Canola, Rapeseed, and Spring Pea As Enhancers of Soil Nutrient Available and Crop Productivity in Cereal Rotations (Cropping Rotations Directly Linked to Barley Yields)

Canola, Rapeseed, and Spring Pea As Enhancers of Soil Nutrient Available and Crop Productivity in Cereal Rotations (Cropping Rotations Directly Linked to Barley Yields)

Summary

Objectives
1.Study nutrient accumulation by brassica, cereal hay, and cereal grain crops; and legume, brassica, and legume-brassica green manure combinations.
2.Determine the effects of the above rotations on nutrient availability to a subsequent barley crops.
3.Identify the influence of rotations including brassica spp. differing in glucosinolate levels on populations and activities of soilborne plant pathogens and integrate rotational sequencing with biological control of seedling diseases.
4.Conduct on-farm demonstrations of legume-rapeseed green manuring.

Abstract of Results
A three-year rotational study of brassica cropping and green manuring effects on nutrient cycling and plant diseases in cereal-based cropping systems was conducted at three Research Centers of the Montana Agricultural Experiment Station. The sites included a sandy loam, a silty loam, and a clay loam soil and all had access to supplemental irrigation. In the first year of the study, canola or rapeseed grown as oilseed crops were generally at least as efficient as barley in acquiring plant nutrients from the soil, but a greater portion of these nutrients were concentrated in the seed. As a consequence, brassica oilseed production resulted in greater removal-by-harvest of nutrients than barley in some cases. Brassicas managed as green manures, which remained vegetative throughout the growing season, were highly efficient in plant nutrient uptake. Total amounts of nitrogen (N) ranged up to 145 to over 300 kg N/ha in brassica green manures, in excess of a legume pea green manure at the same sites in some cases. Nutrient content of brassica green manures in general was three to six times the amount found in a barley crop.

Grain yield responses in the second year of the rotation (1993) were site-specific. At a site where indigenous levels of nitrogen and phosphorus were relatively high, no significant response to first-year cropping systems was elicited. At a site where soil nitrogen but not phosphorus or sulfur were limiting, barley grain yields showed a significant response to the amount of nitrogen returned in crop residues or green manures the previous year, but no significant response to rotational phosphorus or sulfur was evidenced. At the site where both nitrogen and phosphorus are limiting to yield, second-year barley yields were dependent upon the amounts of plant nitrogen and phosphorus returned in the first year. It is rare in rotational or green manuring studies to find these strong relationships between the amounts of cycled nutrients and subsequent yield responses.

In addition, one site elicited significant increases in available soil potassium in the spring of 1993 due to green manuring, and subsequent crop potassium uptake was correlated with these levels. This study presents evidence that legume green manure mixtures can be utilized to increase the availability of soil nutrients other than nitrogen to subsequent crops.
Two sites elicited significant barley grain yield and nutrient uptake responses through a second year of barley cropping to first-year rotational components, indicating that the effects on nutrient availability can be expressed for two years of subsequent cropping. Notable is the continued effect on potassium availability.

We believe that the brassica green manures, because of their glucosinolate content, have the potential to aid in the control of soil-borne plant pathogens. However, the results obtained in this study indicate that a single year of cropping to brassicas is insufficient for a reduction of Pythium ultimum and other soil-borne pathogens to occur. Research involving the long-term production of these crops is needed to define their role in disease suppression.
Potential Contributions

The potential benefits to agricultural producers include 1) increased cropping diversity through inclusion of brassicas in cereal-based cropping systems, 2) new green manure crops that provide expanded benefits to soil nutrient availability beyond those of traditional legumes, and 3) a decreased reliance on off-farm inputs by use of brassica rotations to more efficiently cycle nutrients on-farm.

Farmer Adoption
Seed samples of brassica forage and green manure varieties have been distributed to over fifty individuals in response to educational talks on brassica production for forage and green manure. The largest response has been for sheep forage. It's too early to assess adoption, as producers continue to explore niches in their systems for brassicas.

New Hypotheses
We are gaining the first evidence that brassicas can be managed to increase nitrogen, phosphorus, and potassium availability to subsequent cereal crops, and that cereal responses are specifically related to the amounts of nutrients cycled through previous year's crop residues or green manures. The high nutrient uptake capabilities of brassicas should be adapted to improve nutrient cycling in sustainable agricultural systems.
Reported in 1995