Opportunities for Soil Nutrient Cycling in Montana Cropping Systems

Mal Westcott

Crop harvest and transport to end use necessarily result in the loss of soil nutrients from all cropping systems. Sustainable approaches to soil nutrient management focus on methods for cycling these nutrients back to the site of production as much as possible to offset the losses. Examples include soil amendment with animal manures or with residues from crop processing. This approach becomes impractical, however, as facilities such as feedlots and food processing plants are further concentrated and removed from the site of crop production. These nutrient sources may ultimately be used as soil amendments, but not frequently at the site of removal.

A more direct approach is to utilize green manures at the site of production, thereby avoiding the costs of transport and application. The concept of green manuring is very traditional and simply entails the growth and management of a crop for its direct incorporation as a soil amendment. With the exception of nitrogen (N) fixation, green manures do not replace nutrients removed in harvest since they do not manufacture them. However, benefits from soil incorporation of plant materials are well recognized, not only in terms of increased nutrient availability for succeeding crops, but also in terms of soil C balance. Legumes are most commonly used for their N-fixing capabilities, but non-leguminous species may also be used.

The challenge to incorporating green manures into existing cropping systems is in recognizing and developing opportunities for their growth. This becomes problematic when considering that the economics of crop production, particularly the relatively low cost of commercial fertilizers, do not generally justify sacrificing an entire growing season for the production of a green manure crop. Successful examples of green manure practices, therefore, fit into windows of opportunity that do not disrupt the cycle of production. Examples are the use of winter cover crops in orchards or vineyards, or early spring planting of legumes for green manure followed by cropping of a summer annual. These systems are possible where winter or early spring conditions allow for the growth and survival of the green manure species.

The options narrow when considering cropping systems in Montana. Climatic conditions do not allow for the growth of a winter or early spring green manure with sufficient production prior to planting of the intended crop. The problem is further compounded in dryland small grain cropping systems where green manure production can, over the production cycle, compete with the crop for water, the primary yield limiting resource. A flexible green manuring system has been proposed and researched for summer fallow. The idea is to plant a spring legume in the fallow year, with cessation of growth after a prescribed level of water use. This may become more effective with the development of hardy winter annual legumes.

Irrigated systems tend to be more diverse in the state. Integrated systems of grain and forage production may provide the greatest possibilities for green manuring. For instance, research has shown that annual legumes such as berseem clover or non-hardy alfalfa may be grown for dual use as both hay and green manure in a single season. These crops can be harvested twice for hay with the second cutting in late August. The fall regrowth will provide enough plant material for green manure to supply all the N needs of a barley crop in the subsequent year. The system is flexible in that the producer has the option to utilize the regrowth as green manure or as forage. The latter is the more likely option in light of current market values for forage and fertilizer.

These proposed green manure systems have not gained wide acceptance in Montana, at least in part due to the reasons cited. Far greater contribution of plant-based nutrient cycling comes from crop residues not specifically intended as green manures. A good example is in perennial alfalfa production where, after stand removal, significant amounts of N are made available to a subsequent crop, most frequently a small grain. More subtle effects have been measured in terms of K or S availability to small grains following the production of a brassica such as canola. And where legumes such as peas are harvested for a crop and followed in a subsequent year by a small grain, there can be measurable increases in N availability. The most likely avenue for future beneficial research in this area should therefore focus on methods to enhance residue nutrient cycling through crop diversification.

Mal Westcott
Western Agricultural Research Center
Montana State University
531 Quast Lane
Corvallis, MT 59828
Tel: (406) 961-3025
Westcott@Montana.edu

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