Agriculture Reference
In-Depth Information
limited extent. This can be contrasted with farms with little or no livestock, in which case
commercially desirable crops may dominate the crop rotation (Bulson
et al
. 1996). From the
production point of view, the rotational system has to meet requirements for the maintenance
of soil fertility, nutrient supply and control of weeds, pests and diseases (Robson
et al
. 2002).
Economic aspects also inf luence decisions about crop rotations, such as labour management
and maintaining the continuity of farm cash f flow through f fluctuating market and circum-
stances (Smith
et al
. 2004b). In organic farming, the tension between short-term (economic)
demands and long-term (ecological) needs creates challenges for farmers, and complex skills
are required to balance the competing interests by managing a sequence of crops that provide
direct cash benefits as well as indirect ecological services (e.g. Sandhu
et al
. 2005).
Nutrient management
The structure of organic rotations generally consists of two parts. First, legumes are used as a
soil fertility-increasing component, from yearly to multi-year crops, mainly in the form of
forage legumes, with much less in the form of grain legumes (Herrmann and Plakolm 1991)
and, second, periods with non-legumes such as cereals, root crops or field vegetables, relying
on the accumulation of humus, organic nitrogen (N) and depleting the resources.
In organic farming, the emphasis is on developing healthy, biologically active soil through
the use of certified inputs and land management practices. Inputs used in rotations include
plant residues, animal manures, rock dusts and biological activators (IFOAM 2002). Addition-
ally, biological activity is promoted so that soil processes such as nutrient cycling and soil
structural development are facilitated (Schjønning
et al.
2002, Shepherd
et al.
2002). Crop and
pasture plants then rely on the mineral nutrients that are made available in the soil. Stockdale
et al.
(2002) have indicated that the underlying soil processes and nutrient pools are similar in
organic and conventional soils although the relative rates and importance of individual proc-
esses differ between farming systems. While many soil changes under organic management
may be clearly significant, other research shows that changes are often 'subtle rather than
dramatic' (Shannon
et al.
2002) (see Chapter 2 for a review of soil fertility).
Careful N management is needed for most crops in order to maximise yields while mini-
mising nutrient leakage. In organic farming, this mobile nutrient is usually not difficult to
supply, whether through legumes or animal manures, but is easily lost from the system (van
Delden
et al.
2003). Phosphorus (P) is a very immobile nutrient, especially at higher pH levels,
and organically certified sources of P commonly have low solubility (Oehl
et al.
2002). As a
consequence, several recent studies have highlighted the lack of sustainability in existing
organic P management in different settings (Gosling and Shepherd 2005). Using farm-scale
nutrient budgets, Watson
et al.
(2002) confirmed that N was generally not a problem but that
P and K were being depleted in some systems, particularly arable cropping (Figure 3.1). Dairy
farms had low nutrient budgets as a result of the very low reliance on external inputs, while
horticultural farms had noticeably high budgets, presumably as a result of large externally
sourced inputs of manure. The longer-term decline of P soil reserves in some organic farms in
Germany has been highlighted by Lampkin (1990), where the older organic farms have less P
than newer organic farms (Figure 3.2), a trend not observed in paired conventional farms.
By including greater carbon (C) inputs in the soil, organic producers are able to store
nitrogen and other nutrients for future crop use, while minimising the risk of environmental
pollution (Poudel
et al.
2001). Indeed, Shepherd
et al.
(2002) asserted that improved soil struc-
ture was dependant on frequent, and presumably large, inputs of fresh organic matter, a
common practice in organic agriculture. Edmeades' (2003) review of several conventional field
trials comparing the long-term (20-120 years) effects of fertilisers and manures (farmyard
