How to Interpret a Food Plot Soil Test
I was in a meeting the other day with a phD soil scientist Dr. Spargo at Penn Stat and my agronomist friend Derec Markle of Yonder Mountain Conservation, discussing soil testing and how to interpret a food plot soil test. I shared with them some examples of soil test results I had on my laptop. I like to be in a discussion with guys who know more than me so I can constantly increase my understanding of how things work. I picked up some tips and got a new perspective on food plot soil tests and how to interpret them.
How to interpret a food plot soil test is a little different than how to interpret a farm crop field soil test. First of all, the recommendations from a soil lab will give you fertilizer inputs that are considered to be a rate where more inputs would not yield an economic return on that crop you are testing for. Secondly, since we are not removing the plant material from the field as we would in a crop field, we are not losing hardly any of our nutrients and probably not changing the pH much as we grow our plants. Much of the time, all we really need to concern ourselves with is the pH of the soil and the general health of the soil. If we have our pH and add enough inputs to get our initial crop started, we probably don’t have to fertilize after that as long as we keep a good cover of growing plants on the site.
The other thing that came to light is that the soil test is only a chemical analysis. It does not address the site and soil conditions.
Excessive Nutrient Levels – is There Such a Thing as Too Much?
This soil test report shows excessive levels of pH, P, K and Mg. It has huge levels of Calcium at over 4,000 ppm and good levels of Sulfer. The CEC is looking good as anything up around 15 is good for PA soils.
Now, take a look at the video of this plot as I have struggled to get anything to grow there: Video of the food plot where this soil test came from.
AS you can see, there is not much growing there despite my planting a cover crop cocktail of peas, beans, brassicas, buckwheat, clover.
So, the soil nutrient levels are not the whole story. This site has other problems such as seasonal flooding, extremely dry, very stony soil and a LOT of browse pressure. The combination of these site conditions prevent a good looking stand of forage to grow.
Having said that, the Landowner reports seeing 20 deer come into the plot during an evening sit. This is important to point out – I often see pictures of guys showing off their wonderful huge turnips like they are growing truck crops for market. If they look that great, then deer aren’t eatin’ them. A food plot that is getting food into deer and attracting lots of animals looks like you went over real fast with a brush hog – all chewed up. It is a waste of time and money to pant more food than is necessary for the deer population or plant things they are not going to eat.
How to Interpret This Soil Test
At first glance, I would surmise that this test has too high a pH, too high nutrient levels. Which made me wonder if the excessive bar graph readings showed a soil that had toxic levels of nutrient elements. This is what an NRCS guy told me and he advised me to grow some corn and take the corn off to get the levels down.
Talking to my soil scientist friend about this, he informed me that you would have to be way higher than this to have any problems with nutrient uptake. While it is possible to have too high magnesium levels interfere with Potassium uptake and vice versa and too high Calcium interfere with availability of other nutrients, having your pH correct should solve this and the plants themselves will figure out how to get what they need at the correct levels.
The Bar Charts on the Soil Test Reports
Remember that the bar charts show the levels are excessive but that doesn’t necessarily mean they are bad. It just means that at these levels, there would be no economic gain in adding additional fertilizer to the soil.
Potassium and Clover
Often my soil tests give me a recommendation for potassium when I test for clover even when no other nutrients are required. For a food plot, according to Dr. Spargo at the Penn State soils lab, it probably won’t make any difference in your clover if you add it. The test recommends adding K assuming the forage will be taken off for hay or grazed by cattle.
The Magic of Compost
The most important characteristic besides pH in your food plot soil is the level of organic matter. The ability to hold and cycle nutrients depends on the amount of clay and organic matter.
What is Organic Matter and why Does OM Matter?
O.M. is anything that was alive and is now decomposed in the soil. This can be dead animals (including bacteria), animal waste, leaves, crop residue and dead plants. It is what gives rich soil that nice brown color and cake-like texture. It is what the soil biota live on. Soil biota is another article but it is critical to soil health and its ability to feed plants and, in turn, feed animals.
Clay and O.M. also hold soil moisture. Once water is gone from the pores, it is very hard to grow a plant. Another difficult site I work with is shown in this video. The soil tests out good but the site is not very productive, yet attracts deer anyway. The site conditions will not allow a nice looking food plot to develop. Dry, stony, silty soil with no clay, and high deer pressure prevent a good crop from developing. Plants are slow to develop and when they do, the deer mow them down to the dirt in short order.
You should strive for at least 5% O.M. but that is not easy with abused soil that was poorly managed by farmers for decades. Some field soils have levels at around 2 or 2.5 or even less. OM is the lifeblood of the soil, home for much of the soil biota, the stored water and the cation exchange sites. Small increases in OM have huge increases in water holding capacity and nutrient holding capacity.
Constant tillage and chemical inputs, leaving soil bare 7 months per year will reduce OM greatly.
Another thing to watch out for is that the test involves cooking out all the OM from the soil and comparing the weights before and after. So, they may have burned off something like sticks and sawdust to come up with the numbers, not necessarily good composted material.
