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Know Your Soil

Issue: March 2011

One of the most important factors in any fertilizer management program is application timing, which can vary widely among growing regions, soil textures and potato varieties. But there are fairly standard practices across the industry and knowing the timing and methods of these applications can streamline any fertilizer management program.

Soil nutrient content is always important for efficient fertilizer applications, and the first step in finding the nutrient content is through soil tests, which can be sent to soil labs at land grant universities and private institutions. Soil scientists can perform an in-depth analysis of the nutrient content. Those institutions also have fertilizer guidelines for every major crop in the state, pulled from years of extensive research. Those guidelines, coupled with the soil assays, will help you in your applications.

But knowing variability across the field can also guide applications.

"It is important to look at soil tests to determine what the nutrient needs are, but also spatial variability across the field, and assessment of areas that have poor soils, to have an idea of what the variability is," said Bryan Hopkins, certified professional soil scientist, Brigham Young University, Provo, Utah.

"Also, look at previous seasons to determine what tissue analyses showed to know what to do this year. The initial recommendation comes from soil tests, but I'll customize the recommendation based on tissue samples."

Two of the more important nutrients are potassium and phosphorous. They can be applied together in whole¸ pre-plant applications or as a starter fertilizer at planting. The ease lies in their immobility in the soil - they largely stay in the root zone through the season.

However, both nutrients have disadvantages that should be taken into consideration.

"The problem with phosphorous is it precipitates and forms minerals in the soil, so we have to do things to make it available," Hopkins said.

Banding phosphorous with nitrogen is one method used to make it available within the root zone.

Potassium is typically available in two forms, potassium chloride and potassium sulfate. Both can be applied in single, pre-plant applications, or split between pre-plant and starter fertilizers.

"Some research has shown split applications (of potassium) help bulking, but if you apply it later, it will reduce the specific gravity if you apply the potassium chloride form," said Carl Rosen, extension soil scientist, University of Minnesota, St. Paul.

"Potassium sulfate will not decrease specific gravity and does supply sulphur, but the disadvantage is it's more expensive. You will need to balance the increased fertilizer cost with potential effects in tuber yield and quality."

Nitrogen is considerably more difficult to manage than potassium and phosphorous, largely because nitrogen is mobile within the root zone, leading to potential leaching problems in irrigated systems or regions with higher precipitation. Nitrogen can be split into several applications to decrease the leaching potential and make the nutrient available when it is needed most, specifically emergence and tuberization.

"If you look at nitrogen uptake rates by the potato crop, the largest demand is during the late vegetative state to early bulking stage," Rosen said.

"If you have too much early, there is evidence in some varieties that you'll delay tuberization. If you have too much late, when plants are trying to shut down, you'll delay bulking, though it's somewhat weather and variety-dependent."

Nitrogen is typically applied first as a starter fertilizer, in many cases combined with phosphorous and potassium. Pre-plant applications are generally discouraged in areas with higher precipitation rates due to the leaching potential, but there are exceptions.

Pre-plant applications can work in dry land production systems because there is less leaching potential. Also, because irrigation is not used, having the nitrogen in the soil at planting ensures adequate nitrogen during dry periods. Controlled-release nitrogen is also an option in both irrigated and dry land production systems, with polymer-coated granules that delay solubility and release the nutrient much slower to match crop nutrient demands, staying in the root zone longer.

For irrigated systems in wetter regions, researchers recommend no more than 25 percent to 30 percent of the crop's nitrogen needs be applied at planting, with 50 to 60 percent at emergence. The remainder can be applied through in-season applications on a week to 10-day basis, in either fertigation or side-dress applications, guided by petiole samples.

Nitrogen, phosphorous and potassium are the three most important nutrients for plant health, but there are also many other nutrients to be considered during the growing season. Most other micronutrients and secondary nutrients can be applied in granular form as a starter or a foliar application in-season, guided by petiole tests.

Sandy soils tend to be sulfur-deficient, so many scientists recommend sulfur as a starter, as ammonium sulfate, potassium sulfate or potassium magnesium sulfate, with rates based on pre-plant soil tests.

Most soils are not low in calcium, but research has shown it can have beneficial effects during storage, leading many growers to apply it, specifically for stored crops.

"Adding calcium has been show to help storage properties, though you don't normally see a yield response, it's usually quality. If you're not storing potatoes, you don't need to worry as much about calcium," Rosen said.

—Everett Brazil III, Contributing Writer

Originally posted Friday, Mar. 11, 2011

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