Wednesday, March 25, 2015

Soil Test

Last November I saved soil from the garden for a soil test. I finally got around to sending the sample in to the UMass Extension Service at the beginning of February. I got the results in March. I like to do a soil test at least every two years. Mostly it is just for the pH value. I could buy a pH meter, but honestly that is way too much work when I only need to know what is going on every year or two. The garden soil currently has a pH of 6.8 which is just about perfect, so I don't need to lime. Last time I limed (2013) I over did it so it has taken a while to get back to normal.

There are things to learn besides a pH level though. When I first got the soil in the garden I wanted to make sure it didn't have heavy metals like lead, cadmium, or arsenic. I am free of the latter two. And lead is very low. My soil was made from compost and crushed rock or so I'm told from those I bought it from. My lead levels are about 10 ppm. All of the natural soil around my town of Arlington is filled with lead. So in a way I guess I was lucky that I had nothing but subsoil when I moved in. That way when the soil was put in it was good soil, not something toxic.

I'm always curious as to how much organic matter is in the soil. In the past this was part of the main test, but now you have to request it separately if you want to know. My soil is 10%, which is pretty high. But I like it that way because it really holds on to the nutrients. I make almost all of my compost, so adding organic matter isn't expensive. It is just a lot of work.

One of the reasons that organic matter holds onto nutrients is it raises the cation exchange capacity of the soil which is another thing they test for. I have a sandy/gravely soil. It is not the best for holding onto nutrients or water. But adding a lot of organic matter every year (as organic matter decays over time and needs to be replenished), increases both the ability to hold water and the cation exchange capacity.

What is the cation exchange capacity (CEC)? It is the ability of the soil to hold and exchange positive ions, like C++, Mg++, K+, and NH4+. There are other ions, but those are the ones we are usually interested in as gardeners though there are lots of micro nutrients in this category too. So it measures the ability of the soil to hold on to those nutrients and supply them to the plants when they need them. My CEC this year is 21.3. It has ranged anywhere from 20-23 over the years. Which is pretty good for a sandy soil.

Along with the CEC they supply the base saturation numbers for C++, Mg++, and K+. This is the percent of the CEC that is held by each of those nutrients. Some people try to get a very particular ratio between them for the perfect fertilization, but most extension services (I tend to read the UMass and Cornell sites most often), now say as long as there is enough of each nutrient, it doesn't matter greatly what the exact ratio is. But you do want to it stay within a certain range. I'm well within the range for all three. Though calcium is higher in the range and magnesium and potassium are lower in the range. All this means to me is that when I do need to lime I'll be using dolomite limestone which is high in magnesium. But I'm not there yet. My pH is fine right now. Here in New England our soils get acidic over time so in a year or two I'll need to lime.

Another major thing the soil test provides is a list of macro and micro nutrients. The only one they don't provide is nitrogen as the number doesn't mean much. The amount in the soil changes too much with different soil conditions. The other nutrient values do have meaning though. My numbers are very high. Most are listed as above optimum. Which means I shouldn't be putting anything on the garden except nitrogen. The fertilizer that I buy isn't a balance one. I buy one that is something like 7-3-3. So I can mostly add nitrogen when I need to. When I need phosphorus I use bonechar. And when I need potassium I add some kind of rock dust, either greensand or azomite, which also provide a lot of the micro nutrients. But not this year. I'll just use the 7-3-3.


  1. Great info, and timely. I just came up with a plan for our garden based on the soil tests. It needs more work than yours!

  2. I need to get a soil test done this year too - I did one on my original 4 beds & the pH was high & potassium was low, so I have been adding peat moss and kelp meal to help with that. I really should get them tested again, but I think I will wait until next year and test my new beds this year instead. The new soil sure was different (in texture anyhow) from the original batch I purchased, so it will be interesting to see what the analysis says.

  3. You do garden scientifically don't you? We have never had our soil tested.

  4. Good explanation. Looks like your soil is in good condition. I have read that a 10-11% organic matter content is pretty typical for northeast soils. The higher organic matter is good because it can buffer anions as well as cations. I am working on another post about the soil in our community garden which will be quite different from your results. It was over-limed and has a pH of 7.6, so fertilizer choice is a challenge.

  5. I admire that you are so good at testing your soil and it matches your instinct of what is going on. It has been a while, but I will be testing mine this year. I am curious to know how my amending has affected the soil quality.

  6. I was rather proud of myself for purchasing a PH meter recently - previous to this I've not really bothered to test anything. It would be great to get to your level of soil testing at some time, but I suspect it will be a few years for me.

  7. I haven't had my soil tested in 6 years, when the beds were built and several yards of compost were added. The initial test gave a pH of 7.6, not surprising since bedrock here is limestone. I've been adding composted oak leaves every year, maybe the pH has come down a little.