Types of Compost
On this food plot, I applied several truckloads of sewage sludge compost as well as town compost made of leaves and kitchen waste. I did not add any lime to this plot. The sludge moved the pH from 5.8 to 7.3. It also got my CEC, macros and micros all up in the excessive range without any fertilizer inputs. Sludge compost has wood chips in it and may need to be cooked down some but its loaded with goodies. The town compost is great OM and is more expensive. Sludge is wonderful if you have reclaimed strip mines that are down in the 4s and low 5s on pH.
What Else is Important to Look at on a Soil Test Report
Calcium
Calcium should be up over 1,000 ppm. It can usually be supplied by adding enough lime (CaCo3) calcium carbonate to get the pH to 6.5.
Sulfur
Sulfur is found in the protein molecule so is needed to build plant tissue and provide protein to the animals that eat them. Interestingly, Sulfur here in the Northeast has been in great supply from smokestacks in the factories of the Great Lakes Region precipitating out. Since we put scrubbers on factories to reduce “acid rain” deposition, we have a shortage in the soil and it keeps going down. Sulfur should be around 15 ppm or more. One way to get some more on the land is to fertilize with Nitrogen Sulfate fertilizer if you really need N or straight sulfur if you are planting legumes and don’t wish to add N. They say that the wonderful black cherry that was grown in the Alleghany region was due to high sulfur content acidifying the soil. But that brings with it things like Aluminum toxicity and really low pHs. The nice cherry is gone anyway and so are the markets for it.
Phosphorus
Most important thing this is for is ATP, or the currency of the cell – the molecule that transfers energy for cell function and is necessary in seed germination. Pretty important. It should be at 30-50 ppm, higher end if harvesting forage.
Potassium
K is used in water utilization, acts as a catalyst in cell chemistry and is important in disease resistance. Levels should be 100-200 ppm. For food plots, the lower end of this range is fine.
Magnesium
Mg is a macronutrient that is one core element along with nitrogen in chlorophyll and plant structure. It is a also shows up down in the base saturation panel. Target levels should be 60 and up to 240 ppm and base saturation percentage of 15%. I will get to the CEC and base saturation in a minute.
Mg is found in many enzyme functions in animals.
I acts to glue loose soil particles together. When soils are “tight” you want to counteract this with increased calcium and when “loose” increase Mg. The two should be in the correct ratio in the base saturation percentage. 4:1 Ca:Mg is good for tight soil and 7:1 for loose soil to firm it up… too much Ca can interfere with uptake of Mg.
To get Mg levels up, we apply dolomitic limestone which contains magnesium. Or it can be obtained in other forms for fertilization.
Nitrogen
N is one of the most important elements, so why is it last on this list? Because it is a complicated thing to try and add it to the soil. Suffice it to say that proper soil OM, use of legumes, healthy soil biota and cover cropping to have the soil covered and growing a living root and shoot as much of the year as possible will provide all of it that you need. There are tens of thousands of tons of N above every acre of land as the air is 80% N.
What is CEC?
When I asked Dr. Spargo to give me his layman’s explanation of the bottom panel of the test report he said “ignore it.” I won’t ignore it and will give it to you straight.
Cation Exchange Capacity is the ability of the soil to hold positively charged atoms like K, Ca, Na, Mg as well as positively charged (NH4 which can’t be used in this form by the plant). It is a relative number because it depends on pH, %OM and clay component of soil. Clay particles are where these sites are located (they have a negetive charge.) If soil is too acid H+ molecules take up the space and there is no place for the nutrient ions to sit and get picked up by plant roots. This is why pH (the measure of H+ molecules free in soil) is important.
Roots exchange H ions for nutrient ions and take them up into the plant. So, the higher it is the better. Good CEC is in the teens. In Pa, 11 to 15 is generally what I see. Pure humus or pure clay have CEC values in the hundreds.
Using Soil Test Reports to Repair Poor Soils
When I first began working on this field, it could not grow anything but deer tongue grass, pine trees and sweet ferns. After years of tillage (I would not do that today) manure applications, truckloads of lime, planting food plots and keeping cover crops on the land, this plot went from a pH of 4.8 and unable to grow any forage to a 7 with good nutrient levels. This acre feeds about 20 deer per day. When I was planting 5 acres (I put 4 in switchgrass) the neighbor once told me he spotlighted the area one night and quit counting when he got to 50 deer. That’s the power of correcting the soil through testing and the smart application of amendments and plantings.
When you have your soil chemistry correct, you can start pounding it with cover crop cocktails. This will continue to increase the productivity of your soil and in turn, attract more deer and other wildlife to your property.
One way to get marcos, micros, lime and organic matter in one application, consider the use of composted manure. Manure compost can be mixed with lime or in the case of sludge compost, the pH is very high and will move the pH quite a bit on your land.
Video about chicken manure: https://youtu.be/BFee-vi8MrI
Another compost video: https://www.youtube.com/watch?v=BGjPOJgKdvs
Dirt to Soil by Gabe Brown is a very straight forward easy read that will change the way you think about fertility:
Neal Kinsey is a good read as well
I consider Craig Harper’s book the Bible of wildlife habitat improvement
