Secrets of the Soil: Part 2
This is the second part of a two part series. This part examines the particular soil found here at Lilliputopia and how you can learn about the soil at your own site.
The story of our soil
Luckily, the soil at Lilliputopia has been largely untouched by the corrupting forces of poor management. I was told that it had been a fallow field for several decades and before that, grazed by cows. One season of observation has revealed a variety of beneficial plants including many types of clover, vetch, grasses and daisies. These plants grew high in the summer and made a beautiful meadow. Unfortunately, we do have some areas prone to nasty weeds including Himalayan blackberry, thistle, poison oak, and bindweed. Overall, we are fortunate to have a healthy field that is suitable for agriculture and especially dry farming.
As part of our farming trial in collaboration with OSU, we were visited by a soil scientist who discussed our local soil (referring to the published soil maps) and took a 5 foot core sample from our vegetable plot. Andy was fantastic and explained to us how the gentle slope was formed, with a sandstone "backbone" and soil "flesh," which is a common geology of the area. He also introduced to us the term "smectitic" clay, which represents the type of clay formed by the accumulation of mineral deposits at the bottom of the hill.
Below is the description of the 60'' soil core from Lilliputopia. Disclaimer: you may need a dictionary to fully interpret this-- at least I did.
Andy showed us how to use other soil resources as well. You can find your soil type at the NRCS website here (and mentioned that you can also use Google Earth). According to the NRCS data, the soil types found in our field include Santiam and Dupree, which appear to be good agricultural soils that can retain substantial moisture. However, as Andy noted, this map can be inaccurate at smaller scales, and said that our plot soil wasn't Santiam but "Willamette." Fortunately, most of the Willamette valley is considered fertile loam, so we feel lucky to be here!
Soil tests: an example and interpretation guide
Soil tests are available from various companies (or universities) and can measure many different variables. OSU provided a free soil test for each trial farm. Take a look at our soil test results and then read about how to make sense of each measurement.
Organic matter (OM): This is a proxy for the amount of carbon in the soil and in my opinion, one of the most useful metrics when evaluating soil quality (and improvement). This is because OM is positively correlated with water holding capacity, plant available nitrogen, cation exchange capacity (see below), and microbial activity. Thus, you want as much OM as you can get! Typically, soils have between 1-5% OM and a 2% increase over time is considered great. OM can be increased by the addition of organic matter including mulch, compost, and microbes. One farmer told me that he got his fields to upward of 15% OM just by incorporating leaves every year for many years!
Nitrogen (nitrate (NO3-N), phosphorous (NaHCO3-P), potassium (K): These are the three primary nutrients required by plants and typically are used to determine if you need to add the appropriate amendments. The chemical formulas are the measured compounds used to assess the relative quantities of available N, P, and K.
Micronutrients: Includes the elements B, Zn, Cu, Mn, Fe, and Mo. These are required by plants in smaller amounts.
pH: The pH (stands for potential of hydrogen) is a measurement of how acidic or basic a solution is. Pure water is neutral (pH 7) whereas acids have lower pHs and bases have higher pHs (ranging from 0 to 14). Because pH is measured on a logarithmic scale, pH 5 is ten times more acidic than pH 6. An adequate pH (usually neutral) is crucial to the success of your plants because it determines nutrient availability. Your plants will suffer from nutrient deficiencies if your pH is off, regardless if they are present in high quantities in the soil. In the PNW, acidic soils are common because of the high amount of rainfall. Agricultural lime is commonly used to raise the pH and prevent deficiencies. This figure shows how macro- and micronutrient availability changes with pH.
Cation exchange capacity (CEC): Cations are positively charged molecules. Many elements that are required for plant health have a positive charge, including potassium (K), calcium (Ca) and magnesium (Mg). Clay particles that are found in soil are negatively charged and will hold on to these cations, which can later be exchanged with the plant roots. Thus, the CEC measures the ability of soils to retain available nutrients and acts as a buffer to acidification. A high CEC is beneficial, and will increase with clay content and OM. Similarly, the percent cation saturation shows the actual amount of available cations in the soil at the current pH.
ECe: measures the electrical conductivity via salinity. High salt concentrations are bad and inhibit plant growth.
Our soil test: Looking at our soil test, we can see that we are lucky to have a great starting OM of 5.2%. The nitrogen level look a bit low but phosphorous and potassium are at adequate levels. As Andy mentioned, there are no critical deficiencies and our pH is fine. There are still many things that I don't understand and hope to learn in the future. We hope that our organic no till and mulching practices will continue to build up our soil resources, which should be evident in future soil tests!
Soil moisture data
We participated in a program with OSU to measure soil moisture over the growing season at four different depths in order to assess dry farming site suitability. Here is a graph and description of the soil moisture from our field from June to October. Note that over time the soil dries out and the meters read higher instead of lower values.
Andy believes that the water loss is primarily due to the action of the plant roots. I was surprised by this, because it seemed like the soil would just dry out over the summer regardless of roots. However, this presents an excellent point that I believe is critical for interpreting these data: we need a negative control that measures the moisture in a location that is not planted and free from roots! Hopefully we can set this up for next season. Additionally, I have continued to take measurements so that I can see the soil moisture profile over the whole year-- and it is quite interesting (more on that later)!
I am intrigued that Andy thinks that there may be subsurface flow, like an underground stream or spring. I do remember seeing water coming out of the hillside near the other end of the property. That should be great news for dry farming here!
Andy also observed that there were four "types" or patterns of profiles when he analyzed the data from approximately thirty farms, indicating that the soil moisture levels are affected by many different factors. We look forward to learning more about why each site looks the way that they do.
Well, that is the end of the soil blog for now. Hopefully you have learned a lot about why the soil is important and what you can do to help it thrive!