Fertility Management of Soybeans

Transcription

1 Fertility Management of Soybeans Gyles Randall Soil Scientist and Emeritus Professor, Univ. of Minnesota Southern Research and Outreach Center Waseca, MN Abstract Providing an optimum nutrient environment through fertilizer management is critical for producing high-yielding, profitable soybeans. Often fertilizer-management decisions are quite specific to the soils and growing conditions of the crop and do not fit a one size fits all approach. This is particularly true for Manitoba where soybean production is not as developed as in the 60+ year history of the Corn Belt. Input costs and higher soybean prices may also influence these decisions. Soybeans are a legume, which fix most of their needed nitrogen from the air. Consequently, inoculation of soybean seed is important in developing production areas where rhizobium populations in the soil are limited. Nitrogen (N) fertilization has historically not been recommended, but infrequent responses to fertilizer N have been reported recently. While phosphorus (P) fertilization is often considered to be a non issue in the Corn Belt, where soybeans generally follow corn (a crop requiring greater amounts of P), optimum P fertilization can be most important on the colder soils in Manitoba. Because soybean seeds are very sensitive to salt burn from fertilizers, placement and timing of fertilizer is important. Residual P from past P applications is very important for soybean production in the Corn Belt. Potassium and sulfur needs of soybean are generally met by the soil. Iron deficiency chlorosis is best addressed by choosing resistant/tolerant varieties. Zinc, molybdenum, and boron-deficient soybeans are rare. Although there is conflicting literature on the effects of glyphosate on the mineral nutrition of glyphosate resistant (Roundup Ready) crops, most of the literature indicates that the mineral nutrition and yields of glyphosate resistant crops are not affected by either the glyphosate resistant trait or by the application of glyphosate. Introduction There are many factors to consider when making fertility management decisions for soybeans. Normally, soil test levels and soybean yield responses in experimental trials are considered dominant factors when making a decision. When doing so it is also important to consider the probability of a yield response. Is the response to a nutrient a low probability (<25%), a 50:50 probability, or a high probability (>75%). A high probability of a yield response should be the major factor when making the decision, but often a large yield response at one location is used as a marketing tool even though the probability of a yield response is less than 15%. The price of soybeans relative to the price of the fertilizer nutrient or the inoculant also is a major factor, especially with the current high price of soybeans. When prices are high, farmers strive to produce the highest yield possible because they do not want to leave any potential yield in the field. Price, therefore, becomes a factor because they don t want to lose any potential profit and they want to maximize return-on-investment when soybean prices are high. Nitrogen Because soybeans get much of their needed nitrogen (N) from the air through a fixation process by Rhizobium bacteria, inoculation of soybean seed is important in developing production areas where Rhizobium populations can be low. In these areas inoculants should be applied with the seed for as much as five years to build up the N-fixing bacterial population in the soil. In well

