Soil acidity management of long-term no-till fields in Montana to prevent crop failure

Under Objective 1

Soil pH and Al impacts from liming

We found that sugar beet lime application raised soil pH in the 0-2- and 2-4-inch depth layers if incorporated with tillage (Figure 5). Four ton/ac beet lime application increased soil pH 2.2-2.4 and 1.2-1.3 units in the 0-2- and 2-4-inch depth layers at Fort Benton and Geraldine. Sugar beet lime was left on the surface and not incorporated with tillage at Big Sandy.  Soil pH increased only in the 0-2-inch depth layer at this location, and even this observed effect was likely an artifact of the sampling approach as considerable undissolved lime was mixed with soil material in the 0-2 inch sample cores.  In summary, our results demonstrated that beet lime does not readily wash into the soil in Montana’s semiarid climate, and that incorporation with tillage was necessary to correct soil acidity in the surface 4 inches of soils. 

Figure 5. Soil pH depth-profile in fall 2017 (before) and 2019 (after) sugar beet lime application (4 tons/ac) at Fort Benton (incorporated with till), Geraldine (incorporated with till), and Big Sandy (no-till).

We found sugar beet lime applications (fall 2017) at Fort Benton and Geraldine raised soil pH (0-4 inch depth) over a 1-year and 2-year time window according to the curvilinear relationship of Figure 6.  The dominant soil series at both field sites was Bearpaw-Vida clay loam, and so it was not surprising that the pH change with lime was similar at the two locations.  Overall, the relationships demonstrated beet lime was effective at ameliorating soil acidity, and that most of the pH changes occurred in the first year if the lime material was incorporated with tillage and therefore validating Hypothesis 1. Sugar beet lime requirements to raise soil pH by 1.3 units to a target pH of 6 was approximately 2.5 tons/ac, or 2750 lb CaCO₃/ac.  We estimated the beet lime application costs at $100/ac assuming a transport cost of beet lime to the farm at $35/ton, and field applications cost $12/ac for spreading and tillage incorporation. While our $100/ac estimate represented a considerable cost input, we believe the costs were modest when viewed over a long-term time horizon (e.g., 15-20 years), meaning annualized costs of closer to $5 to $7/ac per year.

Recently, we have created models (from related long-term cropping system investigations and not supported by WSARE) of soil pH change vs. cumulative fertilizer-N inputs. Our models indicate that the acidification process from fertilizer-N occurs slowly over time and is directly related to the cumulative amount of fertilizer-N applied. For example, a 14-yr study in Montana on a silt loam soil found that approximately 2200 lb/ac of fertilizer-N was required to lower pH 1.0 unit. Therefore, presuming soil has been restored to pH 6.0 with lime, it may take many years before cumulative fertilizer-N inputs can again acidify the soil to a pH < 5, and crop Al toxicity problems return (discussed below).

Figure 6. Soil pH change from 2017 to 1- and 2-years after application of sugar beet lime.

Aluminum toxicity in crops is typically found in soils with pH < 5 because of the increased solubility of Al-containing minerals in the soil.  A strong relationship was found between soil Al saturation percentage in (or % of exchange sites occupied by Al) and pH at our Fort Benton and Geraldine beet lime field sites (Figure 7).  The relationship in these figures is typical of most mineral soils and illustrates that crop available Al rises sharply as pH falls below 5.  Crop Al toxicity symptoms have been associated with soil Al percentages that exceed 15%, although a precise critical level will be impacted by crop species and cultivar selection.   The results from our trials indicate the application of beet lime was effective at bringing Al saturation percentages below this threshold for all of our soil samples in the 0-2-inch depth and most of the samples in the 2-4 inch depth.

Figure 7.  Aluminum saturation percentage vs. soil pH relationships in the 0-2" (left) and 2-4" (right) depth soil layer for sugar beet lime applications (tons material/ac) at Fort Benton and Geraldine in the fall 2019 (2-year post-lime application).

Crop growth and liming

In 2018, we observed obvious differences in growth of lentil at our sugar beet lime strip-trial near Fort Benton. Lentil top growth was greener, and biomass was greater in areas receiving lime compared to the non-limed area (Figure 8).  Also, an aerial drone flight on June 24, 2018 revealed the limed strips were visible from the air using NIR imagery, though differences in growth between adjacent lime and non-lime areas were not apparent along the entire field transect (Figure 9).  The benefits to lentil growth, biomass and color were believed to reflect greater Rhizobia activity that translated to improved nitrogen nutrition.  Surprisingly, these early season growth differences did not translate to greater seed yield at harvest, likely because yield was more water limited than N limited. 

