Managing Soil Phosphorous

Originally posted as my September blog as an Illinois Soy Envoy Under the title of  "Three Phosphorus Tests and Which to Use on Your Soil"

Phosphorus was recently in the spotlight as the American Society of Agronomy celebrated Phosphorus Week, September 15-21. It seems appropriate to celebrate the discovery of one of our most important crop nutrients and focus on how to manage it better.

Discovered 350 years ago, phosphorus is the 11^th most abundant element on earth. Because it is so plentiful, it would seem it should be one of the easiest to manage for crop production. However, it is not evenly distributed and not always readily available for plant growth. According to Emerson Nafzinger, University of Illinois Extension, soybeans remove 0.75 pounds of P₂O₅ per bushel of grain. Some of that phosphorus comes from the soil and some comes from added fertilizer. 

The first step to better phosphorus management is testing. Over the years, many tests have been developed to help producers determine if they need to be adding phosphorus fertility to maximize crop production. One of the first universally accepted tests was the Bray P1 test. Ideal P1 levels have been established and the test has been in use since the 1930s, so we have lots of data on removal and ideal levels. P1 ideal level is 70 pounds per acre. Bray P2 uses a slightly more acidic extractant. It was developed to help with application of rock phosphate as a fertilizer. Today, in soils with pH below 7.0, P2 can be used to determine reserve levels of phosphorus in the soil. The Bray tests are colorimetric, which means the amount of light passing through a liquid is measured to determine P levels. 

Mehlich 3 is also a commonly used test for phosphorus. The Mehlich 3 extraction is used to analyze many of our nutrients including potassium, calcium, magnesium, sodium and the micronutrients. The test is read by ICP which is a type of atomic absorption spectrometer. Mehlich 3 readings tend to average a bit lower for P, so ideal levels could be adjusted higher, although University of Illinois Extension does not do so. I have run both P1 and Mehlich 3 on many samples over the years and find that the comparison is not always linear. Use whichever you or your lab prefers and use the same test every time. Beware that some labs run Mehlich 3 but calculate a P1 test—this is not ideal.  

Olsen P is another test I use on high pH soils. The calcium in high pH soils can tie up phosphorus and make it less available for plant growth. Olsen gives a better idea about P availability on high pH soils and you should consider running this test if you are in that situation. Ideal Olsen Test is 30 pounds per acre. 

Iowa State University presents a good comparison of the P tests.  

While low soil phosphorus can have a negative impact on crop production, high soil phosphorus is associated with high levels of dissolved phosphorus in our water. When I was matriculating at University of Illinois at Urbana-Champaign (UIUC), we were taught that phosphorus did not leave the field except by erosion, and movement was not really a problem because it did not affect crop production.

We have since learned that about half the phosphorus that leaves the field is by erosion and the other half is dissolved in runoff and drainage water. The other problem with phosphorus loss is that relatively low levels can cause harmful algae blooms as we’ve seen with Lake Erie in recent years. The phosphorus itself is not harmful for human consumption, but the toxins produced by the algae are harmful. The algae bloom caused by excessive nutrients can also remove oxygen from the ecosystem and cause eutrophication, or the death of animal life due to lack of oxygen. The eutrophication can negatively affect a lot of aquatic life. Some of our most desirable game fish are the most vulnerable. Excessive phosphorus can also interfere with zinc uptake. 

Managing the phosphorus levels in our soils is important to crops and the environment. Phosphorus is already one of the most highly regulated nutrients in the United States. Places like the Chesapeake Bay and Lake Erie watersheds are the most prominent. The regulatory levels of phosphorus may be reasonable enough, but the problem with regulations is that they remove management flexibility for the producer. The target now for more regulation is focused on the Midwest because of hypoxia in the Gulf of Mexico.  Perhaps we can hold back those regulations by responsible management.

Here is a list of steps to take to better manage phosphorus

• ·       Keep phosphorus test levels in the 70 pound per acre range.
• ·       Don’t add fertilizer when soil test levels exceed the ideal amount.
• ·       Placing fertilizer below the surface can be effective in managing losses.
• ·       Keep soil pH in the 6.0 to 7.0 range.
• ·       Do not apply manure on snow or frozen ground.
• ·       In manure systems, keep soil test levels below 300 pounds per acre. 
• ·       Keep soil loss at or below tolerable levels.
• ·       Keep sediment on the field using structural and vegetative methods.
• ·       Soil test often to monitor levels.
• ·       Use soil tests appropriate for your soils. 
• ·       Use 4R principles in making decisions about how much phosphorus to apply and when. 

As we celebrate the discovery of phosphorus, we need to understand that levels that are too low can have a negative effect on crop production, but levels that are too high can have a negative effect on our planet. If you’d like to learn more, Pete Kleinman, U.S. Department of Agriculture, gives a good overview in his blog on why phosphorus is needed on farms. The site also has additional resources for added context and perspective. 

Visit rpmsoils.com to learn how we can help you manage your phosphorous.  

