A DASH of Gardening: Soil Testing

How to complete soil testing in your garden on your journey to providing the best soil for plants.

What’s in your soil?

Soil is made of of a variety of things, but the main components of soil are sand, silt, clay, and organic matter in a variety of states of decomposition. The balance of the nutrients play heavily in the efficacy of nutrients and their absorption by the plants that grow in them. For plants to be able to absorb nutrients properly and grow they need a soil with a balanced pH, plenty of organic matter, mineral balance, adequate moisture, and appropriate growing temperatures. The best type of soil to grow food in is loam, which is a mix of 30-50% sand, 25-50% silt, and 10-25% clay. Organic matter should be about 2-6% of the soil composition. The pH should be between 6.4 and 7 for most kitchen garden vegetables.

Most soils will tend to be either on the sandier side or more clay side, each of which is workable and has different advantages and disadvantages. Sandy soils are easy to work with and warm quickly in the spring.  However, because they do not hold water well, they also don’t hold nutrients well.  This means that if you are growing in sandier soil you need to do more frequent applications of fertilizers, compost, and other amendments to maintain healthy soil. One might regularly mulch with compost to maintain plant health.

Clay soils are hard to work with given their density and that they dry to a hard concretious surface in the summer. Generally, they take a long time to warm up and dry out sufficiently in the spring for planting and then you need to regularly work and water it so that it stays moist.  One advantage is that clay soils do hold nutrients well. If you are growing in a more clay based soil, you will not need to do as much fertilization and compost application once the growing area is initially established.

For any soil composition or nutrient imbalances, most of the time a variety of composts and other organic material can be used to address soil nutrient issues.

Nitrogen is one of the main three nutrients that plants rely on to grow.  It encourages leaf growth.  Nitrogen typically needs to be added to soil because the natural process to break down nitrogen from available organic material takes too long compared to the rapidity the plants need it as they grow.  This is the most important nutrient to ensure is immediately available to your plants immediately after they are planted.

Fertilization is especially important when the average temperature is below 50D F, because this is when the microorganisms that support the mineralization of organic matter into things like nitrogen are not very active and the nutrient and others are even less available in the soil.  However, it’s very easy to overfertilize during the cold months in fear of this issue which can also create toxicity in the plant.

Phosphorus is required for root development but is more dangerous to the plant and the environment if there is too much of it present in the soil.  It lingers in soil and so tends to accumulate, especially because it is often in the more nitrogen rich fertilizer sources.

Potassium improves root vegetable storage time and flavor, size, color of all vegetables as a whole.  It also improves growth rate, disease resistance, parasite resistance, and climate resilience.  Potassium though is super mobile and tends to quickly wash out of soil.  If you have a sandier soil, this is a particular issue and so additional potassium fertilizer application is likely needed throughout the growing season.

Evaluating Soil Condition

There are a couple things that you can immediately look at to get a better understanding of your soil issues. Good soil should crumble into clumps in your hand when it’s moist.  If it holds together in one big clump or does not clump at all, the soil will need amending with some organic material to improve its condition.  You can also look for good ongoing decomposition of organic matter.  Lack of this could indicate issues with the soil pH or drainage that is contributing to the non-decay.

Additional Soil Evaluations

There are a couple other things you can do to understand the more about your soil. You can look up your house on the Natural Resources Conservation Service website.  There are maps of soil types and classifications for the entire U.S..

Another things you might do, especially if you are concerned your soil might be especially slow or fast at draining, is to conduct a soil drainage test. This is best done a little later in the season, but can help you determine if you need to do something more drastic than just amending with compost to address soil composition issues.

Soil Test Methods

There are several different types of tests that you can do in order to determine your soil condition.  Earlier in January I reviewed starting the effort of preparing my garden bed for the growing season. In the following sections I will review how I approached testing my soil at home.

