While many of us have our garden plans laid out for the year, it’s not too late to do what we can to improve our soil before we get to planting. WSU Extension-King County and the UW Center for Urban Horticulture help you get started.
The best way to know what your soil needs is to test it every two or three years. Soil tests that are commonly offered include:
pH – determines the acidity of your soil and estimates how much lime is needed to adjust the pH to an optimal level.
Nutrients – determines the levels of available plant nutrients. Often, labs don’t bother to test for nitrogen, since this test is often misleading. In this area, you can assume your soil needs moderate inputs of nitrogen every year. Some labs test for calcium. This is unnecessary too, since you will be adding lime (calcium carbonate) for pH balance anyhow.
Organic Matter – determines the percentage of organic material in your sample.
Soil Texture – determines the percentages of sand, silt and clay in the soil and classifies it by texture according to the USDA system (for example, silty loam). Since your soil texture is not going to change, you need to request this test only once.
Heavy Metals – determines whether your soil contains abnormally high levels of toxic heavy metals. Sometimes the test is strictly for lead, the most common garden heavy metal contaminate. Usually if levels are elevated, recommendations will be included on steps to take to counteract the danger.
How to Take a Soil Sample
Call the lab you intend to use for an order form and specific instructions. However, following are general instructions on taking and submitting soil samples.
There will be variations even within a rather homogeneous area, so the sample you send should be a mix of several samples. Using a clean trowel or spade, take thin vertical slices of soil from about 10 locations within the area they are to represent. Put them in a clean bucket and mix thoroughly. From this, take the cup or whatever amount your lab requests. For most purposes you will want to sample the top 6-8 inches. For growing fruit you should go down to 8-10 inches.
Knowing exactly what your soil needs helps prevent over applying fertilizer. This saves you money and protects the quality of our surface and groundwater.
Organic Fertilizer Requirements
The results of standard soil tests do not always translate easily into recommendations for using organic fertilizers. In addition to managing nutrient levels, organic growing methods rely on practices such as crop rotation, green manure and compost applications, and the use of cover crops and microbial inoculants. As a result, data on biological soil health as well as mineral composition is necessary for managing soil fertility. Some soil tests offer data on biological parameters, such as organic matter content and microbial activity.
A Few Tips
- Be sure you understand what units of measure, parts per million (ppm) or pounds per acre, the soil lab you choose uses.
- Results from soil tests do not always translate easily into actions to take. Ask your soil testing lab if it offers advice or interpretation of results.
- Be careful with home soil testing kits. Often these kits are not very sophisticated and offer only information on relative levels of nutrients or acidity. The chemicals in home tests have a limited life span and can be inactive by the time you use them.
If You Can’t Get a Soil Test Before You Plant
Here are some general recommendations based on average soil needs in the Puget Sound area. Clay soils and silt soils can take more fertilizer and lime than sandy soil, but sandy soils need it applied more often. All amendments should be mixed thoroughly in the top 8-12 inches of soil.
Lime, per 100 square feet use (100 square feet equals four 4’x6′ beds):
Sandy Soil – 4 pounds (4 pints) every 2 years
Loam – 6 pounds (6 pints) every 2 years
Clay Soil – 8 pounds (8 pints) every 3 years
Compost (including manure, yard waste & kitchen waste): About 2 wheelbarrow loads, or ten 5-gallon buckets, per 100 square feet. New gardens may need more.
Fertilizers usually contain three primary nutrients – nitrogen, phosphorous, and potassium. They are listed on the label in that order. The numbers on a bag of processed fertilizer indicate the percent of each nutrient in the product and the guaranteed amount of available nutrients. For example, 5:10:10 means 5% nitrogen, 10% phosphorous, and 10% potassium. The analysis for organic fertilizers represents the total amount of nutrients rather than available nutrients. This is because organic fertilizers are release slowly and the amount of immediately available nutrients is less than the total.
Different plants require different fertilizers. Some, such as woody landscape plants, generally require none at all. Find out what type of fertilizer your plants need and apply according to the instructions on the label. Use recommended amounts only! More is not better.
If you’re in San Juan County, you can pick up a soil sample packet at the WSU Extension Office in Friday Harbor. WSU soil tests are conducted through the University of Massachusetts; read more about the process here.