Heat in Brazil

By Eduardo Paim

I waited with good expectations to tell you about the start of our soy planting. It is not happening so far, the heat is very strong, far beyond what I remember. Our planting did not start well. Who is planting is risking a lot because the rains are irregular. I believe it will be a difficult year if the heat continues like this!

Cover Crop Selection

Cover crops are being widely promoted and slowly adopted. Farmers don’t doubt the benefits covers play in soil health, but often question the economic prospects of planting one and if there will be a return.



Since cover crops are still the exception, here are seven reasons to plant cover crops. 

1.       Cover crops can help you implement a no-till system.

2.       Cover crops can help reduce erosion by improving aggregate stability.

3.       Cover crops support soil microbial activity.

4.       Cover crops can help with weed control. 

5.       Cover crops help build soil organic matter.

6.       Cover crops can reduce the need for commercial fertilizers.

7.       Cover crops can extend the grazing season for livestock.

As with any new practice, you want to do everything possible to make it successful. Selecting what will work best with your operation will support your decision to use cover crops. Often, we hear suggestions that you should first decide what you want the cover crop to do, but that may be difficult as one of my clients said, “I want it to do everything.”



One of the big things to focus on is to increase soil microbial activity. Soil microbes drive the soil system.  The more microbes we have, the more we release nutrients, and the more we promote aggregate stability. There are many microbial products on the market, but why not grow your own? Multispecies mixes will best support microbial activity. Each species added to the mix promotes different microbes.  Five or more species would be considered ideal. Nitrogen-fixing species should be part of the mix, too. There can be pitfalls to growing a lot of different crops so you might want to consider something simpler to start with.



Extending your grazing season sounds simpler. Lots of producers like to graze turnips in the wintertime.  This a good choice because the cattle love them. Ideally, turnips should be planted by early September.  How do you do this? Fly them on or use a high clearance applicator adapted to seeding. Sounds expensive? Maybe you can start with cereal rye or triticale drilled after harvest. Both are suitable for winter grazing and both are also suitable for green chopping in the spring.



Maybe you want to start by growing your own nitrogen. Legumes are the way to go. Crimson clover, red clover and hairy vetch are popular choices. One traditional way to use clover is to plant it in a small grain crop in spring. Then after harvesting the grain, let it grow out for hay. Next spring, once again it needs to grow into May to maximize nitrogen production. Hairy vetch or clover may be planted into soybeans aerially or by high clearance equipment by September 1. It should also grow into May to maximize nitrogen fixed and stored in the soil for the next corn crop.    



If you want to open your no-till ground to increase water infiltration and improve root penetration, you may want to try oilseed radishes or annual ryegrass or maybe both. Radishes experience winter-kill, so that is a bonus. Annual ryegrass can be difficult to kill, so work out how you plan to terminate it in spring.



Some producers like to keep it simple by planting cereal rye or triticale after corn. Soybeans can be planted or drilled either before or after termination. A simple cover after soybeans is oats and radish. It winter-kills and can be easy to plant into in spring. Oats and radish should be flown on or planted into early maturity beans after harvest.



Consider soil health testing to provide yourself with some scientific data to prove that you are accomplishing what you want to with cover crops. Run a before test and another maybe two or three years down the road, and notice the difference you will see.



There are way too many options to cover them all in this short blog. Michigan State University maintains the Midwest Cover Crop Tool, which is a good place to start when picking your species of cover crop to plant. Don’t be afraid of choosing a more complicated mix, but do your homework on how to make it work. Contact us at rpmsoils.com 

Road trip Effingham

I made a short road trip to Effingham today to do a septic tank evaluation.  Most of the corn along the way was pollinating or past pollination, although not in some of the uneven parts of fields.  Some fields are starting to run out of nitrogen.

Soybeans looked better than corn.  We had a lot of rain earlier in the week and the soybeans were putting on a lot of top growth.  Two issues that will cut into yields at this point are weeds that were starting to show through, and poor stands.

Zombie Weed Apocalypse

By: W Kevin Nelson, CCA 4R NMSSenior Agronomist, Prairie Agronomics, LLC 

The growing season of 2019 just keeps on giving. Without going over all the morbid details, let’s just say that it’s been challenging from the very beginning, and looks like the challenges will continue on through to harvest and beyond.

As we get into the latter part of summer we are still contending with weeds that have refused to die. For lack of a better term, zombie weeds. These weeds have had burndown herbicide treatments, pre-emergent residual treatments, one or more post treatments, and still refuse to die. Some are newly emerged, some have just not succumbed to the programs we had planned before the spring weather went off the tracks. And now they have gone to seed, or soon will.

At this point, another spray trip becomes little more than an attempt at revenge. The plants are way above label size restrictions, and have hardened off with the dry conditions of the last few weeks. The chances are they won’t die, and the seed on them is probably already viable to bring on the next generation.

Am I going to give you the solution to the zombie weed apocalypse? Probably not. But I’ll point out that this weed control disaster isn’t a failure in any one program or system. We can find fields under all types of management that is experiencing problems, owing mainly to the conditions of the growing season. What I will say is this: start right now addressing the weed problems that will probably be coming our way in 2020.

