We got our soil test reports back a couple of weeks ago, but due to schedule craziness, this is the first chance I’ve had to share them with you. As I expected, the results are quite interesting.

I sent in 7 samples. I sent in two samples of just the sand (load 1 & load 2). Then I sent in two samples of just compost (load 1 & load 2). Then I send in two mixed samples. The first soil mix sample was load 1 of sand and load 1 of compost. The second soil mix sample was load 2 and load 2. The seventh sample was from the bagged raised bed mix.

For your reference as we go through the results, vegetables prefer a pH of 6.3-6.8 (slightly acidic), phosphorus of around 100 ppm, and potassium of around 250 ppm. Low Exchangeable Sodium and Electrical Conductivity are good, high is bad.

Here are the results by the numbers:

Sand #1:

pH = 8.3

Organic Matter = 0%

Nitrogen = 1 ppm (parts per million)

Phosphorus = 4 ppm

Potassium = 21 ppm

Exchangeable Sodium = 0.50%

Electrical Conductivity = 0.44 mS/cm

Sand #2:

pH = 8.6

Organic Matter = 0%

Nitrogen = 1 ppm

Phosphorus = 4 ppm

Potassium = 31 ppm

Exchangeable Sodium = 0.07%

Electrical Conductivity = 0.40 mS/cm

As you can see, the two sand samples are very similar, both with astronomically high pH levels (yikes!), very little in the way of nutrients (not a big deal once we see the compost numbers), and very low sodium and salinity rankings. Since I was concerned about the sodium levels, I’m relieved that they are very low.

Compost #1:

pH = 8.0

Organic Matter = 13.0%

Nitrogen = 12 ppm

Phosphorus = 585 ppm

Potassium = 2,110 ppm

Exchangeable Sodium = -5.81%

Electrical Conductivity = 6.05 mS/cm

Compost #2:

pH = 7.8

Organic Matter = 13.0%

Nitrogen = 4 ppm

Phosphorus = 385 ppm

Potassium = 1,740 ppm

Exchangeable Sodium = -4.81%

Electrical Conductivity = 4.31 mS/cm

With the compost, we are still seeing a very high pH. I was rather surprised to see that, and I’m definitely not thrilled at that situation. You can probably guess what the pH of the soil mixes is going to be. The available nitrogen is very low, but the purpose of compost is that the organic matter breaks down and releases nitrogen slowly throughout the summer and in future years. The organic matter percentage is only an estimate because of the nature of testing compost. The phosphorus and potassium levels are astronomically high, which does counter-balance the extremely low results from the sand a bit. Excess of these nutrients isn’t usually considered a problem, we just don’t need to add more, probably for many years.

I’m not really sure what a negative exchangeable sodium means, but I’m not going to be concerned about it, since it certainly isn’t high. Unfortunately, the electrical conductivity is higher than I’d like to see. Not too surprising though, given the amount of nutrients.

Soil Mix #1:

pH = 8.2

Organic Matter = 6.0%

Nitrogen = 3 ppm

Phosphorus = 228 ppm

Potassium = 980 ppm

Exchangeable Sodium = 1.18%

Electrical Conductivity = 4.84 mS/cm

Soil Mix #2:

pH = 8.1

Organic Matter = 7.0%

Nitrogen = 3 ppm

Phosphorus = 223 ppm

Potassium = 820 ppm

Exchangeable Sodium = -1.33%

Electrical Conductivity = 5.83 mS/cm

The two mixes ended up being pretty similar as far as results go. That highly alkaline pH of 8.2 and 8.1 means we certainly have a lot of work to do on our pH. We will also be planning to use some supplemental nitrogen all year as needed. The available nitrogen should increase as the compost breaks down, but with our very sandy soil and the watering and drainage system, we will probably lose it to leaching pretty quickly. The phosphorus and potassium levels are down in a more reasonable range, but we certainly won’t need fertilizer for awhile. The sodium levels are still low, which is good, but the electrical conductivity is still in a damaging range, so we may see some problems from that until some of the salts leach out. (There is a benefit to that sand and the drainage system!)

So far, we have been using a water soluble fertilizer formulated for acid-loving plants to help in the zone right around each plant. This fall, we will probably incorporate quite a bit of sulfur. The recommendation is to start with 3 lbs sulfur per 100 sq. ft. on a sandy soil with a pH of 8.0. I think we’ll do that this fall and then continue testing our soil pH regularly for a couple of years and add more sulfur as needed.

Now for the other interesting result, the bagged Raised Bed Mix!

pH = 7.7

Organic Matter = 13.6%

Nitrogen = 3 ppm

Phosphorus = 692 ppm

Potassium = 6,200 ppm

I have to say that I was really surprised by these results. Usually bagged mixes are somewhat pH balanced! 7.7…wow! It’s a little bit better than our sand-compost mix, but not by much. Sulfur will be in order here too. I shouldn’t have been so surprised at the organic matter percentage, because the coconut coir was really prevalent in the mix. The phosphorus and potassium levels, again…WOW! They are even higher than the straight compost we got! It seems like a waste of nutrients to put into a bagged mix. I’m now very curious how they developed this mix…if they developed the mix to meet a certain soil texture and structure without regard to nutrient levels, or if these levels were intentional. I’m all in favor of adequate fertilization, but this goes well beyond adequate.

So, now we know where we’re starting from. It will be very interesting to see how our garden does this year, how things change as we continue to amend the soil in the next couple of years, and if we notice a difference over time.