SPUDSMART – Understanding P in your Soil
Spudsmart | by Justin Miller | January 2017 – Phosphorus plays an incredibly important role in a potato crop’s ultimate yield and quality. Ensuring a crop’s access to adequate phosphorus is not always easy, however. Because phosphorus has
complex, inconsistent soil solubility, it can sometimes be unavailable to plants even in fields where
it exists at relatively high levels. Simply dumping on more phosphorus to compensate brings economic,
agronomic and environmental repercussions. For these reasons, carefully managing one’s phosphorus
needs to be a high priority for every potato producer. A 300 to 500 cwt potato crop requires between
60 and 90 lbs/ac of phosphorus, most of which is absorbed during bulking and about two thirds of
which is removed from the field at harvest. While the same crop requires significantly more nitrogen
and potassium (about 130 to 215 lbs/ac and 145 to 240 lbs/ac respectively), phosphorus can be tied
up in soil, resulting in much lower fertilizer use efficiency.
To overcome phosphorus’ low efficiency, farmers often over-fertilize at the beginning of the season, or top up phosphorus via multiple in-season foliar or fertigation applications. Unfortunately, excessive phosphorus fertilizing is economically inefficient, can negatively affect the availability of zinc and other essential crop nutrients, and can lead to damaging nutrient leaching or run-off into ground or surface waters.
Since potatoes have relatively low phosphorus foraging ability, they benefit from aggressive, proactive phosphorus management. Optimizing phosphorus availability requires a combination of practices, fertilizer sources, application timings and placements determined by both the growing environment and the crop’s agronomics. Phosphorus is typically band or broadcast applied at or near planting. Broadcasting is only effective if the broadcast phosphorus is incorporated evenly throughout potato’s 12 inch root zone to allow the highest possible root interception. Side-dressing phosphorus post-planting is the poorest application method as surface applications will not reach roots and incorporated applications can damage roots.
Water soluble phosphorus fertilizers applied at or near planting tend to release their phosphorus rapidly, often before peak potato demand. Often, the delay between the nutrient’s release and the crop’s peak uptake is long enough for the nutrient to become fixed in the soil and unavailable. Some growers choose to apply foliar sprays and fertigation applications multiple times throughout the growing season. While these can be effective, the crop must be extremely carefully managed to ensure phosphorus levels do not drop below critical levels between applications. Other growers opt for a combination of an early-season water soluble phosphorus combined with a slow release mid-late season phosphorus in order to ensure adequate nutrition from vegetative growth. Not sure if your phosphorus application is optimizing your crop potential? Soil testing is always wise and should be the foundation for all of your fertilizer investment decisions across all your crops.