2 established soybean production areas, inoculants are seldom applied. Under current high soybean prices and relatively low inoculant prices, inoculants are more frequently applied by growers. Soybean yield responses to fertilizer N seldom occur. Recently however, responses have occurred on low organic matter, coarse to medium-textured soils with irrigation where yields have been >70 bu/a. Ancedotally, yield responses have been reported in northern Minnesota in areas where soybean production is relatively new. Studies conducted at 11 sites in Minnesota showed significant responses to N at two sites --- both low organic matter sites (personal communication, Dan Kaiser, Univ. Minnesota). The results from four northern Minnesota sites are shown in Table 1 (adapted from Kaiser, 2012). An 8 bu/a yield response to preplant broadcast ammonium nitrate was found at the Warroad site (within 20 miles of the Canadian border) on a low organic matter soil. Yield responses were not found at the other three sites. Table 1. Soybean yield response to fertilizer N in northern Minnesota. Site Zero-N Check N as AN 1/ 24 lb/a P > F bu/a Hallock Warroad 11 2/ Crookston Hallock / Broadcast-applied in spring before planting. 2/ A low OM soil. Historically, because of the very low probability of a yield response to N, fertilizer N has not been recommended for soybeans in Minnesota. However, due to recent findings, the suggestion that Fifty lb N/A may be beneficial if soil nitrate-n is low or nodulation is poor occurs in the latest Univ. of Minnesota fertilizer bulletin Fertilizer Recommendations for Agronomic Crops in Minnesota. Phosphorus Phosphorus (P) management, including application rate, timing and placement, is especially critical for profitable soybean production. Application rate is generally dependent on soil test P (STP). Many years of experimentation have led to calibration curves between STP and yield response. Yield responses are greatest at low STP and least or negligible at high to very high STP. Two long-term P studies were conducted at Morris and Waseca, MN with corn and soybean yields taken from (Randall et al., 1997). At Morris, the soil was an Aastad cl with a ph of 7.6 and a Bray P 1 test of 10 ppm (Low). At Waseca, the soil was a Webster cl with a ph of 6.0 and a Bray P 1 test of 22 (V. High). Continuous corn was grown from and was followed by 12 years of a soybean-corn rotation. Phosphorus treatments of 0, 50, and 100 lb P 2 O 5 /A were broadcast-applied annually before fall tillage from A 150-lb P 2 O 5 rate was broadcast triennially in 1973, 1976, 1979, and The residual effects of these P treatments on yield were measured in (4 years of corn and 4 years of soybean). Soybean yields taken in the two application years (1982 and 1984) and the four residual years (1986, 1988, 1990, and 1992) are shown in Table 2 (except 1988 at Morris when no yield was obtained due to drought). At Morris with the initial STP of 10 ppm (Low) soybean yields were optimized at 46.0 bu/a with the 100-lb annual P 2 O 5 treatment --- a 13.4 bu/a response

3 over the control. At Waseca with an initial STP of 22 ppm (V. High), yields were optimized at 48.7 bu/a with the 50-lb annual treatment --- a 6.5 bu/a response. Soybean yields were 2.2 and 1.0 bu/a greater for the 150-lb triennial treatment compared with the 50-lb annual treatment at Morris and Waseca, respectively. Economic return during the application phase (10 years of corn and 2 years of soybean) was greatest for the 150-lb P 2 O 5 rate applied every third year at both sites. These data clearly show the effect of initial STP, P rate, and P timing on soybeans grown on a calcareous soil and a slightly acid soil. Table 2. Soybean yield as affected by P treatment at Morris and Waseca. P treatment Morris (5 yr) Waseca (6 yr) lb P 2 O 5 /A bu/a annual annual triennially Yields were affected significantly in 10 of 11 site-years. Soybean yields as influenced only by residual P from P applied continuously for 12 previous years are shown in Table 3. On the calcareous soil at Morris where Bray 1 STP ranged from 5 to 29 ppm, 3-yr average yields ranged from 27.0 bu/a on the control to 43.0 bu/a for the 100-lb P 2 O 5 treatment. Additionally, yields were increased 4.7 bu/a when the STP was 29 ppm compared with 17 ppm, showing the influence of residual P on this soil. At Waseca, where the initial STP was greater, residual P only resulted in a 7.0 bu/a advantage. These data dramatically showed the influence of residual P on both of these soils. Table 3. Soybean yield average as affected by residual P. Annual Morris (3 yr) Waseca (4 yr) P treatment STP Yield STP Yield lb P 2 O 5 /A ppm bu/a ppm bu/a Yields were affected significantly in all 7 site-years. Another on-going trial at Waseca is being conducted to determine the effect of STP on corn and soybean yield. Fig. 1 shows the soybean yields in 2009 as a function of STP from residual P, ranging from 3 to 41 ppm Bray P 1. Yields were optimized at 52 bu/a at the threshold STP value of 19.5 ppm. Yields declined to <40 bu/a when STP was <5 ppm. At STP values >19.5, yields were not increased above 52 bu/a.

