According to Sullivan et al. 1999, during the active growing season, nitrogen (N) uptake can be divided into three phases in hops production. During Phase I, which occurs in early spring, N uptake is slow and corresponds with slow plant growth. Phase II corresponds with rapid N uptake and pronounced biomass accumulation. Phase III is a time of minimal N uptake.

In Phase I, initial spring growth is slow and primarily fueled by reserves stored from the previous year; only around 10 percent of total biomass is accumulated through mid-June. In the Willamette Valley, OR, the rapid N uptake associated with Phase II generally occurs mid-June through mid-July, and is responsible for the generation of 3-4 lbs. of plant biomass/acre/day. Through the end of July, a typical hop yard will have accumulated 80-150 lbs./N/ac in the trained biomass, depending upon cultivar and age of the hop yard.

Rates, Timing, and application method

N fertilizer rates, timing, and application depend upon soils, climate, age of plant, expected yields, and cultivar. For first year plantings, recommended N rates are 75lbs/acre for Oregon hop yards (Gingrich et al. 2000). In mature Oregon hop yards N application rates, based on N uptake data, range from 100-150 lbs./N/acre. However, in and around Yakima Valley, WA a common fertility regime involves application of 100 lbs. of N by May, increasing to over 200 lbs. of total N applied through mid-July.

Research presented by Doug Walsh at the 2014 North American Hop Convention, compared the effect of different N application rates, timing, and methods on hop yields in the Pacific Northwest. While the yield differences were not statistically significant, the fertilizer regime that produced the highest average yield included a 90 lbs. N/ac as a spring application, followed by 90 lbs. N/ac administered through fertigation, ending in June (180 lbs. of N/ac total). Statistically, this treatment did not result in greater yields compared to the ‘standard fertility’ regime-120 lbs. N/ac in the spring followed by 120 lbs./N/ac fertigation through mid-July (240 lbs./N/ac total). While a single spring application of 240 lbs. N/ac was statistically similar to the prior treatments, the yield variability was greater. This data also indicates that an extra 60 lbs. of N/ac may not confer additional yield benefits and may be lost to the environment. If N-fixing cover crops, compost, or hop residue is applied to the yard, growers should reduce N applications accordingly.

N management considerations

Soil and tissue testing is an appropriate method for determining plant N needs. Hop petioles from mature leaves (5-6 ft. off the ground) should be collected just prior to the plant reaching the wire (likely early July in Northwest Michigan). Petioles should be sent to a tissue testing laboratory such as the Michigan State University Soil and Plant Nutrient Lab, to be analyzed for nitrate-N. Research has shown that additional N applications do not confer yield benefits if petioles contain more than 4000ppm nitrate-N.

Please continue to visit the Michigan State University Extension hop webpage or the MSU Hops News Facebook site for up to date information and stay tuned for Hop fertility: Part 2, which will cover additional hops nutrient needs.