CO-Horts Blog

Monday, November 15, 2021

Healthy Soil- Too much of a good thing?

Recently I attended our annual forum where all the Extension staff across the state come together and we learn and share.  The sharing of programs, county issues and among us plant folk, plant and soil issues are the most telling.  There seems to be a common theme this year, gardens being over amended and a misunderstanding of organics and fertilizer versus compost is evident.  Both compost and fertilizer can be organic, but they are not the same thing. 

Compost image, Colorado State

I want to point out before I go further, that Extension and Extension staff have conventional and organic options on all our factsheets and that we all care about our environment, it’s why we have the jobs we do because we are passionate about our world.  I think it is very important understand and get the facts about your soil prior to adding amendment or fertilizer.  This is very similar to practicing IPM- Integrated pest management where you observe / monitor, proper id, learn more about the issue (or system), determine action, choose tactic(s), evaluate.  IPM is monitoring the situation, gathering the facts and doing something when needed.  So think of these similar steps in your landscape and how to improve your soil.  Get the facts before applying anything and to improve when you need to improve, plant the right plant in the right place and you will be successful.  If you have never done a soil test, this is the foundation of the garden and a good place to start.  The CSU lab is currently moving so not testing currently, other great laboratories that understand our Western soils are Ward Laboratories, Servitech and American Agricultural Laboratory Inc.

Once you pick a lab, stick with them as their procedures might be slightly different.  You want to be able to compare apples to apples down the road.  Please read how to take an appropriate sample and tell them exactly what you are planting (your crop): buffalo grass, tomatoes, plum tree- the more specific you are, the better the recommendation they can give you.

University of Hawaii Soil Management

Let’s go back to the word organic.  It was first used to describe the organic matter (basically plant material: leaves, roots, stems, & microorganisms) that is in the soil.  We still use it this way, but it has also morphed into products that are naturally based.  Here I am sticking to the original definition.  Our native soils in Western Colorado are naturally 1-2% organic matter.  This is why our native plants do not prefer high amounts of compost being added to our soil, it’s not what they are acclimated to.  Then somewhere along the way organic morphed into meaning using naturally sourced fertilizers and pesticides.  We know from experience and as shown in the soil pie chart that we want up to 5% organic matter in the soil for the general typical landscape plants and vegetable and flower gardens.  When soil test results get closer to this 5% organic matter, the need for nutrients like nitrogen will drop because it is being released of some nutrients from the organic matter (plant based and microorganisms).  This article from Minnesota gives a good explanation of other reasons too much organic matter is not good. Of course our soil is going to be different, but the having too much organic matter is the same.  We general have good minerals (phosphorus, potassium, sulfur) in our soils.  Occasionally we will see a micronutrient low due to the pH of the soil, high pH affects the accessibility of certain nutrients to specific plants.  Example: Silver maples have trouble accessing iron in high pH soils resulting in chlorosis.  Adding more iron to the soil doesn’t work unless it is chelated. 

Microbes in the soil, invisible to the naked eye-
                                                                    Colorado State
Example 2: Someone was having trouble growing vegetables, since the pH was high they continued to add sulfur, which was already at a good level.  This caused a toxicity of sulfur in the soil which causes a reduced rate of growth and necrosis.  Since we have high amounts of calcium carbonate in our soils and there already is enough sulfur, our soils are buffered which means it is very hard to change the pH so we focus on what we can change and improve.  So please do a soil sample, talk to Extension if you need more explanation, pick the right plants for your soil and you will be successful. 

Lastly, pick the right soil amendment if your organic matter is below 5%.  This includes using compost and it is ok to ask for a sample of compost and have it tested before you buy or use it.  Compost is a soil amendment, though it may help with nutrients down the road, it is not considered fertilizer or a major source of nutrients.

If your plants are needing fertilizer, than use the appropriate fertilizer. Here is understanding fertilizer.

Hopefully that helps in understanding that you can have TOO MUCH of a good thing.  It is like finding out that watermelon is good for you and you eat the whole watermelon, you might not feel so good afterwards.  It is important to keep things in balance.  By adding too much of one thing, other things have to become out of balance.  Look back at the soil pie chart, if there is too much water, there would be less air.  If too much organic matter, there would be less mineral particles, water or oxygen.  ALL are important. 

So hopefully if nothing else, you will do a soil sample and think about what your soil needs before you add something.

Susan Carter, Tri River Area Horticulture and Natural Resource Agent