4 60 Soybean yield, bu/a Y = x x 2 Plateau at 19.5 ppm Bray P 1 Soil Test P, ppm Fig. 1 Relationship between soybean yield and STP at Waseca in Fertilizer P was applied for corn to a very low testing site and a high testing site (initial STP values of 5 and 19 ppm, respectively) in years 1, 3, and 5 of a corn-soybean rotation at Waseca (Randall and Vetsch, 2004). In years 2, 4 and 6, soybean was grown to determine the residual effect of fertilizer P. No fertilizer P was applied for corn in year 7 or for soybean in year 8. The rate of P 2 O 5 application was 50 lb/a/yr when applied with the seed or banded 4 to 5 below the seed for the VL testing plots and 40 lb/a/yr for the high testing plots. Broadcast rates of P 2 O 5 were 100 lb/a and 80 lb/a for the VL and H testing sites, respectively. The corn was strip-tilled and the soybeans were no-tilled planted. In year 8, soybean yields showed about a 10 bu/a and 19 bu/a response to residual P from the 50-lb and 100-lb P 2 O 5 rates, respectively, applied for corn 4, 6, and 8 years earlier on the VLtesting site (Table 4). On the H-testing site, soybean yields were not different between the zero- P control treatment and the 40-lb with seed or deep band (4-5 below seed) treatments. However, yields were 5 bu/a greater for the 80-lb broadcast rate compared to the zero-p treatment. The higher yields for the broadcast treatment are thought to be related more to the higher rate of P 2 O 5 than to the method of application. In summary, these data vividly illustrate the ability of soybeans to feed on fertilizer P applied 4 to 8 years earlier, especially when the initial STP was very low.

5 Table 4. Soybean yield response to residual P in year 8 from P applied to corn in years 1, 3 and 5 on a very low and a high P-site at Waseca. P 2 O 5 Treatment Soil test P Placement Rate Very low High lb/a bu/a None W/seed 50/ Deep band 50/ Broadcast 100/ Soybeans are much less responsive to placement position of P than is corn. Soybean yield response data from numerous studies over the last 40+ years in Minnesota show broadcast application of P to have a high probability of producing the greatest yields with least risk and least effort. Thus, soybeans almost never receive starter, deep band, or seed-placed fertilizer in Minnesota. However, the question of seed-placed or placement very near or below the seed comes up periodically, especially when soybeans are grown on highly calcareous or infertile soils. Because germinating soybeans are much more sensitive to salt damage from fertilizers, causing stand loss, one must ask whether the potential yield gain is worth the risk of loss. In Minnesota, broadcast placement of P is the gold standard for soybeans as much of the P is applied for corn in the year prior to soybeans. But, seed or near-seed placement of P fertilizers may be a good research opportunity on the highly calcarious and cold soils of Manitoba. Importance of high Soil Test P The previous P response data indicates STP plays an important role in soybean and corn production in Minnesota. Yet we see some farmers mining soil P from land they rent. Because of high land rental costs, they do not apply P and K and thus reduce their production costs, especially when crop prices were lower relative to the cost of P and K. Thus, a study was started at Waseca to determine if very high corn and soybean yields can be produced on low P-testing soils when applying P rates 40% above the U of Minn. recommendations. Two sites, one with a low STP (7 ppm Bray P 1 ) and one with a very high STP (25 ppm Bray P 1 ), were located on a tile-drained Webster clay loam soil. The low P site had been mined with no P or K applied the previous 8 years. Potassium was applied to all plots at 120 lb K 2 O/A in the first year and 200 lb/a in the second year. Corn was grown at both sites in 2005, 2006, and 2007 followed by soybeans in 2006, 2007, and Corn hybrids, soybean varieties, and planting dates were the same for both the L and VH P sites each year. Corn was strip-till planted while soybeans were no till planted. Fifty lb P 2 O 5 /A (40% more than the 35-lb U of Minn. recommendation) was applied each year for corn at the low P site. For the VH P site, a 40 lb P 2 O 5 /A rate was used. Four P placement methods were used at each site: deep-band (4-5 directly below the seed), pop-up (seed-placed), broadcast, and a 50:50 combination of broadcast + pop-up. The source of P was No P was applied for soybeans allowing us to determine the residual effect of the P treatments. Corn yields shown in Table 5 indicate a significant 3-year average yield response to P at the low STP site with no difference among P placement positions. No response to the P treatments occurred at the VH P site. The most striking finding was the 25 bu/a yield advantage for the VH P site when no P was added compared to the average of the 50-lb P 2 O 5 rates on the L P site when all other variables between the two sites were eliminated. Soybean yields shown in Table 6 exhibited very similar response characteristics as for corn. In these three years, soybean yields from the zero P treatment on the VH P site averaged about 10 bu/a greater than for the 50-lb P 2 O 5 treatments that were applied for corn the previous year. In other words, both corn

