Phosphorus status on Canadian organic farms

Abstract

In eastern North America, many conventional livestock farms, especially dairy farms with high inputs of feed and fertiliser have excess soil phosphorus levels and an annual phosphorus surplus. However, a Canadian dairy farm in transition to organic, without fertiliser inputs reduced its farm P surplus to a marginal level. On long-term organic dairy farms in Ontario, most soils tested low to very low in available P as measured by a standard soil test. Canadian Prairie organic grain farms also consistently demonstrate deficiencies in available soil P. Organic producers have few viable alternatives for P management. Phosphate rock can be acceptable to organic standards (provided they are low in heavy metals, and not processed synthetically), but the P in these becomes available slowly, especially in high pH soils common on most organic farms in Canada. An alternative is to increase soil P availability. Enhanced microbial activity in organically managed soils may make P more available. Livestock manures are rich sources of available phosphorus, but a majority of organic farmers in Canada do not keep livestock. Off-farm manure sources are subject to organic regulations and hauling costs, both of which may be prohibitive. Furthermore, manure from conventional farms in Canada may be contaminated by genetically modified material from corn and soybean feed. Additional research is required to improve short-term availability of soil P and long-term replacement.  2007 Society of Chemical Industry Keywords: phosphorus deficiency; phosphorus surplus; phosphate rock; manure LIVESTOCK FARMS IN EASTERN NORTH AMERICAIn eastern North America, most conventional livestock farms use high inputs of feed and fertiliser, and have excess levels of available soil P levels. Even pasture-based farms with lower stocking rates and milk production than confinement farms, had an average annual P surplus of 11.3 kg P ha −1 (Table 1). 1A crucial consideration on dairy farms is the impact of dietary P on the manure N to P ratio. As the dietary P increases in feed, the N to P ratio drops in manure.2 High dietary P makes it more difficult for a farmer to meet N requirements of a crop by fertilising with manure. Nutrient management plans in eastern North America are increasingly based on P inputs from manure, rather than on N, in order to address the issue of excess soil P levels. 3,4

Phosphorus availability on many organically managed farms in Europe

“…In this study we used data sourced from European countries, primarily Germany and Norway, to address the immediate question: What is the soil P status of land under organic management? We used the results of Mallarino et al (2013) and Schick et al (2013) Nutr Cycl Agroecosyst (2018) 110:227-239 229 this analysis to explore the wider question: What factors, including farm type, affect the soil P status on organic farms? We also explored the limitations associated with the use of soil test results to evaluate P status on organic farms.…”

Residues from a buckwheat (Fagopyrum esculentum) green manure crop grown with phosphate rock influence bioavailability of soil phosphorus

“…Low soil test P concentrations are common to organic farming systems in Canada (Martin et al 2007;Roberts et al 2008). Chemically untreated phosphate rock (PR) has been proposed as an external input of P in these systems in place of readily soluble synthetic fertilizers that are not permitted under organic production standards.…”

“…Regardless of rock characteristics, dissolution of PR can be significantly slowed in soils of high pH and high calcium concentration (Bolan et al 1997). The high pH characteristic of most soils under organic production in Canada (Martin et al 2007) poses a significant challenge for the effectiveness of direct applications of PR. However, certain crops that can acidify their rhizosphere have been found to effectively obtain P from PR (Van Ray and Van Diest 1979), even in alkaline soils (Chien et al 2003).…”

Optimal seeding rate for organic production of lentil in the northern Great Plains

“…(1 in Saskatchewan and Manitoba (Martin et al 2007). Martin et al (2007) also found that the farms that had been managed organically for more than 30 yr had the lowest levels of soil P.…”