Phosphorus fertilization of snap beans for processing: revisiting current recommendations (2013)

Aaron Heinrich
OSU Department of Horticulture

Current snap bean phosphorus (P) fertilizer recommendations for Oregon (Bush Beans: Western Oregon—West of Cascades Fertilizer Guide publication #FG 28) are higher than those given in other extension publications across the US. Oregon is the #2 snap bean producer in the US and yields per acre are consistently about 1.5x greater in Oregon than any other state (USDA NASS 2012). Despite the scale and importance of snap bean production in Oregon, little attention has been paid to the phosphorus (P) utilization of this crop over the past 30 years in the Willamette Valley. Due to changes in production practices, the increase in the price of P fertilizers, trends towards greater sustainability, and soil test P (STP) values commonly greater than 50 ppm Bray (the level at which a crop response to P fertilizer is unlikely), there is interest in revisiting the current recommendations to better balance P inputs/outputs.

Due to limited available research on P fertilization of snap beans in Oregon, a review of fertilization literature and university extension publications across the country was done (Table 1).  This review indicated that the critical level (the level at which increasing STP levels do not result in a yield response) for snap and dry beans is approximately 50 to 60 ppm Bray. Above this level, all literature reviewed recommend applying no P fertilizer. Except at low STP levels, OSU’s recommendations are much higher than other regions, suggesting that Oregon farmers may be able to cut back on their P applications. However, due to different soil types, growing conditions, varieties grown, and target yields, Oregon specific data is needed. For example, Oregon yields are approximately 1.5x higher than other states, and as a result, P requirements may be greater.

Table 1. Comparisons of OSU’s current P recommendations with other sources. Very low (VL; >90%), Low (L; 60-90%), Moderate (M; 30-60%), High (H; 10-30%), and Very high (VH; <10%) indicate the probability of a fertilizer response to applied fertilizer

To better understand the relationship between P inputs and outputs on farms growing commercial snap beans for processing, information was collected on fertilization practices, STP levels, and harvest data from 6 fields on 6 farms in the Willamette Valley in 2013 (Table 2).  The complete research report with methods can be found here.  Five of 6 sites had very high (>120 ppm Bray 1P) STP levels. Even at these high levels OSU guidelines recommend applying 60-90 lb/acre P2O5 (Table 1), but at such high levels a response to P fertilizer would be highly unlikely. At all 6 sites, more fertilizer P was applied than was removed in the harvested pods. On average, growers applied 76 lb/acre P2O5 (range 30-158) while total plant uptake and removal in harvested pods was 35 (range 25-51) and 12 (range 8-18) lb/acre P2O5, respectively.  On average 76% of applied fertilizer P remained in the field after harvest and will contribute to the increase in STP levels and to the long-term pool of soil P (Table 2). This fertilization above crop removal explains why STP levels have increased, sometimes to very high levels. Although STP levels were ≥159 ppm at sites 5 and 6, these farms applied >130 lb/acre P2O5. This was the result of applying a low N, high P analysis fertilizer (10-34-0) to meet crop N needs. As a result, P was applied in well in excess of crop removal. Given the high STP levels and that P uptake and removal in harvested pods is a small fraction of current P fertilizer rates, growers can likely cut back on P fertilizer applications withourt suffering a yield loss.

Table 2. Soil, fertilizer, and harvest data for 6 commercial snap bean fields from the Willamette Valley in 2013.

In 2013, we also conducted a replicated P fertilizer trial at site 4, which had a STP level of 44 ppm (Table 2). P fertilizer was banded at planting at a rate of 0, 15, 30, and 60 lb/acre P2O5.  At harvest, maximum pod yield was obtained at 30 lb/acre P2O5 (Fig. 1). Using the current guidelines given in Table 2, the recommended rate for site 4 would be 110 lb/acre P2O5, almost 4 times higher than the rate at which maximum yield was achieved.  Although the results are from only one trial, they support reducing fertilizer applications below current recommendations.

Figure 1. Fertilizer response curve for banded P fertilizer at planting for site 4. Bray 1P= 44 ppm.

Based on the literature review and the results from the 2013 research, fertilizer P rates could be significantly cut back on most snap bean fields in the Willamette Valley of Oregon without suffering a yield loss. Although the P requirements for snap beans grown in Oregon may be greater due to yields that are 1.5x higher than other states, they are not high enough to justify applying 60-90 lb/acre P2O5 when soil test levels are >60 ppm. Because there is a small probability of a P fertilizer response when soil test P levels are >50 ppm, P fertilizer should be eliminated or the rates drastically reduced when feasible. This can be done by shifting away from low N, high P analysis fertilizers (i.e. 10-34-0) to higher N, lower P products.  For growers that are uncomfortable with completely leaving off P fertilizer when STP levels are >50 ppm, a suggestion would be to apply just what will be removed in the pods (Table 2; ~15 lb/acre P2O5). Research addressing this issue will continue in 2014.

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