6 and soybean yields from the low P testing site, even when P was applied at 40% more than recommended, never measured up to the yields from the VH P-testing site with no applied P. Economic analyses of these yield data using average corn and soybean prices of the period showed an economic return of $111/A for a VH P-testing soil on a 50:50 split corn and soybean farm (Table 7). Table 5. Three-yr average corn yield as affected by soil P test and P placement. P Treatment P Test Rate Placement Low VH lb P 2 O 5 /A bu/a /40 Deep-band 1/ /40 Pop-up /40 Broadcast /40 DB + Pop-up / 6-7 below soil surface under row. Table 6. Three-yr average soybean yield as affected by soil P test and P placement for previous corn crop. Residual P Treatment P Test Rate Placement Low VH lb P 2 O 5 /A bu/a /40 Deep-band 1/ /40 Pop-up /40 Broadcast /40 DB + Pop-up / 6-7 below soil surface under row. Table 7. Yield and profitability advantage for a VH P-testing soil. Advantage Crop Yield Econ Return 1/ bu/a/yr $/A/yr Corn Soybean Avg / $4.50/bu and $11.00/bu, not counting fertilizer cost. This study identified three important points that should be valuable to farmers: 1) High and profitable corn and soybean yields could not be produced on L P-testing soils even through the P rate for corn was greater than the Univ. of Minnesota recommendation. 2) There was no advantage to deep-band placement of P over broadcasting. 3) It is important to know the soil test P when acquiring or renting new land. Potassium Soybeans respond well to potassium (K). However, K management can be quite simple when a K soil test is used. In Minnesota, exchangeable soil test K should be >120 ppm to optimize soybean yields. Since soybeans follow corn in much of Minnesota, where >150 ppm K is

