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Understanding P in Your Soil

Phosphorus plays an incredibly important role in a potato crop’s ultimate yield and quality. Ensuring a crop has access to adequate phosphorus is not always easy, however. Because phosphorus has complex and inconsistent solubility in soil, 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 with it multiple economic, agronomic and environmental repercussions. For these reasons, carefully managing one’s nutrients to ensure adequate and efficient phosphorus in potato fields 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 during tuber 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 than potato’s other macronutrients.

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. Excess fertilizing with phosphorus is not only economically inefficient, it also leads to a host of other negative consequences. A high level of phosphorus in the soil can negatively affect the availability of zinc and other essential crop nutrients. And, excessive phosphorus can lead to damaging eutrophication when water-soluble phosphorus leaches or runs off into surface or ground waters.

Since potatoes have relatively low phosphorus foraging ability, they benefit from aggressive and proactive phosphorus management. Optimizing phosphorus availability in a potato crop requires a combination of practices, fertilizer sources, application timings and placements determined by both the crop’s agronomics and the growing environment’s unique characteristics.

Phosphorus is typically applied in a band or broadcast at or near planting. This latter option 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 after planting is the poorest application method as surface applications will not reach roots and incorporated applications can damage roots.

Unfortunately, water soluble phosphorus fertilizers applied at or near planting tend to release their phosphorus rapidly, often before peak potato demand. Very commonly, the delay between the nutrient’s release and the crop’s peak nutrient uptake is long enough for the nutrient to become fixed in the soil and, therefore, unavailable to plants.

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 (such as mono ammonium phosphate, diammonium phosphate or liquid ammonium polyphosphates), 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.

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