Thursday, April 11, 2019

Soil pH and Nitrates

I got the lab results back two days ago on my soil samples.  As any good grower will tell you, getting your soil tested is the starting point to growing a big pumpkin.   If you don't know what is in your soil then you don't know what to add and you need to keep everything in balance.   Yesterday I called Western Labs to talk with John Taberna.  I usually give him a call after getting my reports back to understand pieces of it better and he usually ends of adjusting some of his recommendations based on the questions I ask him.  He is the best soil scientist I know of.  He sometimes gives more detailed recommendations on what specific types of fertilizers to use when I talk to him on the phone than what is in the report and he usually will have recommendations on how and when to best apply them.

One thing I asked him about is why one patches nitrates where way higher than the other and why the nitrates where much higher than I've ever had before on any previous soil report.  I treat both of my patches the same and typically put down in the fall about the same amount of compost on each patch.  The answer was interesting, but should have been obvious.   The nitrates in the greenhouse soil were about 6 times higher than the outdoor patch.  I asked why?  The answer, after some discussion, was that the nitrates in the outdoor patch had leached out from all of the winter snow. 

I don't know what our total accumulation of snow was for the winter, but for the better part of the winter we had 3+ feet of snow in the yard.  That means we probably had at least 4" of precipitation or more that percolated through the soil, leaching out the nitrates from the soil.  The greenhouse soil was covered so it only saw a small percentage of water from the irrigation I gave it from time to time during the winter.  Nitrates don't stay in the soil very easily and can leach out.  John says the nitrates in the outdoor patch are still there, but farther down then the 5-8 inches I was getting my soil samples from.  After knowing that and seeing the amount of organic matter in the soil he adjusted down the amount of nitrogen to give the soil during the season.

The other part of the conversation that was interesting with John was about soil pH.  I think sometimes we as growers get too caught up on some aspects of different parts of growing when in fact is that it may not be as important as one might think.  

When looking at my soil pH which is at 7.6, John said, "That is right about where you want to be."  That surprised me.  He then mentioned that Steve Daletas and Ron Wallace have pH's that are about the same as mine.  A lot of what you read is about nutrient uptake for most plants is best around 6.5-7.0, so most growers do everything they can to raise or lower their pH to something in that zone.  For my soil that is nearly impossible because of the amount of lime in my soil and higher pH waster.  The fact is that it is true that 6.5 is ideal, but not in the way I thought and I have a better understanding of it now.

To some degree, the only thing that matters when it comes to the soil is what is in the rhizosphere  around the roots.   The rhizosphere is the area around the roots and that is where almost all of the magic takes places.  To put it simply, the root hairs excrete acids to get at the nutrients in the soil.  That acidification can decrease pH  by as much as 0.75 pH units.  So although the soil pH may be too high, the rhizosphere that is important might be just right. 

John mentioned that the acidification of the soil typically goes out about a quarter of an inch, but with myco that may be extended as much as a couple of inches.  The ideal soil pH is about 6.5, but that is in the rhizosphere and not the overall soil.   If you have a high pH soil, do you need to add sulfur to adjust the pH then?  Yes for a few different reasons.  But it may not be as critical as some growers make it sound.  There have been lots of big pumpkins grown in higher pH soils.

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