Why Standard Soil Tests Only Show Part of the Picture
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.
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 :)
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!