Figure 8.  Visual differences in lentil growth were apparent from the sugar beet lime applications at Fort Benton with the green strips receiving lime vs. the chlorotic, non-limed areas.  Spring soil pH (0-4 inch depth) was 4.7 in the non-limed strips and 5.9 in the limed (4 ton sugar beet lime/ac) strips. The photograph was taken June 14, 2018.

Figure 9.  Aerial NIR drone imagery of the lentil field on June 24, 2018, revealed darker red coloration in strips receiving sugar beet lime relative to strips without lime. 

Similarly, in 2019 we observed yellow pea growth at Geraldine was more robust where lime was applied (Figure 10).  Unlike lentil, seed yield was significantly improved by lime (non-lime = 2.73 bu/ac vs. limed =30.6 bu/ac).  As observed in our lentil trials, the benefit of liming was believed to result from improved Rhizobia activity, nodulation, and N-nutrition of these pulse crops.  Inspection of yellow pea root samples found large differences in nodule numbers between pea on limed and non-limed fields (Figure 11).  These results were not surprising as Rhizobium nodulation is known to be sensitive to soil acidity.  The soil pH of the non-lime areas was 4.7 and in contrast to a pH > 6 in our limed strips.

Figure 10.  Yellow pea growth and coloration was improved at Geraldine with the application of lime on June 05, 2019 (left) and June 19, 2019 (right) 

Figure 11.  Yellow pea roots exhibited differences in nodule abundance between areas with lime (left) and without lime (right).  June 19, 2019.

Seed-placed P fertilizer trials

In 2018, we observed a large growth response to P fertilization in durum where the soil pH was not amended with Aglime application (Figure 12).  Grain yield was affected by the interaction of lime and P fertilizer (Figure 13).  Briefly, seed-placed P fertilizer mitigated Al toxicity symptoms and improved grain yield by 22 bu/ac over unfertilized controls where lime was not applied. Conversely, durum was unresponsive to P fertilizer when lime was applied to correct soil acidity.  In 2019, a similar response by durum to P fertilizer was evident early in the growing season at the Highwood Bench field site.  However, two hailstorm events during the growing season reduced yield by approximately 50% and minimized the yield response to P fertilizer. Our results indicate that seed-placed P fertilizer provides a method for mitigating Al toxicity at field sites with acidic soils and can help even at field sites with very high soil P levels (50 – 83 ppm).  The response to P fertilizer is not associated with a nutrition benefit as it occurs in soils that test very high in available P, which was the case in both 2018 and 2019.  Rather, the benefit results from a reduction in Al concentration in the root tips as a result of Al-P precipitation (reference).  The fertilizer strategy of placing P with the seed to mitigate Al toxicity in crops should be viewed as a short-term approach to manage acidic soils as soil pH is not remediated.  In Montana, this strategy might best be suited for a situation where a farmer is renting land under a short-term lease agreement. 

Figure 12.  Seed-placed P (right) resulted in more vegetative growth and higher durum yields than non-fertilized P areas (left) at our field site on the Highwood Bench.  Soil pH was 4.4 in the plots that did not receive Aglime. 

Figure 13. Durum grain yield on the Highwood Bench was improved with P fertilizer under acid soil conditions (-lime) but was not affected where soil acidity was mitigated with lime applications.   Olsen soil P level = 50 ppm in 2018 and 83 ppm in 2019 (very high).

