Grow Your Own Food Challenge: Understanding Soil Test Results

Although I’m still in the planning stages of my new garden, one step I have completed is a soil test. Whether you’re starting a new bed or simply unsure of the nature of the soil in your garden, having a soil test done is very important.

A soil test tells you how much of the important elements your soil has (excluding nitrogen, which is difficult to measure), how acidic or alkaline your soil is, and what you need to add to bring your soil up to optimal planting conditions. Some tests will also include the clay and humus content of your soil, and others provide the levels of toxic elements such as heavy metals.

To find a soil testing facility in your area, just contact your local Cooperative Extension Service. A basic test usually costs between $15 and $30.

Because my future garden bed is currently covered in sod, I did a soil test before breaking ground to determine which amendments to add come tilling time. Although we use fancy soil core samplers at the farm to collect soil for tests, a regular garden trowel works just fine at home.

I dropped my soil sample off at a local testing facility and within a week I had my results. The full micro-nutrient test I ordered did not come with any explanation or recommendations, so this is my first time interpreting soil test results on my own!

Here are my results and what I think I should do:

Percent Organic Matter – 2.2%

  • The organic matter in my soil is quite low because it’s currently covered in sod. In most climates, 4-5% organic matter is ideal. To increase organic matter and add a good, steady nutrient source that will improve soil structure, I’ll add well-rotted goat or horse manure on an annual basis. Manure is very rich in nitrogen, and also contains potassium, phosphorus, magnesium, as well as trace elements.

Soil pH – 6.3

  • My pH is in the optimal range of 6.2-6.8, which is great because I was a little worried that nearby conifers might have made my soil too acidic. Even though I don’t need to actively adjust my soil’s pH, I will be careful about adding any amendments that would lower the pH further, such as peat moss, sawdust, leaves, or bark.

Phosphorus (P) lbs/acre – 29

  • My phosphorus level is pretty low. Because manure is relatively low in phosphorus, I’ll also add rock phosphate, a naturally occurring mineral deposit, to bump it up.

Potassium (K) lbs/acre – 171

  • My potassium level is also on the low-side. The manure should help a lot, but I’ll also add greensand, a mineral deposit that’s high in potassium and micronutrients.

Calcium (Ca) lbs/acre – 1592

  • Would you look at that – another deficient nutrient! If the manure doesn’t do the trick, I’ll add lime or crab meal.

Magnesium (Mg) lbs/acre – 458

  • This magnesium level is acceptable, and the manure should push it into the optimal range. But if I have any deficiencies down the road, I could use a dried seaweed product such as kelp meal.

Sulfur (S) lbs/acre – 6

  • My soil is very low in sulfur, but I don’t want to add pelleted sulfur because it might lower my soil’s pH too much. Adding organic matter in the form of well-rotted manure and compost should help a lot.

Boron (B) lbs/acre – 0.5

  • Another low micronutrient. If adding organic matter doesn’t help, greensand or dried seaweed might.

Copper (Cu) lbs/acre – 2.4

  • This copper level is in the acceptable range.

Manganese (Mn) lbs/acre – 121

  • My manganese level is very high, perhaps too high. Something to keep an eye on when I test my soil again in the fall.

Zinc (Zn) lbs/acre – 6.1

  • Another micronutrient that’s on the high side but should hopefully balance out with the addition of organic matter.

Percent Sand – 49.2%; Percent Silt – 32.4%; Percent Clay – 18.4%

  • The breakdown of my soil composition suggests that I have loam soil, which is ideal. I don’t have to worry too much about water drainage. Adding organic matter will only make things better!

Because I’m new to this process, I’d love any advice that you could give me on how to amend my soil. Please leave your recommendations and questions as comments!

Image courtesy of Flatbush Gardener via a Creative Commons license.

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4 thoughts on “Grow Your Own Food Challenge: Understanding Soil Test Results”

  1. With a pH of 6.3, that area could stand a little fireplace ash. not too much, but some would help. also, remember that the natural soil has inherent buffering capacity, which means that it doesn’t want to change pH much and it has “tools” to keep that from happening. potting soil generally lacks this ability. really watch adding any lime, especially with low organic matter in the soil. with low organics, lime is much more powerful and will drop your pH out of safe zone. a better trick would be leaves of a dogwood tree, chopped up. those things are seriously loaded with calcium, more than any other leaf. if you do this, pick leaves that don’t have any powdery mildew on them.

  2. Grew up on a Maine potato farm and always a fresh crop of rocks each spring. You want soil that is not compact, holding water like clay but not drying out quickly like sandy, gravelly. Hydroponics where the soil only holds the plant, keeps it upright takes away all the fun!

  3. Your P&K are quite low as some crops such as potatoes care remove in excess of 40 lbs/acre. Add manure/compost to go with the soil testing.

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