Soil Testing - What is it? And why should you do it?
As a Master Gardener I am often asked questions regarding suspected nutrient deficiencies of plants. People will ask how much fertilizer they should add at planting time. Or what to add to fix a plant with yellowing leaves. My answer is usually an unexpected one because my answer is always “I don’t know.” Followed by, “What does your soil test say?”
The truth is that no matter how much a lifelong gardener may suspect that a plant is deficient in X, without a soil test, there is no true way to know. A soil test is undeniably the best way to get a snapshot of your soil in time and to get the exact recommendations for what needs to be amended and by how much.
Many assume that a balanced fertilizer should be added to the lawn or garden every year but that is probably a waste of both time and money and could potentially be damaging the environment.
Let’s start with the basic fundamentals.
NPK
You may be familiar with these three letters as they are on every bag of fertilizer or soil amendment you have ever seen.
N = Nitrogen
P = Phosphorus
K = Potassium
A bag of fertilizer that is listed as 10-10-10 will have equal amounts of nitrogen, phosphorus, and potassium (10% each). This is a very common fertilizer. A “balanced” fertilizer. Many people will use this on their lawns and gardens thinking that because it is balanced, nothing will be getting too much. Unfortunately, this is not true. Nitrogen is the only nutrient that is rapidly used by plants. Phosphorus and potassium are needed in smaller amounts and excess will wash away through the soil into rivers and streams. Excess phosphorus is responsible for algae blooms in natural waterways. Excess potassium is responsible for contributing to soil salts.
Plants also require small amounts of calcium, magnesium and other nutrients like boron, manganese, iron, copper, and zinc. These nutrients, known as micronutrients, are required by every plant but only in very tiny amounts. There are many fertilizers and soil amendments on the market today that advertise these micronutrients but these fertilizers and soil amendments are usually quite expensive. This is a standard case of “more is not better”. More nutrients available to plants does not make a healthier plant if there are already sufficient nutrients in the soil. More will definitely be a waste of money and could also damage the plants, the environment, or both.
The recommendation to lime your soil every year is common but is intended for soils that are naturally acidic. Here in the Okanagan, where we have naturally alkaline soil, adding lime without recommendations from a soil test can lead to phosphorus and iron deficiencies in your plants.
So how do you determine what is actually required and wade through the vast area of fertilizers and soil amendments on the market?
A soil test.
Soil tests are the only accurate way to determine what is in your soil and what nutrients, if any, are lacking.
In Canada, soil tests can be done at private labs. They are easy to do and inexpensive. On a commercial basis, I would recommend getting a soil test every year but for the home gardener every 3-5 years is fine.
Regular soil tests are also a baseline with which to measure your soil in years to come. They let you know how your soil has changed and adapted over the years so that you can definitively measure the difference that compost application, for example, has made.
Each lab will have a slightly different method of testing so to maintain continuity, use the same lab for your future soil tests.
Although there are many DIY soil test kits on the market, I would highly recommend getting a professional soil test. The DIY tests seem easy to use but they are difficult to understand the results. They also don’t provide any information on what needs to be added in order to achieve the desired amount of a missing nutrient. Different plants also require different nutrients so when you are sending off a professional soil test, you are able to indicate what plant(s) you are growing in this area and the lab will send back specific instructions for these particular plants.
I’d love to hear about your experience with soil tests. Pop your questions and experiences into the comments section.
Nadine