Update on Sorghum Sudangrass & Cowpeas Cover Crop, and Groundhogs Show How Bad our Soil Is

Unfertilized sorghum sudan cover crop 30" tall seven weeks after planting

In this earlier post, I laid out all our hopes and dreams for our very first cover crop attempt.  This cover crop begins our restoration effort to help our neighbor’s fields recover from abuse and years of no-till roundup ready soybeans.  We want the cowpeas and sorghum sudangrass to fight weeds and to provide a lot of biomass that will eventually turn into organic matter.


The sorghum sudan and cowpeas emerged and established nicely.  We’ve had nice amounts of moisture this season.  The sorghum sudan is about 30 inches tall and the cowpeas are about 6 inches shorter.   We planted both on April 30th; this is slow growth for sorghum sudan.  Its color is a kinda sickly yellow-green, and its leaves are narrow, signs we interpret as nitrogen deficiency.  With organic matter around 1.5 to 2%, the soil has very little capability to supply nitrogen to growing plants.  The cowpeas, which look better than the sorghum, are probably supplying some nitrogen, but the demand likely far outpaces the supply.

Weed Fighter and Biomass Provider

Cowpeas well-established, but getting shaded by sorghum sudan

Plenty of weeds are growing too, especially common ragweed and roundup resistant marestail.  But so far, the sorghum sudan is outpacing them.  Regarding biomass, we decided not to fertilize even though we know the sorghum sudan loves nitrogen because fertilizer is so expensive and because the rate of growth is meeting our low expectations for our first cover crop attempt.   We want lots of biomass to till in later this summer, but not so much that we feel completely overwhelmed in our first go at rotovating.  We also want the biomass to break down quickly so we can plant the winter cover crop by the end of August.    

Groundhog Phenomenon

Around groundhog holes, sorghum sudan is twice as tall, thicker, and a deeper green

The groundhog dens have a story to tell!  Around each entrance, the sorghum sudan is twice as tall, a darker green, and has much thicker stems and leaves.  It feels like the groundhogs are mocking us!  What’s going on?  Obviously the soil around the entrances is much better than the “topsoil” in the fields.  Louis Bromfield in Pleasant Valley wrote about how groundhogs drag subsoil up to the surface, and if your crops look better around groundhog holes, that’s proof that your subsoil is better than your topsoil.  Sad, sad, sad!  I’ve also heard that groundhogs poop around their den entrances to fertilize what’s growing there so it provides better cover.  Based on my limited knowledge, I’m going with the poop.  Mr. Bromfield could be right, especially since some nutrients like calcium, sulfur, and boron leach down into the subsoil so easily, but I’ve soil tested the groundhog soil, and it’s worse than the surrounding field’s soil test.  I believe that the fields’ limiting factor is microbes – bacteria and fungi living in the soil.  Groundhog poop can supply that, and microbes help the soil come alive and help deliver nutrients to plants.  We see the same amazingly better plant growth around compost piles on our fields.  Even though it’s a bummer, it’s helpful to see this difference.  It’s a reminder of where we’re headed and how much better our grass and crops will grow… eventually.

Our Plan

Man, I wish we had hoardes of animals to provide these microbes!  We don’t have the time to take care of animals right now, so we’re investigating soil inoculants.  They’re pretty cheap (around $5 per acre), and they can help jumpstart soil life.  We plan to inoculate the sorghum sudan and cowpea cover crop just before we rotovate it in.  Also, when we plant the next cover crop, we plan to mix the seeds with mycorrhizal fungi.  This is probably the most important inoculant, one that is tied closely with organic matter creation in the soil, and one that early evidence shows is easily destroyed by lots of roundup.  I’ll talk more about this in a later post.

What’s Next

As described in the earlier post, we want the sorghum sudan to grow to about 3 to 4 feet tall, then mow it so it will set down deeper roots.  We’re hopeful it will grow that tall by the time the ragweed and the marestail start to set seed.  No matter how tall the sorghum is, we’ll mow for weed control.  We’ll wait one to two weeks for a green regrowth, spray the crop with inoculants, then rotovate it in.  I’ll let you know how it all turns out.  Thanks for reading!


8 responses to this post.

  1. Posted by Rich on June 23, 2011 at 11:09 pm

    “…but I’ve soil tested the groundhog soil, and it’s worse than the surrounding field’s soil test…”

    If the grass is around the groundhog holes is taller, darker green, etc., how can it be said that the soil is worse than the surrounding soil?

    Have you considered fertilizing some “test” strips across your field to see if fertilizing would be worth the expense in future years?

    Some teat strips might show you that fertilizing at a 25 lb/acre rate gives you 65% of the growth as a 50 lb./acre rate.


    • Rich, thanks for commenting! I’m puzzled too by the groundhog soil test results. With growth differences that dramatic, you’d think that soil would be great. But the test came back with a low pH and low in all nutrients. I could be my sampling error, maybe I’ll try testing it again.

      We’ll definitely fertilize in future years, and thanks for the idea on test strips. We have a few bags of ammonium sulfate, so I might spread that and see.


  2. I would highly recommend cutting based on the ragweed, not the sorghum. It is the limiting factor as it will become too woody sooner for good breakdown in the soil.


  3. Posted by Kevin Hulbert on March 13, 2013 at 2:32 pm

    The darker green color of the sudangrass near groundhog holes could be from nitrogen given off from manure of the groundhog. The cowpeas may not have given off enough nitrogen to offset the needs of the sudangrass.


    • Hi Kevin! Yes, I believe you’re right. I’m planning to grow sorghum-sudan again this summer (2013), so we’ll see if the soil can provide some N on its own now that the soil is healthier than it was in 2011.


  4. Posted by Kacee on May 22, 2013 at 10:02 pm

    Hi we are growing sorghum sudan this year to make silage bales for our cows. I can’t find this anywhere online… How long does it take to germinate?


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