By Brian Hefty

Phosphorus is an incredibly important nutrient for plants, but just because you apply it doesn’t necessarily mean it will get into your crop, especially this year.  Here are some of the issues you need to be aware of:

  • In soils with a pH less than 7.0, we believe the Bray tests are the most accurate measurements of P.  The P1 test, or weak Bray, will show you approximately how much phosphorus is available in your soil.  The P2 test, or strong Bray, gives you total soil phosphorus.  If there is a big gap between these two numbers, that means much of the phosphorus in your soil is sitting there unavailable to your crop due to a tie-up issue.
  • Tie-up means that your phosphorus is bound up with another nutrient.  Calcium and phosphorus together is often in the calcium phosphate form, which is insoluble in water and unavailable for plant uptake.
  • Tie-up can occur in any soil, but it is more likely if your soil pH is outside the ideal range of 6.3 to 7.3.  In effect, your phosphorus dollar is getting wasted if you don’t address your pH problems.  There are many estimates as to how severe this tie-up can be, and that certainly varies with a number of other factors, but if your pH is outside the ideal range it is likely that at least 25% of your phosphorus is tied up; but seeing 50% or more wasted is also a lot more common than you might think.
  • Because soil pH is such a huge factor in this and almost everything else that happens in your soil, it should always be the FIRST THING YOU LOOK AT ON A SOIL TEST!
  • Soil pH also has a lot to do with microbial activity.  If the microbes aren’t active in your soil, that hurts your phosphorus uptake, too.  For example, mycorrhizae fungi have been proven to attach themselves to plant roots and help bring phosphorus in.  If you have a condition like fallow, flooding or poor soil pH, the mycorrhizae won’t do the job you are counting on them for.
  • AVAIL has an 1800 CEC so it can tie up calcium, magnesium and other nutrients that could bind with phosphorus.  AVAIL can definitely help, but in the long term we still want you to fix your pH and other soil issues so tie-up is less of a concern.
  • Perhaps the biggest issue we have talked about in the last 18 months has been nutrient stratification.  As tillage has been dramatically reduced in the U.S., there is less erosion, which is helping to keep phosphorus in our fields.  Since phosphorus is virtually immobile in soils, it won’t leach down like nitrate, sulfate or boron will.  If your soil blows away or washes off your fields, there goes your phosphorus.  If all your P is in the top inch or two of soil, not only is erosion a huge concern, but so is drought.  Just because you have water deep in the soil for your roots doesn’t mean you’ll get top yields.  You also need plant food, including ample phosphorus, so your plants get fed when they take a drink.
  • Dry phosphorus is cheaper, but it also requires rainfall to break down; and it is certainly not available as quickly as liquid.  If you are using liquid phosphorus in-furrow, just make sure to pick a low-salt product and keep your rate low to avoid issues.
  • Phosphorus should be kept in balance with other nutrients.  Don’t get too hung up on this, but just understand that if you apply lots of P, you should probably also apply lots of other nutrients depending on what your soil tests look like.  For example, if you put on lots of P and no zinc, you may have fixed your phosphorus problem but created a zinc deficiency.
  • If you want great yields, you have to find a way to get lots of phosphorus into your crop.  I know everyone is talking about cutting expenses this year, but if you cut the inputs that help your yield, that’s a problem.