Crop Rotation – Soil Health Gift That Keeps on Giving
One basic soil health concept is that of plant diversity – a diversity of plant species grown in your fields will benefit soil health. Crop rotation is a well applied example of that soil health concept. The impacts of crop rotation on weeds, diseases and insects are numerous and help to explain how rotation raises yield of corn and soybean. I remember that in the 1980s, Johnsongrass control in soybean benefited the following corn crop. Take-all disease has long prevented growing wheat after wheat. Soybean cyst nematode reduces our ability to grow soybean after soybean. Corn rootworm can hinder continuous corn production.
When changes in weed, disease and insect pressure don’t explain the ‘rotation effect’, changes in soil chemical (pH), physical (aggregation/tilth), and fertility (available N, P and S) properties are often talked about. But the ‘rotation effect’ can occur in the absence of all the previously described causes/ mechanisms – this means that the effect is probably due to differences in soil microbiology that are induced by rotation versus monocrop cultivation. The differences in soil microbiology associated with this phenomenon are not well understood, but a buildup in mycorrhizal fungi is suspected by some researchers (Johnson et al., 1992; Hendrix et al., 1995).
What does this mean in Kentucky? Before I came to Princeton, I used to manage (Dr. Hanna Poffenbarger has that pleasure now) a grain crop rotation research trial at the Spilndletop research farm near Lexington. Besides continuous corn, continuous soybean, and the 2-year corn-wheat/double crop soybean rotation, there was a 4-year corn-corn-soybean-soybean rotation. All crop rotation components were grown every year. I’m going to use those yield results to illustrate some long-term observations.
Corn benefits a great deal from rotation. Figure 1 illustrates the ‘rotation effect’ in the context of corn grain yield response to fertilizer N. In this figure, three corn rotation components are shown: 1st year corn after 2 years of soybean, 2nd year corn after 1 year of corn and 2 years of soybean, and continuous corn. Corn yield rises and then levels off as the N rate rises. The ‘rotation effect’ is shown at the far-right side of Figure 1, where 1st year corn exhibited greater maximum yield potential (203 bu/acre) than 2nd year corn (193 bu/acre) and continuous corn (191 bu/acre). Interestingly, the larger portion of the ‘rotation effect’ was lost with 2nd year corn, whose maximum yield potential was not very different from that for continuous corn. And as noted by many, more fertilizer N was needed to achieve maximum yield in the corn after corn systems; 141, 169 and 177 lb N/acre for the 1st year, 2nd year and continuous corn, respectively. That said, the greater corn after corn fertilizer N requirement did not overcome the ‘rotation effect’.
In this long-term field study, the continuous corn and corn-wheat/double crop soybean systems have been around for the longest time, over 25 years. Corn yields in each of these systems, as related to the seasonal/yearly average yield in the trial, are shown in Figure 2. The negative impact of continuous corn was generally apparent across all seasons – good, average, and bad – though not all.
There were years where continuous corn outyielded corn after wheat/double crop soybean. The impact was greater in the better seasons. In a 50 bu/acre season the yield loss is nearly 11 bu/acre. In a 250 bu/acre season the yield loss is around 21 bu/acre.
For those of you considering an expansion is soybean acres next spring - full season soybean is not immune to the ‘rotation effect’. Figure 3 exhibits the 1st year, 2nd year, and continuous full season soybean yield as related to the seasonal/yearly average yield for the 11 years that all 3 rotation components were present. This long-term field study area does not have soybean cyst nematode (I regularly took soil samples for cyst nematode detection). Again, there were some years when soybean after soybean outyielded soybean after corn. However, the general yield trends indicate that soybean after soybean yield potential was inferior to that for soybean after corn and that the rotation effect was larger with a greater seasonal yield potential. Again, 2nd year soybean yield potential was not very different from that for continuous soybean.
The ‘rotation effect’ is one of the earliest known manifestations of soil health – reported in ancient Roman agricultural texts. Most of us understand the benefits of crop rotation without knowing exactly how/why the ‘rotation effect’ occurs. The ‘rotation effect’ is derived from the soil, likely a change in soil microbiology brought on by changing the crop species production sequence and thereby improving soil health and increasing grain crop productivity. Most grain producers are promoting soil health every production season.
Hendrix, J.W, B.Z. Guo, and Z.-Q. An. 1995. Divergence of mycorrhizal fungal communities in crop production systems. In The Significance and Regulation of Soil Biodiversity. Eds. H.P. Collins, G.P. Robertson, and M.J. Klug. pp. 131-140. Kluwer Academic. The Netherlands. Johnson, N.C., P.J. Copeland, R.K. Crookston, and F.L. Pfleger. 1992. Mycorrhizae: Possible explanation for yield decline with continuous corn and soybean. Agron. J. 387-390.