Crop Rotation and N Rate Effects on Net Returns and Soil pH

This plot-scale study began in 2012 at Big Sandy (sandy clay loam), managed as low-disturbance no-till (disk drill), and included four cropping treatments in rotation with winter wheat, ranging from low to high drought risk: 1) Chem fallow, 2) Cover crop (herbicide-terminated, except hayed in 2018), 3) Pulse crop harvested for grain, and 4) Spring wheat (Table 1). 

All wheat varieties were considered sawfly-tolerant and rotations were managed with four target N availability levels for wheat ranging from no fertilizer applied to 150% of a 3 lb available N/bu rate. The 3 lb N/bu rate is midway between winter wheat and spring wheat MSU guidelines (Jacobsen et al. 2023). 

Soil nitrate N was measured 3 ft deep prior to wheat planting and additional legume credits were used for pulse crops harvested for seed (9 to 13 lb/ac) and for pea manure (18 to 26 lb/ac). Target wheat yields after fallow were set to reflect a high potential yield for that field (50 bu/ac); wheat target yields after fallow and cover crop were set equal while the continuous wheat yield goal was reduced by 33%. The yield goal for the pulse–wheat rotation was set at 10% greater than continuous wheat, thus requiring correspondingly greater available N. But rarely was more N fertilizer needed due to the N provided by the pulse rotation. 

Net returns to land and management were calculated ignoring crop insurance or USDA Farm Program payments. Fertilizer prices were set as a 2-yr average based on actual fertilizer prices monitored bi-annually by the Montana Department of Agriculture, while pesticide/adjuvant and custom machinery rates were taken from NDSU sources. Seed and rhizobial inoculants were based on farmer-reported costs in similar situations. The base price for pulse crops was set at a 24-month average for publicly offered prices in Montana centered on harvest. The base price for wheat was set at a 36-month average from July prior to wheat harvest until June two years after wheat harvest. Hay price for the cover crop in 2018 was considered $45/ton ‘on the stump’. Protein discount/premium schedules were applied in two ways based on data collected from 40 Montana grain elevators from 2001-2012. During those 12 years, ‘Steep’ discount schedules occurred in five years and a ‘Flat’ market response in seven years. Equations were generated to reflect the average of the ‘Flat’ and ‘Steep’ response scenarios. 

In mid-September 2019, two top 3-inch soil cores were collected with a hydraulic probe in each subplot and mixed for laboratory pH analysis (1:1 water:soil by volume).  

All treatment differences are reported with 95% confidence (P < 0.05). 

RESULTS

Pulse- and fallow-wheat rotations had the highest (and similar) net returns. Pulse-wheat had higher net returns than fallow when no N was applied under Flat protein discounts (Fig. 1A) and with no added N and 100% N for Steep protein discounts (Fig. 1B). Cover-wheat and continuous wheat rotations had similarly lower net returns. Continuous wheat had net loss when no N was added during Flat protein discounts (Fig. 1A) and when less than 100% recommended N was supplied during Steep protein discounts (Fig. 1B).  

In all rotations, net returns were greatest at 100% of recommended N under a Flat protein discount, but were greatest at 150% of recommended N under a Steep protein discount. Although this suggests that about 4.5 lb N/bu should be applied to maximize profit when protein discounts are steep, the profit gains between 100% and 150% might not be enough to offset the negative effects of increased soil acidification measured at greater N rates on this sandy loam soil (Fig 2.). 

Soil acidification can lead to yield losses and is a growing issue in Montana (Jones et al. 2019; Jones and Olson-Rutz, 2020). Continuous wheat had lower soil pH than the pulse-wheat system (Fig. 3), a result also seen in the wetter Bozeman study (Jones et al. 2022). Fallow-wheat also had lower pH than pulse-wheat at Big Sandy possibly due to greater nitrate leaching, a result not seen at Bozeman. Due to high costs of mitigating low pH with lime, the short-term benefits of greater N rates might be partly or fully erased by liming costs at least on soils where the pH is already low. In addition, applying high N can lose money when discounts are flat.

Figure 1. Total net returns after 8 years to land and management under assumed ‘Flat’ (A) and ‘Steep’ (B) protein discount schedules. All net returns are different than fallow EXCEPT pulse at 150% N at flat discounts and pulse at 75% N and 150% N at steep discounts.

Figure 2. Soil pH after 8 years of different N rates.

Figure 3. Soil pH after 8 years under different rotations.

FERTILIZER FACTS

• 100% of recommended N was most profitable under a Flat protein discount, while 150% of recommended N was most profitable under a Steep protein discount scenario.  
• Pulse-wheat rotations had equal or higher net returns than fallow-wheat systems during this 8-year study. 
• Greater N rates led to lower soil pH, which could hurt yield or require lime.  

ACKNOWLEDGEMENTS

This study was funded by the Montana Fertilizer Advisory Committee and the Montana Agricultural Experimental Station.  We thank Jeff Holmes and Terry Rick for making this project happen in the field and lab. 

REFERENCES

Jacobsen, J., et al. 2023. Fertilizer Guidelines for Montana Crops. EB0161. MSU Extension. https://store.msuextension.org/Products/Fertilizer-Guidelines-for-Montana-Crops-EB0161__EB0161.aspx 

Jones, C., et al. 2019. Soil Acidification: An Emerging Problem in MT. Montana Fertilizer eFacts Fertilizer 78. https://landresources.montana.edu/fertilizerfacts/html/FF78.html 

Jones, C., and K. Olson-Rutz. 2020. Soil Acidification: Problems, Causes, & Testing. MSU Extension Soil Scoop. https://landresources.montana.edu/soilfertility/soilscoop/ss_AcidifProbsCauseTest.html 

Jones, C., et al. 2022. Dramatic soil health changes after 18 years of different nitrogen rates and cropping systems in the northern Great Plains. In Great Plains Soil Fert Conf. Proc., pp. 21-26. https://greatplainssoilfertility.org/files/2022_GPSFC_Proceedings.pdf   

Miller, P., et al. 2015. Pea in rotation with wheat reduced uncertainty of economic returns in southwest Montana. Agron. J. 107(2): 541-550. https://doi.org/10.2134/agronj14.0185 

 

posted November 2024