The fertility stories I've been writing recently have generated some interesting food for thought from readers. Here's a question on broadcast versus banding from Marengo, Iowa: "We are in eastern Iowa and are all no-till and would like your advice as to surface application versus banding with a low disturbance opener and is it worth the cost and time over a conventional surface spread program."
A little review of how nutrients move in the soil and roots intercept nutrients sheds some light on how to best manage nutrients to improve efficiency. The three pathways to consider are: root interception, mass flow and diffusion.
Roots grow through the soil and root hairs intercept nutrients. There will often be areas with a greater density of roots where nutrient concentrations are higher in the soil solution.
As water evaporates from the soil or is transpired through plants, it pulls along chains of water molecules that physically adhere to each other. This water carries along nutrients to roots as it flows through the soil.
When you apply fertilizer, it dissolves into its inherent salts such as potassium (K+), chloride (Cl-) or phosphate (PO42-). The salt concentration will be highest at the fertilizer prill's point of contact in the soil and lower as you move away. Since water molecules are connected through films of water, salts migrate from areas of high salt concentration to areas of low concentration.
Nutrients reach roots in different ways. Nitrogen is soluble and mobile in the soil and uses all three mechanisms to move through the soil to be picked up by roots. That is why nitrogen can be broadcast and roots can still intercept much of it.
Phosphorus is considered immobile and fixes to minerals such as calcium, iron and aluminum in the soil. Potassium (potash) is considered slightly mobile and doesn't get tied up by minerals. Instead it binds to the soil particle's internal structure (matrix).
Therefore, phosphate, and for practical purposes potash, don't move with soil water or along concentration gradients because they tend to be bound in place. P and K uptake relies strongly on root interception.
That's a long way of saying that for phosphate and potash, banding is better than surface broadcasting. Concentrating fertilizer in a zone helps ensure that it doesn't all tie up with minerals or the soil matrix. Root growth will concentrate to some degree around the zone and capture more of the nutrients. Nutrient efficiency should go up and that means more bushels per pound of applied nutrients.
Banding also means you can reduce fertilizer rates a bit and still see an improvement in yield and recover investment in equipment. In addition, the crop should emerge more quickly and evenly. A good start in the spring almost always puts more grain in the tank come fall.
Dan Davidson can be reached at AskDrDan@dtn.com
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