Since many, though not all, weed control systems now combine seed traits with herbicide products, now is the time to start putting those combinations together. These systems must start with variety and hybrid selection, and seed companies will be on your doorstep early this fall. Choose a system that fits your needs from a production standpoint as well as weed control ability.

Consider cover crops to suppress weeds this fall. Some cover crop mixtures are better than others at keeping weeds down, and the consideration on how to terminate these crops needs to be factored in. Seed supplies may be an issue, as well.

Talk to your retailer about a fall burndown treatment. This can make a tremendous difference in weed pressure in the spring. With what looks like an extended harvest, it is crucial that you make your plans and let your retailer know with plenty of time what you intend to do.

So as you look out over your fields, frustrated with the zombie weeds poking through your crops or covering your Prevented Planting acres, remember that they aren’t coming after your brains, but you will need to put your brain to work now to prepare to hold them in check in the coming seasons.

Corn Harvest in Brazil

By Euardo Paim:

 Here in MT we are harvesting corn and the yields are very good! In an average harvest we harvest 90 bags of corn per HA, this year the yields are at 135 bags of corn per HA. In some regions we had rain breakdown, but it won't be a big problem

Managing Micro-Nutrients in Soybeans,

Written for IL SoyAdvisor,  

Are you paying enough attention to all the nutrients your crops need?

Byline: Dave Rahe

Topic: Nutrient Management

There are 18 essential nutrients [JF1] in plants: Nitrogen, phosphorus and potassium are the most likely to be added as fertilizer. Water, carbon and oxygen come from air and water. Calcium, magnesium and sulfur are needed in large amounts as well, and in Illinois we generally take what we get from nature.  Sulfur now commonly needs to be added along with the big 3. We add calcium and magnesium when we lime, although in Southern Illinois we generally come up short on magnesium in our liming materials. And micronutrients are also essential, but needed in smaller amounts. Boron, iron, manganese, zinc, copper, chlorine, molybdenum and cobalt are generally considered essential micronutrients and round out the 18. Some would add silicon to that list as well. 

The first step in getting your micronutrient levels in order is to get your soil pH right. Soil pH affects the availability of all plant nutrients, but in order to keep micronutrients available, pH needs to be in the sweet spot between 6.0 and 7.0. The chart above is from Michigan State University.

Once soil pH is corrected, the next step is to soil test for micronutrients. It is surprising how many people still say that testing is not needed for micronutrients. In the modern world, it costs very little to get the extra data. Another rub is, micronutrient testing is thought to be unreliable in determining a plant or yield response to soil test levels. My response is that you need to start somewhere. 

If you still want more data, I have found tissue sampling to be useful in determining whether micronutrient deficiencies exist. Last year, I had some soils with relatively low manganese test levels, but by tissue testing, I found that manganese wasn’t an issue. Tissue sampling in soybeans is done at R-1 or R-3 growth stages. North Central States has a publication on Managing Micro-Nutrients for Soybean Production in the North Central Region.

A few cautions are in order concerning tissue testing:

·        Time of day sampled can make a difference.

·        Sometimes dust on the plant can skew results.

·        Don’t test when the crop is stressed. Of course, it will show deficiencies, but the plants are unlikely to respond to treatment.

After collecting your data and determining you have a problem, what do you do about it? First off keep in mind that micronutrients can be toxic if over applied. Soybeans are especially sensitive to boron.  Copper can be toxic at as little as 10 ppm. I work on some soils where copper is over 100 ppm caused by industrial pollution, which is very difficult to manage.

Many researchers report that a blanket application of micronutrients without some indication they are needed isn’t economically wise. If treatments are needed during the growing season, foliar applications can be effective in treating symptoms. I have also found that soil applied micronutrients can be effective in treating low soil test levels. Sometimes soil test levels remain at desired levels for several years after treatment. I suspect we are stimulating microbes who will consume and then recycle micronutrients back into the soil. 

This leads to the question about biological treatments for micronutrients. One of the reasons that pH between 6 and 7 is so important is that it is ideal for microbial activity. But the big question is, can I improve micronutrient availability by using one of the new biological seed treatments? I think biological treatments can work, but it seems to be on a farm by farm and year by year basis, so if you want to try biologicals, you should also be prepared to do some strip trials to see what works on your farm.

There are also some farmers using silicon as a fertilizer. Silicon is one of the most abundant elements.  The building framework of our soils is alumno-silicate clays. There should be enough available in most soils, however some trials have shown a good response to fertilizer silicon. The downside is that we have little idea on response or economics and currently there is no soil test for available silica. To learn more about this element click Silicon Fertilizer Gaining Ground.  

In summary, micronutrient deficiency isn’t a common problem in soybean production, especially when pH is maintained between 6 and 7. Soil and tissue tests can be useful in identifying micronutrient issues, and microbial treatments may be useful as well. Finally, keep your eye on silicon as an emerging fertilizer.