7 suggested for optimum corn yield, soybeans rarely show K deficiencies unless the soils have been mined to very low levels of exchangeable K. Broadcast placement is preferred. Sulfur As of this writing I am not aware of any research showing soybean yield response to sulfur (S). But, S deficiencies in corn are becoming more common in Minnesota, especially under highyield production. Gypsum applied at 15 lb S/A provides sufficient S to eliminate the S deficiency. Some gypsum-applied S likely carries over to the next crop, suggesting that soybean producers in Minnesota may not experience S deficiencies. Micronutrients Iron deficiency chlorosis (IDC) commonly occurs in Minnesota on high ph (>7.4) soils. Much research work has been concluded to solve the problem. In the late 70 s, many field trials were conducted on farmers fields to evaluate various foliar-applied products. Significant yield increases (up to 37 bu/a) to foliar-applied sequestrene iron chelate -138 (EDDHA) were obtained at 22 of 31 sites. However, timing was critical and other logistics were complex. When coupled with an expensive product, adoption by farmers did not occur. More recently, spring planting of oats followed by killing after planting soybean has shown some effectiveness at reducing IDC. Logistical problems also exist with this approach, and adoption has been minimal. Thus, we are back to the same best management practice that we had in the early 70 s: choose an IDC resistant/tolerant variety. Perhaps in the future using this BMP plus technological advances involving seed-placement of iron products, planters capable of planting at least two varieties according to IDC-mapped fields, etc. will solve the IDC problem. Dr. Dan Kaiser, Univ. Minnesota Soil Fertility Extension Specialist, has conducted numerous onfarm cropping trials around the state to determine the yield response of soybeans to micronutrients (Dan Kaiser, personal communication, dekaiser@umn.edu, ). In one experiment conducted at 11 sites in 2011 and 2012, he used a drop-out experimental design to determine the response to Zn, Mn, Mo, and B compared to a control with no micronutrients and a treatment that received all four of the micronutrients. Significant differences among the treatment yields were not obtained at any of the sites. Average yields across all 11 sites were 46.9 bu/a for the control treatment and 47.4 bu/a for the treatment that received all four micronutrients. In a dropout design, when a micronutrient treatment is dropped out, the yield should be lower than the treatment where all micronutrients were included if a micronutrient deficiency is present. In this study the average yield when Zn, Mn, Mo, or B were dropped out were 47.0, 47.7, 47.5, and 48.9 bu/a, respectively. Since these yields ranged from 0.4 bu/a below to 1.5 bu/a above the 47.4 bu/a all micronutrient treatment, there was no yield advantage when applying Zn, Mn, Mo, or B. Soil tests for these four micronutrients did not provide useful guidance when interpreting the data. In another field study conducted on 11 on-farm sites in 2011 and 2012, Dan Kaiser, U of M Extension Soil Fertility Specialist, found soybean yield responses to N and P at three sites, but no response to S or a combination of S and Zn, using MicroEssentials SZ (MEZ), at any of the sties. (The MEZ contains 12 lb N + 40 lb P 2 O K 2 O + 10 lb S + 1 lb Zn per 100 pounds of product.) In these trials, neither S nor Zn limited soybean yields (personal communication, 2012). Summarizing the results of these micronutrient studies: soybeans did not respond to sulfur (S) or any of the micronutrients applied [Zinc (Zn), manganese (Mn), molybdenum (Mo), or boron (B)] in these 22 field sites.

8 Glyphosate (Roundup) Effects on Mineral Nutrition of Soybeans In the last few years a controversy has developed on whether glyphosate (Roundup) has affected the mineral nutrition and yield of soybeans. Two scientists (Dr. Huber of Purdue and Dr. Gordon of Kansas State University) have reported reduced micronutrient concentrations and soybean yields when glyphosate was applied to soybeans. Many other scientists have not seen evidence of this effect. Recently a paper in the Journal of Agricultural Food Chemistry (Duke, et al., 2012) reviewed the large body of evidence and concluded the following: 1) although there is conflicting literature on the effects of glyphosate on mineral nutrition of glyphosate resistant (GR) crops, most of the literature indicates that the mineral nutrition of GR crops or crop diseases are not affected by either the GR trait or by the application of glyphosate and 2) yield data of GR crops do not support the hypothesis that there are substantive mineral nutrition or disease problems that are specific to GR crops. Literature Cited Duke, S.O., Lydon, J., Koskinen, W.C., Moorman, T.B., Chaney, R.L., and Hammerschmidt, R Glyphosate effects on plant mineral nutrition, crop rhizosphere microbiota, and plant disease in glyphosate resistant crops. J. Agri. Food Chem. 60 (42): Randall, G. and J. Vetsch Don t overlook effect of variables on P use in corn-soybean rotations. Fluid J. Vol. 12, No. 1, p Randall, G.W., S.D. Evans, and T.K. Iragavarapu Long-term P and K applications: II. Effect on corn and soybean yields and plant P and K concentrations. J. Prod. Agric. 10: