8. The Native Meadow Experiment - Soils

Forging ahead with my plan to replace my backyard lawn with a native meadow, I am now taking a look at the soil. (see this earlier article to read about the first few steps of this experiment.) If you’re new to gardening you may not realize how important the dirt under your feet is to happy plants. Especially in urban areas where the land has been worked over for a long time, the soil is likely to be degraded and not ideal for planting. So amending your soil is usually a good idea, and in order to know what to amend it with, it is best to do some soil testing.

I won’t be able to do a full discussion of all the intricacies of soil here, but it is definitely worth reading up on - there is a lot more to it than you might think. Soil is a living system in its own right, full of all kinds of living organisms and complex chemical reactions.

Soil Testing – Go Dig a Hole!

For my yard, I’m doing some basic field testing myself, but also chose to send a sample off for professional laboratory analysis. For larger projects, especially those involving a new building or other major soil disturbance, I would always have the soil tested by a professional lab. A lab report can provide very detailed information and precise recommendations for how to improve the soil. However, in a small yard, unless you think something is not right about the soil, you can probably get away with some simple do-it-yourself tests.

Your obvious starting point is, of course, to go dig a hole in your yard. For all soil testing you want to scrape away any mulch or organic matter on the surface, as well as about an inch of the top dirt layer to get at the “root zone” soil where most of the plants roots will be. The main aspects of soil to consider are Soil Texture, Drainage, Nutrient Levels, and pH, so let’s take a look at each in order:

1. Soil Texture

Your soil’s texture will have a big influence on what you want to add to it and what plants you choose to grow in it. There are three basic soil types: Sand, Silt and Clay. Sand pretty obviously has the largest particles while clay has the smallest particle sizes and the smoothest, stickiest texture. Sand is ideal for drainage but clay tends to hold more of the nutrients that plants need, so what you are looking for is called Loam, something in the middle of these 3 types.

Testing soil texture is relatively easy to do on your own. Try the ribbon test – an easy do-it-yourself method to get a rough idea of your soil texture, with the bonus that you get to play in the mud! Take a small handful of your soil from below the top layer, remove all the rocks that you can and then moisten it slightly. Knead it around in your hands and feel the texture of it. You will immediately get some idea of its sandy-ness just from kneading it. Then try to form it into a ball (add a little water if it starts to dry out). If it can be formed into a ball, then it is closer to clay than sand. Next try to squeeze it out into a ribbon until it naturally breaks off from its own weight. There is a bit of an art to making the ribbon - you can find online videos to help. The longer the ribbon, the more clay content in the soil.  Generally it is thought that if the ribbon is less than 2” you have sand, between 2” and 3” is loam and 3” or greater is clay. I was a little surprised because I couldn’t get a ribbon longer than about 2”, I was sure I had heavy clay, but it seems like there is some loam in there. The professional lab analysis classified my soil as Sandy Clay Loam which generally backs up my ribbon testing. Soil texture is closely tied to its drainage capacity, so let’s look at that next.

2. Soil Drainage

To test your soil’s drainage capacity, you can do the percolation test – this is very useful even just for the visual on how long water sits in your soil. Dig a hole about 6” diameter and 12” deep. Fill it full of water and time how long it takes for the water to be absorbed all the way to the bottom. If the soil is particularly sandy and/or dry, you may need to fill the hole with water a second time to get a real idea of the percolation rate. In my backyard the first time I filled the hole it took so long to drain completely that the sun had gone down – approximately 8 hours. This is not a good sign, generally I’ve read that you want it to drain in less than 3 or 4 hours. This would indicate that I’m definitely going to need to amend my soil to improve this drainage. The professional lab test did not flag drainage as a major issue. However, they estimated the infiltration rate as 0.29 inches per hour, which means that it would take my 12” deep hole a long time to drain, so the testing seems consistent. A recommendation is made by the report to add organic matter to the soil to improve the structure, a common practice to effectively dilute the clay content. (Really heavy clay is a special case that I won’t be able to get into this time.)

3. Soil pH

The relative acidity or alkalinity of soil is important for plant growth, mainly because it effects the availability of nutrients. pH is measured on a scale from 0 (acid) to 14 (alkaline) and most plants want the pH to be right in the middle at 7, called neutral. A notable exception is a class of acid-loving plants, such as Rhododendrons, that like a pH from about 5.5 to 6. I used a common home testing kit to check the pH in 6 different locations in my yard. It wasn’t as easy to read as I thought, but it looked like my soil was fairly neutral in the range of 6 to 7.6. However, when I got the professional analysis done, they identified the pH as strongly acidic at 4.7 (a test they ran twice to be sure). I trust the professional test over my home testing, so I’m going to be adding some lime to the soil to bring the pH up.

4. Soil Nutrients

There are quite a few different nutrients that healthy plants need, starting with the big three: Nitrogen, Phosphorus and Potassium (the N-P-K numbers you see on most fertilizers). These are followed by three main secondary nutrients: Calcium, Magnesium and Sulfur, and beyond that there are 14 micronutrients.

At my local nursery, right next to the soil pH testing kits, I saw some kits for testing the major soil nutrients (Nitrogen, Phosphorus and Potassium). I didn’t try it myself since I was definitely planning on the laboratory test. The lab test shows that I have a pretty good availability of all the nutrients, with sulfate on the low side, and interestingly Zinc is very high. This is partially because of the low pH and other complex chemical properties, but I’ve also heard that my yard is on or near the site of a farmhouse for this area, so maybe there’s some interesting history to be “dug” up on that – but another time.

5. Other Factors

There are other things to keep an eye out for when it comes to soils in the San Francisco Bay Area. One is salinity – some soils can be high in salts because of our coastal location, and this can have a big effect on plant health. You also want your soil to have a good population of the organisms that keep it healthy, starting with worms. I have heard about a basic do-it-yourself soil fertility test called the worm test. You take some of your soil and sift it to look for worms. The presence of worms is of course a good sign of soil fertility and aeration. It didn’t sound too scientific, but what I read online gave a sort of scale for how many worms per amount of soil to tell you how good the soil is. However, when I dug out my soil, from 3 different locations, I only saw a single worm, so I didn’t take this test any further and will be looking to improve the soil for all the beneficial organisms.

Soil Preparation

Based on all of this analysis, I have some decisions to make about how to prepare my soil for planting. The laboratory report recommends two major amendments: “Nitrogen Stabilized Organic Amendment” which can be nitrogen treated Redwood or Fir sawdust, and “Calcium Carbonate Lime” which commonly comes as Oyster shell flour. These get mixed into the soil through tilling, in my case by hand or with a small rototiller. The report recommends going down to a 10 inch depth, which also generally loosens up the soil in the plant’s primary root zone. Time now to start looking for those amendments, estimating the quantities, and getting ready to do the work!

Especially when talking about soil, I think it is important to emphasize that this is a very brief overview. Soil Science is a complete profession in its own right, the interactions of different characteristics of soil are very complex, and I’m not able to provide a complete textbook of information here. When in doubt, consult a professional, and get them to do a complete laboratory analysis and provide you with their expert recommendations.

References:

Bay Friendly Gardening, published by the Alameda County Waste Management Authority, web site: www.StopWaste.org

Soil Matters, from Bay Nature Magazine, Jan-March 2005

The New Sunset Western Garden Book, from Sunset Publishing, Feb. 2012