Posted by: Mahdi Ebrahimi | March 29, 2008

Fueling organic soils with forages

Organic farming has traditionally depended on biologically active soils to recycle nutrients and minimize inputs of purchased feeds and fertilizers. The unpaid work of nitrogen-fixing bacteria, fungi, earthworms and other soil organisms, fueled by forage crops, generate a level of biological efficiency that many believe forms the cornerstone of sustainable agriculture.

Meanwhile, the relentless climb in energy prices is a warning sign that the sustainability of fossil fuel dependent agriculture will be put to the test. With the growing tendency to shorten crop rotations, usually to the detriment of forage production, organic agriculture has a challenge on its hands in order to retain its biological advantage.

The most glaring sign of trouble is the uncoupling of crop and livestock production. The organic farming movement was founded on a mixed farming model, yet the same daunting economic hurdles faced by conventional farmers are leading to greater specialization in organic farming, as well. In the past, manures could be easily imported from conventional farms to shore up the nutrient balance on a cash crop or vegetable farm. In Atlantic Canada, where beef and pork production are in long-term decline, importing manure is becoming increasingly rare as the overall number of farms continues to shrink and farmers are inclined to keep manure on-farm as fertilizer prices rise. Exceptions to the manure shortage exist in those areas where certifiers permit the use of composted chicken manure from large-scale farms.

Without livestock, the overall production of compost, perhaps the best of all biological soil activators, is eliminated. Secondly, there appears to be less reason to include forage legumes in crop rotations.

Forage legumes are important both for fixing nitrogen (50-200 kg/ha) as well as their extensive tap roots which help aerate and drain soils, and extract nutrients from deeper soil layers. With synthetic fertilizer nitrogen costs ranging from $4-600 per tonne, the dollar value of biologically fixed nitrogen alone should warrant increasing attention Yet many farmers feel the costs and effort of producing, for example, an alfalfa/timothy green manure crop outweigh any improvement in soil quality.

Surprisingly, a 12-year trial in Manitoba showed that an alfalfa-alfalfa-wheat-flax rotation was more profitable than a similar rotation under conventional management and two (organic and conventional) grain only rotations. The comparison did not factor in a premium price for the organic crops. According to researcher Martin Entz, there is growing recognition of the value of forages and manure in crop production and that the rising price of energy will make the economics of forage production increasingly attractive.

Closely connected to forage production are earthworms populations. Their burrowing action improves soil structure, and by ingesting as much as 35 tonnes of dry earth per hectare they leave behind roughly 18000 kg/ha of casts. The casts are all more nutrient rich and biologically active than surrounding soil and are comparable to high-grade compost.

The soil scientist W.A. Albrecht, however, was fond of saying that, “Fertile soils make the earthworms not vice versa.” Farmers must furnish the organic matter to feed the worms, if they want to cash in on the earthworm’s soil improving habits. As high earthworm numbers in grassland soils demonstrate, perennial forages benefit worms more than annual crop residues.

In biological systems, it is important not to discount the small contributions made by a broad number of other soil organisms. For instance, Vesicular-Arbuscular Mycorrhizal (VAM) fungi are thread-like root extensions that facilitate water and nutrient uptake by a variety of crop plants. The monetary value of their contribution may be less easy to pin down than the N2-fixing Rhizobium/legume symbiosis, but they play an important role all the same. The combined work of many soil organisms might be equated to the role of solar energy heating. Mostly unnoticed, their value suddenly rises as fossil fuel prices increase. Not surprisingly, VAMs do poorly with excessive tillage and prefer undisturbed soil – like the soil under a two or three year forage/ legume crop.

The motto “feed the soil, not the plant” is another way of saying that organic farmers shouldn’t forget to feed the below ground livestock.

Not doing so will severely test whether organic agriculture can thrive not because of higher prices for certified products, but despite increasing costs for fossil fuels.



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