Don’t Trip on Triple-19

– Jimmy Henning, Extension Professor, University of Kentucky (modified from Forage Doctor column, The Farmers Pride – Nov.16, 2017)

There are wrong ways to do right things. Repeated use of products like triple-10 (10-10-10) or triple-19 (19-19-19) on hay fields can ultimately make that field unresponsive to the fertilizer that is applied. Don’t get me wrong, fertilizing is a ‘right’ thing. People that fertilize their pasture and hay fields have a special place in my heart. But here is why triple-19 can trip you up.

The nutrients in a hay crop are 100% removed from the field, unless that hay is fed back in the same field. It takes 18 pounds of P2O5 and 50 pounds of K2O fertilizer to replace the nutrients in one ton of grass hay (Table 1). Using triple-10 or triple-19 alone to replace these nutrients is guaranteed to over-fertilize with P or under supply K.

Soils have very different abilities to supply P and K from the mineral parent material. This fact alone is one of the best reasons for a current soil test from hay fields. Repeated use of 200 or even 300 pounds of triple-19 on hay fields will drain the soil of potassium, such that the other fertilizer nutrients are ineffective.

To understand why low K fertility limits other fertilizer benefits, you have to understand what is called the Law of the Minimum, which states that growth only occurs at the rate allowed by the most limiting factor. Think of your field as a barrel and your yield as water. The height of each stave is the level of individual soil nutrients. If your ‘K’ stave is excessively short, your barrel will not hold much water. Low K means low yield in spite of how tall the P stave is.

Fertilizing with adequate K affects more than yield. Potassium is directly involved in stomatal function. Stomates are the openings in leaves where water exits the plant; low K forages cannot effectively regulate water status in drought. Potassium directly affects winterhardiness, especially in legumes like alfalfa. Finally, plants with low K status are more prone to disease when stressed.

Don’t let triple-19 trip you up. Get a current soil test, especially from hay fields, and apply the needed nutrients. Make a visit to your preferred vendor for fertilizer and tell them what you need. Getting a blended fertilizer that fits your needs may take some extra work on their part, but they can do it. If they can’t, maybe it’s time to find another vendor. Really. Happy foraging.