds just as the soil warms a bit. This is when the soil microbia as well as the plant roots begin to seek out these materials. As the soil microbia process the materials, their excretions will contain nutrients in forms that plant roots will happily take up.

pH ~ 6.5 = Slightly Acidic & Great for Most Edibles

pH ~ 6.5 = Slightly Acidic & Great for Most Edibles

But, before I gave up on doing anything with my soil this season, I also wanted to look at the pH of my soil. In the Seattle area our soils tend to run acidic. Rain and leaching contributes to that trend. Many plants live happily in the slightly acidic to neutral range, but too acidic and plants just won’t thrive. Primarily, when living in a pH range they don’t like, they won’t be able to take up soil nutrients/fertilizers properly. So, they won’t thrive. Plus, any nutrients we add to the soil again will just leach out into the water table. It’s a waste.

A pH test is about the easiest test you can run at home. I don’t like the electronic soil test meters. I find they register the same thing everywhere. A little chemistry kit that runs less than $10 will tell you your pH.

When I ran my pH test, my soil was very acidic in a bed that had been filled with acid-loving rhodies and camellias; it came in at 5.0. My other beds came in around 6.3-6.5, which is slightly acidic and generally good for the edibles I plan to plant in these beds. The bed running 5.0 will need to be adjusted by adding lime.

It is very important to test soil before randomly adding lime, which I know many gardeners “just do”. Which kind of lime you add, how much, when you add it, and what you mix (or don’t mix) it with, is important. Too much lime, and your garden may start trending toward alkaline. Less of the plants that we like to grow in our area will thrive in alkaline soils than will in acidic/neutral soils. Plus, depending on the kind of lime you add, you can have a rapid conversion that doesn’t last a long time or a slow conversion that lasts a long time. Knowing what kind of lime you’re adding, and when is very important. But, mostly, you should verify whether you need the lime or not in the first place. And, finally, it is important to add lime before you add composts and fertilizers to the soil. Lime can take anywhere from 2 weeks to a month to start doing its job, and it needs water to do so. This means the lime can/should be added around this time of year while our rains are still steady and before the time arrives to start adding fertilizing soil amendments to the garden.

I realize this article may leave you with more questions than answers. Heck, I didn’t even get into testing for soil texture/amount of sand, silt, clay in the soil. That’s a whole other soil science discussion. If you’ve got lingering questions about your own garden soil or soil testing, feel free to post them here or sign up for a garden coaching session for help learning more in person, in your own garden spaces.


  1. paul says:

    PH test paper
    is easy to find for this purpose :)

  2. Ian Keay says:

    We live in Vancouver British Columbia Canada.
    It is the first of Oct and what do I do first.
    Fall fertilizer or lime


  3. Kathy Boullin says:

    Great info Robin and nice to have an update on where to get soil samples analalyzed. I too should check for the heavy metals content of my edibles beds. You have spurred me on to digging a few samples!

  4. The quality of the soil plays a vital role in making good plant productivity. So, for that you need to identify the features of the soil to determine its ability to support your plants growth and makes better productivity. (links removed at the discretion of Garden Mentors)

  5. Thanks for your comment. We are big proponents of soil testing!

  6. Carole Fariss says:

    What can I use from around the house to add lime to my Bleeding Heart plants

  7. Carole, Thanks for writing in. If you visit a local nursery, they should be able to help you choose the right amendments for your location. Good luck!

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