Standard Soil Test + Saturated Paste Test = Complete Soil Picture

Why Standard Soil Tests Only Show Part of the Picture

Logan Labs Standard Soil Results. Click to enlarge.

At the Acres USA eco-farming conference, we learned a lot about testing plant tissue to determine the health of our pasture grasses and specifically, what nutrients the grasses are picking up from the soil.  We learned that soil tests are usually poor predictors of what and how many nutrients will get into plants.  There’s a lot going on between soil and plants.  Soil biology must be hearty enough to help the plant’s roots absorb nutrients.  The soil is the plant’s digestive system, and if it’s not working at its full potential, plants won’t pick up the quality and quantity of nutrients they need.  It’s the same with us – we can eat all the healthy food in the world, but if our digestive systems are unhealthy, we’re not going to absorb the nutrition.  That’s why healthy soil with super robust biology is so important to plant health.

The Saturated Paste Test

Many conference speakers encouraged the audience to get a saturated paste soil test.   The saturated paste test shows what nutrients are immediately available in the soil’s water solution.  These are the easy access nutrients for plants, so this test better predicts what nutrients (and how many) will get into the plant.  Logan Labs describes the standard test as the soil’s “savings account” and the saturated paste test as the soil’s “checking account”.  Both show nutrients that are accessible, but the checking account nutrients are more easily available.

Logan Labs Saturated Paste Results

We came home from the conference and sent in four pasture samples to Logan Labs.  Each sample was a quart-sized ziploc bag full of soil.  You need to send in more soil than usual if you’re getting both tests.  The pictures show the results of our standard soil test, our saturated paste test, and Logan Lab’s guidelines.

What Both Tests Show Us

Comparing the two tests with the guidelines, it’s clear that magnesium and potassium are pushing out calcium.  On the standard test, it looks like we have enough calcium in proportion to our very sandy (very low) Total Exchange Capacity (CEC), but both magnesium and potassium are too high, especially in base saturation.  The saturated paste results confirm that plants don’t have enough easily accessible calcium.  Calcium is one of the most important plant nutrients, and excesses of magnesium and potassium spell trouble, so we need to figure out how to get this balanced.

Phosphorus is also interesting – the standard test shows we have plenty, but the saturated paste test shows we barely have any phosphorus that’s easily accessible.  This, combined with the low calcium availability, might explain why we have a constant broomsedge problem.  Phosphorus is an anion (negatively charged) that easily locks up with other nutrients.  Active, healthy soil biology is the key that unlocks it.  These two tests are confirming our soil’s biology is lacking.   Like we didn’t already know, thank you 🙂

Our Plan

Logan guidelines. click to enlarge.

Besides maybe some sulfur, zinc, and copper, we’re not going to add any fertilizers.  With our very sandy soil, we’ve learned that it takes very little fertilizer to throw things out of whack.  Take a look at our soil test from last year, for example.  Potassium was lacking, especially in the West field, at 2.6% base saturation.  In Spring 2011 we spread just 250 lbs/acre of potassium sulfate, which is 40% or 100 lbs of potassium, and the base saturation jumped to over 6.5%, way too high.  Perhaps the soil needs more time to straighten out, and December probably isn’t the best month to test.

We need calcium to come down closer to 60% base saturation on the standard test, but that’s probably not wise since it’s so deficient on the paste test.  We want magnesium to come down to maximum 20% base saturation (calcium plus magnesium should not total more than 80%), and we definitely want potassium to come back down under 5% base saturation.  We’re hoping the soil can do this on its own, especially with the fallow cover crops and biology inoculants we plan to introduce in Spring 2012.  More to come on that- thanks for reading!

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4 responses to this post.

  1. Hi Kelly,
    Thanks for all the thoughts on improving your soil, it’s great to see your process. We’re interested in getting a soil test this spring and I was curious why you went with Logan Labs? What were your considerations, etc? Why not use the soil tests provided by the local extension service?
    Thanks

    Reply

    • Hi there! We went with Logan Labs for a couple of reasons. First, they have a very good reputation for repeatable results and taking time with each test. Second, when i first discovered how bad our soil was, I ordered Astera’s Ideal Soil Handbook, and that book recommended Logan Labs. I didn’t know it at the time, but Logan sticks closely to Albrecht’s school of thought, a true base saturation test that focuses on the importance of getting calcium, magnesium, potassium, and sodium in the right balance so the soil can “open up” and let soil life thrive. All other nutrients are just as important, but the idea goes that getting the first four balanced properly will make other soil corrections easier and more effective.
      Our very first soil test was with our extension, and it didn’t give us much to go on besides N, P, K recommendations, which don’t fit well with biological ag. Our extension test calculated calcium as well as base saturation and CEC, but the numbers seem to be screwed up (percentages don’t total 100, etc.) If you have a good relationship with your extension agent and want to use his/her test, ask for a results sample and how he/she uses the results to create recommendations. If all you hear is N, P, K, I’d stay away from that test. There’s so much more info you can get with other testing services associated with biological farming. Good luck and let me know if you have more questions!

      Reply

  2. Posted by Rich on December 30, 2011 at 6:00 pm

    I hope this makes sense, but isn’t a soil test really a “snapshot” of a moving target?

    Using a field similar to yours as an example (i.e soybeans planted the previous year, fallowed over the winter, and a soil test in the spring), this first soil test was at a point in time in which nothing significant was growing, so all the nutrients were theoretically in the soil (and would show up in a soil test).

    Then at a point in late summer, a cover crop of sorghum-sudan and peas has been planted and is still growing. At this point, a portion of the nutrients that were in the soil are now in the plant and wouldn’t necessarily show up in a soil test.

    In early fall, the cover crop has been tilled into the soil, but the residue hasn’t completely broken down. So, some of the nutrients are still present in the residue and will slowly become available (to plants and a soil test) as they break down. A soil test at this point in time will only show what is theoretically available to a plant at this particular point in time, and can’t predict what will available in the future due to rainfall, soil activity, or composting residue.

    I could continue on and on, but the point is can you compare these “snapshots” to each other and get a true picture of your soil’s condition? Or, do you need to compare apples to apples by taking soil tests after fallow periods, etc.

    Again, I hope that makes a little sense.

    Reply

    • Hi Rich, I’m not sure it matters so much because plants are roughly just 5% minerals from the soil (ash). And these fields, except the mccarthy one, have been in the same perennial grasses since 2008, so to me it’s an apples to apples comparison. I’ve heard the base saturation percentages aren’t as useful with very sandy soil like ours, so the paste test gives us another view. I imagine the paste test would vary quite a bit during different times of the year, but less so with the standard test.

      Getting a test from a lab with a good reputation for consistency and quality control enables us to really use the test results. I’ve heard bad stuff about many university and extension labs, where work study students do the testing and don’t feel compelled to actually run the tests. There’s an interesting account of this in Neal Kinsey’s Agronomy book.

      Reply

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