Testing Soil Composition

In order to test soil composition, I first found my inner 5year old and made a mud milkshake in my garden. Ideally, when you are going to make your mud milkshake to complete this test, you will get samples from multiple areas around your garden.

Soil Composition Test Instructions

You will need..

• 1xper sample: clear glass or plastic cylindrical containers, such as a glass or jar or vase
• A spoon or trowel
• Water
• Dirt samples from your garden

To test your soil

Take your glass container and fill it no more than 1/3^rd full of soil in the area you are sampling.  Make sure that the soil sample you collect is free of large organic debris and rocks.  Repeat this with a new glass container for each area that you want to test.  Fill the glass container(s) with 2x as much water as there is soil in each and mix thoroughly.  Allow the mud milkshake to settle, undisturbed, for 30 minutes to 24 hours before you evaluate the composition.  During the settling process, the soil will break up into it’s main parts of sand, silt, clay and organic material and will settle into corresponding layers in the container.

Reading the result

Sand is the heaviest material in your soil and will naturally sink to the bottom of the container.  The next material will be silt, forming a distinctive layer from the sand.  The final layer in this will be the clay, which settles the slowest and is the lightest colored layer on top of the soil trifle.  All the organic matter in the soil will float to the top of the water in the glass. 

I mixed and read my glasses of soil at 3 different intervals to see if there was a significant difference for me in results.  The first I read at 30 minutes past mix and found that the sand and silt had already settled to the bottom of the glass in their respective layers.  The clay layer appeared to have also begun to form, but since the majority of the liquid in the glass was so cloudy still, I figured there must be some more clay settling.  The next reading was 24 hours after mix and I could see that the liquid in the glass was now separating into even more layers, but it did not appear the clay layer had significantly increased in depth.  The last reading was 48 hours after mix, just because I thought that the liquid would eventually settle and become clear if I left it another day.  However, I was surprised to see how murky it still was, though I could now see the top surface of the layer of clay and some organic material that had fallen since mixing.  At this point, I decided I wasn’t going to get much more of a change in clay amount and called this the final readout. 

How to Test Soil pH

There are a couple different methods out there for testing your soil pH at home.  One is a liquid test kit, similar to what you might use to test chemical levels in a pool.  The other is a electronic reader that you can use to measure the soil pH in various areas around your house, almost like you would use a meat thermometer.  Both of these test options can be found at most hardware stores.

I purchased both, though planned to reserve the chemical test for my vegetable garden bed.  I have the pH meter for quick checks around other areas of the yard.  In the chemical kit are a number of color coded containers with a comparative chart inserted on one side.  There are two chambers to the container, the smaller one is the only one that is used for the test.  The instructions provided with the kit are quite clear to fill the test chamber with soil to the line indicated on the side of the container.  I did this then open a provided capsule of chemicals over the soil in the test chamber.  Next, I filled the test chamber with filtered water to the fill line on the side of the container.  Lastly, I put the cap on the test container and shook the crap out of it.  I let the contents of the container settle per the kit instructions before reading the test.

Soil pH Test Result

Based on the color of the liquid and the chart, my soil has a pH of about 5.5 which is acidic. A soil pH of less than 6 begins to inhibit healthy soil microbe growth.  Once the pH is balanced, it can be maintained with regular application of high-quality organic compost.

Testing Your Soil for Nitrogen, Phosphorus, and Potassium

This is like a soil composition test combined with the pH test I did.  My kit had me mix up soil and water in a 1:5 ratio and allow to settle for up to 24 hours (similar to the soil composition test).  Once the water and soil mixture was mostly settled out, I extracted liquid only from the soil mixture and filled the test chambers of each of the mineral sample containers as indicated by the fill lines on the sides.  I then broke open the corresponding color chemical capsules over each test chamber, closed each container up, and shook them up thoroughly.  After 10 minutes of reaction time, I read the results for each. 

Be aware, the results window and its interpretation can be deceiving.  When it says “adequate” it really means that there is enough of the nutrients there for plants to survive, but they won’t really thrive or be vigorous.  Anything that is adequate or less will need some level of amendment.

