The science of water, fertilizer and the perfect potato

The perfect french fry spud is the result of a delicate balance of water, fertilizer and sandy soil.

But growing potatoes on sandy irrigated soil has become one of the largest contributors to the nitrate contamination that is creating a health risk for people who drink water from the shallow Pinelands aquifer in north central Minnesota.

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The sand plains of central Minnesota are a good place to grow potatoes because the soil drains quickly, so it’s easier to manage the water the plants need.

Potatoes grown there need about 20 inches of water during the growing season, according to Andy Robinson, extension potato agronomist for North Dakota State University and the University of Minnesota.

The area receives an average of 17 inches of rainfall during the growing season, but of course the rain doesn’t all fall when plants need it.  Experts say on average, rainfall provides about one-half of the water potatoes need.

Farmers are trying to reduce irrigation use through better timing.  Sensors in fields tell them when the soil is dry.  They can calculate how much water plants need based on each days weather conditions.

R.D. Offutt Co. agronomist Nick David said if plants need water and rain is forecast in the next few hours, farm managers will hold off on irrigating fields.  In the past, they likely wouldn’t have managed water that carefully.

Potatoes also need a lot of nitrogen. The amount varies depending on soil, plant variety and type of fertilizer.

And there’s a close connection between the rainfall and nitrogen contamination of the aquifer.

Soil can hold only so much water.  According to U of M nutrient expert Carl Rosen, sandy soils like those near Park Rapids can hold about  three inches of water in the top two feet. That’s how deep potato roots grow.

Any excess water will run off the field, or move down through the soil to the groundwater, taking nitrogen with it.

Rosen’s research found for every one inch of excess water, 11 pounds of nitrogen is lost on each acre.

A typical irrigated field is about 140 acres.  So if the soil is saturated and a one inch rain falls, that field will lose about 1500 pounds of nitrogen, much of if moving down toward the groundwater.

If the excess rain falls right after fertilizer has been applied the loss is greater, and if time has passed and some nitrogen has been used by the plants, the loss might be less.

Farmers are trying to reduce nitrogen loss. Granules of fertilizer coated with biodegradable plastic keep the nitrogen from quickly dissolving.  Rosen found the slow release fertilizer can reduce nitrogen loss by about 10-percent.

Putting fertilizer in irrigation water can also reduce the amount that “leaks” to the groundwater.

Farmers try to “spoon feed” the plants by applying the liquid fertilizer in small amounts when the plants most need it.

Some farmers are growing potato varieties that require less nitrogen.  RDO agronomist Nick David said using a different potato variety reduced nitrogen use on those fields by almost 50 percent.  But those varieties don’t store as well and they don’t make the best french fries, so the market is limited.

The best hope for solving the nitrogen problem lies in genetics, according to Rosen.  Plant breeders in Minnesota and elsewhere are working on new varieties.

Or perhaps the solution will come from genetically modified potatoes.  J.R. Simplot company has developed a system for moving genes between potato varieties.  The USDA is considering a petition to allow the first GMO potatoes to be sold.  Experts say swapping genes among potato varieties could create more efficient spuds that need less nitrogen. http://www.simplotplantsciences.com/