Activity 14

Managing soil nutrients so that they are available when and where crops need them is critical for optimizing farm productivity and income while minimizing risk of impact to the environment. This is particularly challenging for small scale organic vegetable producers, where nutrient demands can be high and variable across production fields. While there has been some progress in developing cropping system strategies for organic vegetable production, the sector continues to under-yield conventional counterparts (Seufert et al. 2012, Ponisio et al. 2015, Seufert and Ramankutty 2017). Nutrient management has been identified as one of the major contributors to this yield gap (de Ponti et al. 2012). Organic cropping systems have been designed to improve soil health and maximize nutrient cycling on-farm through the utilization of cover crops and organic amendments, to create an environment that will enable the development of healthy plants, high quality food and the overall sustainability of the farm. However, these objectives do not always line up with the most effective nutrient applications for plant growth (and yields), as organic nutrient sources pose some unique challenges.

The first is the ratio and mobility of nutrients in cover crops and manure. Cover crops are unlikely to be effective nutrient sources on their own. Although cover crops can be planted in the off season to capture residual N left in the soil or fix N from the atmosphere, they are unlikely to meet vegetable crop N demands, particularly in BC where climate limits cover crop establishment. Furthermore, although cover crops can help mobilize phosphorus (P), they cannot bring in more P to the system. Therefore, if P is exported from the fields through cash crop harvest, it must be replaced with either mined P or P originating from compost and manure. Alternatively, application of compost and manure (amendments) to meet, primarily, plant N demands, is likely to lead to the buildup of soil P and an increased risk of contamination to water resources, thus calling into question the sustainability of this practice. There are a number of organically certified fertilizers (e.g. blood meal) that can provide readily available sources of N that could also be used to help better match N:P ratios of soil inputs with crop demands. Clearly, some combination of cover crops and soil amendment and perhaps organic fertilizers needs to be employed to meet crop demands, but optimizing their use requires a better understanding of their decomposition and interactions.

The second challenge, given that the availability of N and P is microbially driven, is the timing of their availability. Due to microbial activity being temperature and moisture-dependent, it is inherently difficult to predict, which can lead to the inefficient use of nutrients, subsequently resulting in under-yielding crops and an associated risk of losing unused nutrients to the environment. These challenges are particularly acute for organic vegetable farmers who need to develop a nutrient management strategy for a wide range of vegetable crops, some of which require far more nutrients then others. There is some evidence that organic fertilizers can be used to effectively time nutrient availability with vegetable demand to maximize yields but, given their expense, it is unclear how cost-effective they are for small scale producers. Furthermore, there has been little work assessing their potential environmental impacts. Thus, developing nutrient management strategies to improve nutrient use efficiency may be time-demanding and complex for farmers, and the benefits of utilizing organic fertilizers to optimize production unclear.

We propose to develop the science required to integrate key organic vegetable production practices (cover cropping, crop rotations, and soil amendments) to optimize nutrient cycling. There are a number of planning tools that could help organic farmers navigate some the complexity of managing these components, but these tools were not developed for the soils, climate or highly diverse crop mixes that are typical for organic vegetable producers in BC. Many of these tools utilize generalized nutrient values that are not specific to either BC or even organic production. The objectives of this project are to 1. Identify improved nutrient management strategies for enhanced production, environmental and economic outcomes. 2. Refine models and laboratory methods for estimating plant available nitrogen for British Columbia’s organic vegetable production and 3. Develop an online tool for effective organic nutrient management planning.

de Ponti, T., B. Rijk, and M. K. van Ittersum. 2012. The crop yield gap between organic and conventional agriculture. Agricultural Systems 108:1–9.

Ponisio, L. C., L. K. M’Gonigle, K. C. Mace, J. Palomino, P. de Valpine, and C. Kremen. 2015. Diversification practices reduce organic to conventional yield gap. Proceedings. Biological sciences 282:20141396.

Seufert, V., and N. Ramankutty. 2017. Many shades of gray—The context-dependent performance of organic agriculture. Science Advances 3.

Seufert, V., N. Ramankutty, and J. A. Foley. 2012. Comparing the yields of organic and conventional agriculture. Nature 485:229–232.

Nutrient management is one of the primary reasons that organic farms, particularly those producing vegetables, have lower yields than non-organic farms. Undersupply of nutrients can result in reduced crop yields and income. At the same time, applying nitrogen (N) or phosphorus (P) above what the crop can use can result in nutrient leaching and runoff into water resources, contribute to poor air quality and emit climate-changing greenhouse gases. Managing this trade-off is difficult given that outcomes can vary widely with the types of amendments organic farmers have access to, their climate, and soil conditions. The objectives of this project were to trial various organic nutrient management options to give farmers a better understanding of their potential economic and environmental trade-offs and to develop tools to enhance nutrient management in the future...

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