However, each showed that my soil was drastically under nourished, which I am not super surprised about but means I have a lot of work ahead.  There appears to be more potassium in the soil right now which does make sense because I recently fertilized with potash, but it is also a very sad amount.

Addressing Soil Condition Issues

Heavy feeders, such as broccoli plants, should be fed a much higher amount of nitrogen than other vegetables.  Since I will be doing intercropping and also succession planting throughout the year, I’m opting to amend the soil based on the assumption that I will have a heavy feeder…somewhere in each spot in the garden at some point in the year.  I am using the following guidelines for how I am amending my soil, a combination of strategies from The Market Gardener and recommendations from the soil testing kit and fertilizer bags:

• Granulated Chicken Manure:  1/2oz per square foot of garden bed
• Compost: 1.65 lbs per square foot of garden bed
• Bone Meal:  1/4oz per square foot
• Muriate of Potassium: 1/8oz per square foot

I am assuming that this will also increase the pH of my soil, so once I’m done with my double digging and incorporating these first nutrients into the soil (as an initial treatment), I will be incorporating some limestone into the final steps of my bed prep to help provide more nutrients and address the pH issue (as the longer term treatment).  I will wait to do another soil test until 4-6 weeks after I’m done fully preparing the garden bed to see if I need to make any additional adjustments.

To incorporate the soil amendments, I will be double-digging the garden bed area.  If you have a bed that is already established, you can incorporate soil amendments by sprinkling them over your beds in the appropriate amounts you need to amend your bed, and hoe or rake into the soil until it is thoroughly distributed and incorporated.  Reapplication of soil amendments may be required throughout the growing season depending on how the next round of soil testing goes and how my plants grow and mature once in the beds.  If they start looking sickly, this could be a sign that I still don’t have the soil condition corrected yet and need to retest.

Common Soil Amendments

Here’s a list of common soil amendments as recommended by my resources:

Limestone: Raises pH; adds calcium and magnesium

Gypsum: Lowers pH

Bonemeal:  Good source of nitrogen and phosphorus. Very slow acting.

Bloodmeal:  Good source of fast-acting nitrogen and phosphorus

Feathermeal:  Good source of nitrogen

Hoofmeal:  Good source of nitrogen

Fishmeal:  Good source of fast acting nitrogen, phosphorus, and potassium.

Dried Poultry Manure: Good source of nitrogen and phosphorus, low organic matter.

Wood Ash: Good source of potassium and phosphorus; has an alkaline effect on the soil.

Seaweed emulsions: Good source of potassium with a little nitrogen, some trace minerals

Vermiculite: Assists in holding water in the soil; adds space and porousity to the soil; provides some potassium , calcium, and magnesium.  Has a neutral pH.

Greensand: Contains potassium and other trace elements

Coconut Coir: a good alternative to using sphagnum moss to improving the water retention of soil, but is is not as likely as becoming waterlogged which can be beneficial depending on what you are growing in the soil and your soil nutrient situation.  It’s pH is 5.7 – 6.8 which is less significant of an acid compared to moss and is also more ecofriendly.

Next Steps for the Best Soil for Plants

Once you complete your soil testing, here’s a summary of what you can anticipate as your action items:

1. Use the results of your soil test to inform decisions on soil amendment needs
2. Review your desired garden area and calculate the amount of each of the soil amendments you need so you know how much you need to purchase initially.
3. Consider what you will need longer term to maintain the soil condition once achieved so that you can plan to also purchase those items, or you can plan to purchase them later in the season.
4. Buy your necessary soil amendment products and tools.
5. Amend the soil
6. Maintain the soil per your planned maintenance program or as you observe your plants responding to the enhanced soil conditions and need to adjust.

I hope you continue to follow me as I double-dig my vegetable bed and prune my trees next month!

What are you doing to amend your garden beds? Leave a comment and let me know!