Ensuring a good supply of nutrients, in particular phosphorus, from the soil is the nutritional foundation of high yield. Yield increases after P fertilisation of up to 40% are reported under farm conditions in low P index soils. Good fertilisation practice secures this yield potential while minimising the risk of phosphorus loss to water. Placement of P close to the seed in low P soils supports good P utilisation and ensures optimum use of the investment in fertiliser.
Rate of phosphorous application
The above reported evidence on phosphorus supply being particularly important for high yielding faba bean crops grown under low soil-P supply has important implications for production practice in Ireland and in other countries. Faba bean yielding above 6.5 t/ha is common in Ireland. What are the implications for practice and what are the principles that determine these practices?
Soil analysis for plant available P is the basis of planning phosphorus applications to all crops. This involves laboratory analysis of representative soil samples following national or regional guidelines.
The Irish soil index system categorises soils into one of four soil index levels based on the soil test P result (Morgan extraction). Table 1 shows the P recommendation for each soil index for faba bean.
Soil pH and phosphorous uptake
Phosphorus exists in several different forms in soil and the occurrence of each of them depends largely on soil pH. Plant available inorganic P is most abundant when the pH is between 6 and 7. A whole-farm liming regime that maintains soil pH between 6.5 and 7 over the rotation ensures that the soil phosphorus is most available to crops.
Application time and method
Beans as with other legume crops require P for crop growth, from early development to the end of grain fill. Plants require relatively small amounts of P during establishment but have high P uptake during rapid canopy development. Ensuring the availability of P at the establishment phase is essential. This can be from soil reserves or applied P in low P sites.
Phosphorus is relatively immobile in soil and so applications on low index soils must be made at or before sowing to influence plant growth (Table 2). Placement of fertiliser in close proximity to the seed (either by placement in the same furrow as the seed or by side banding at planting/seeding) is an effective method of fertiliser application, especially to provide a starter source of nutrient for early crop nutrition and growth.
Depending on the soil P status, fertiliser may be broadcast (ideal for higher P sites), with or without subsequent incorporation, or placed close to the seed at planting (which is beneficial on low P sites). Where soil phosphorus levels are adequate, faba bean shows little response to timing and method of application.
Where P requirement is high, placing all the P with the seed at sowing may increase the risks of damaging the emerging plant. Incorporation/placement of P at sowing provides a good basis for high yields, especially in low P-soils.
Key practice points
- Research observations indicate that faba bean is responsive to good P fertilisation due to the effect of phosphorus on nodule formation and function. This impacts indirectly on the nitrogen supply from biological nitrogen fixation.
- As a pre-requisite for the effective application of P fertilisers, soil samples must be taken and analysed according to national or regional standard practices to determine the soil phosphorus levels/indices following national guidelines.
- Application methods should take into account soil phosphorus index and the rate of phosphorus to be applied. Placement of P close to seed is important on low P index soils. This is achieved using combined drilling where the fertiliser is placed in or beside the seed row. On high P index soils, placement close to the seed is less important and broadcasting before or after sowing can be used.
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Grant, C. A., Flaten, D. N., Tomasiewicz, D. J. and Sheppard, S. C., 2001. The importance of early season phosphorus nutrition. Can. J. Plant Sci. 81(2): 211–224.
Havlin, J. L., Beaton, J. D., Tisdale, S. L. and Nelson, W. L., 2014. Soil Fertility and Fertilizers. An introduction to nutrient management. 6th ed. Prentice Hall, NJ.
Henry, J. L., Slinkard, A. E. and Hogg, T. J., 1995. The effect of phosphorus fertilizer on establishment, yield and quality of pea, lentil and faba bean. Can. J. Plant Sci. 75: 395–398.
The Fertilizer Association of Ireland in association with Teagasc, 2019. The efficient use of phosphorus in agricultural soils. Technical Bulletin Series – No. 4, February 2019 (Booklet). www.fertilizer-assoc.ie/wp-content/uploads/2019/02/The-Efficient-Use-of-Phosphorus-In-Agricultural-Soils-Tech-Bulletin-No.-4.pdf
The Fertilizer Association of Ireland in association with Teagasc, 2017. Precise application of fertiliser. Technical Bulletin Series – No. 3, May 2017. www.teagasc.ie/publications/2017/precise-application-of-fertiliser.php
The Fertilizer Association of Ireland in association with Teagasc, 2015. Soil Sampling – Why & How? Technical Bulletin Series – No. 1, October 2015. www.fertilizer-assoc.ie/wp-content/uploads/2015/10/Fert-Assoc-Tech-Bulletin-No.-1-Soil-Sampling.pdf