Under Objective 2

We collected soil samples for the lime-requirement incubation study in fall 2017 from ten locations in Montana (Table 1).  The ten soils were treated with seven rates of CaCO₃ and incubated for 90 days in small pots at field moisture capacity. Soil pH was measured at the conclusion of the 90-day incubation experiment and plotted against the CaCO₃ application rate (Figure 14 – left – only four soils shown).  The observed CaCO₃ (or measured lime requirement) to achieve a target pH of 6.0 was determined by the interpolation of soil pH vs. CaCO₃ rate curves.  We then estimated lime requirement using the four buffer protocols.  Pearson correlations of predicted (buffer protocols) vs. measured lime requirement was 0.84, 0.76, 0.66, and 0.63 for the modified Mehlich, Sikora, Adams-Evans, and Woodruff buffer protocols, respectively, with one outlier soil (Gerber silty clay) deleted from this analysis. The modified Mehlich protocol provided the best estimates of measured lime requirement.  A plot of lime incubation (measured or observed) vs. modified Mehlich lime (predicted) requirement shows the predictions (y-axis) were closely aligned to the observed (or measured) lime requirements (x-axis) as evidenced by their proximity to the 1:1 line (Figure 14 - right). The Gerber silty clay soil behaved differently than the nine other soils we examined.  The incubation lime requirement for the silty clay was much higher than predicted by the modified Mehlich protocol as well as for the three other buffer protocols. The reason why the Gerber soil behaved so differently may be related to a large drop in pH observed for the 0 CaCO₃ treatment over the incubation period (pH_{0 day} = 4.55 vs. pH_90day = 3.88).  It is possible Al³⁺ ions may have desorbed from the soil and then were ‘hydrolyzed’ creating more acidity than would be predicted by the buffer tests.

Table 1.  Series name, location, pH (1:1 water), and measured lime requirements to a target pH 6.0 of ten soils used in the lime incubation experiment. Four highlighted series appear in Figure 14 (left).

 Figure 14.  Soil pH after a 90-day lab incubation as affected by CaCO₃ (left).  Only four soils shown for clarity.  Predicted lime requirements to pH 6.0 using the modified Mehlich6 buffer vs. measured lime requirements determined from the 90-day incubation. 

Under Objective 3

In 2018, we conducted Aglime (+,-) x cultivar selection trials with four crop species (pea, canola, spring wheat, and barley) at two farms (Geraldine and Highwood) to determine potential benefits to lime applications and to identify selections with tolerance to soil acidity.  At Highwood in 2018, early and mid-season differences in growth from lime applications were visually evident in barley, pea, and spring wheat with greater biomass occurring in areas receiving lime.  However, the greater top growth likely created more drought-stress at grain-fill resulting in smaller mature kernel sizes and compromised yield. This phenomenon was particularly evident in barley, which actually showed a significantly lower yield with +Aglime vs. -Aglime control. At both locations in 2018, certain cultivars of pea and canola incurred severe (>15%) pod shatter losses.

In 2019, we conducted Aglime (+,-) x cultivar selection trials with four crop species (pea, canola, spring wheat, and barley) at Highwood, and two crop species  (spring wheat and barley) at Geraldine. Residual herbicide issue prevented the seeding of canola and pea at Geraldine.  At Highwood, two mid-season hailstorms severely damaged all crops, but in particular pea and canola. However, it was assumed that hail damage and aluminum toxicity did not interact to affect yield or quality. Yield and quality for limed and non-limed plots were analyzed separately across site-years, with cultivar as a fixed effect and rep nested in site-year as a random effect. In Table 2, yield and quality results are summarized for cultivars that were represented in three (pea, canola) and four (wheat, barley) site-years.

Table 2. Lime-amended (+Aglime) and unamended (-Aglime) protein (oil for canola), protein or oil yield (yield x oil or protein) and seed yield for cultivars and experimental lines of barley, canola, pea, and wheat. Results are averaged across three (pea, canola) and four (wheat, barley) site-years.

Under Objective 4

Outreach activities and outcomes are summarized in the Education and Outreach section.  Briefly, our activities have included 39 oral presentations including talks to growers, fertilizer dealers, private consultants, and scientists at international and regional meetings.  Also, we have presented our research project on Youtube, Montana AgLive and Yellowstone Public Radio (https://www.ypradio.org/post/worm-november-6-2019#stream/0).  Our research has been reported in print media including Montana Grain News, The Prairie Star, Crops & Soils Magazine, and Montana Fertilizer eFacts.

We had 1539 direct contacts over the course of the project totaling 1189 direct contact-hours (Table 10) and over 10,000 indirect contacts.  In the three events with attendees being predominantly producers (>80% of participants), on average, each person would share their new information with 9 others. If we include the presentation dominated by certified crop advisers in the audience, on average each attendee would share information with 38 other people. In the three events where the majority of attendees were producers, 81% said they would likely change management practices as a result of what they learned that day.