Asparagus: Maximising yield by optimising establishment and agronomy

 

• Due to the high initial investment required to establish this perennial crop (approx. £5,000/ha), growers need confidence that they can sustain economic and high quality production over several years.
• All growers know from practice that the best (consistently highest yielding crops) are those which were established optimally in year 1.
• Premature loss in asparagus productivity (‘decline’) is widespread and can shorten the economic life of a crop by 7 years, with associated financial losses due to poor returns on original investment, lower prices obtained for reduced spear diameters and the extra expense of re-planting crops more frequently.

The opportunity

• The UK asparagus industry is currently expanding with the latest market estimates suggesting that the total planted area in the UK has increased from 1,000 ha in 2005 to approximately 2,100 ha today,
• Supermarkets are increasing their total volumes of fresh asparagus by taking an increased level of imported product, and would like to source more British asparagus,
• Retailers are also placing demands on growers in terms of reducing carbon footprints, and improving the environmental sustainability of crops, therefore growers need to demonstrate that fertiliser (particularly N) use is appropriate to crop requirements.

The need for the project

• There has been little work on nutrition of the asparagus crop in the UK since FV 152 (1996; study of nitrogen (N) responses) and P and K (1996: FV 153), and none at establishment; the above work was all on established crops,
• With P being an important nutrient for root growth, and high yielding asparagus crops requiring large root systems to be established as early in the life of a plantation as possible (Wilson et al., 2008),
• Some growers believe that modern high yielding varieties of asparagus need more N in the establishment years, and that higher rates are not supported by the current fertiliser recommendations; hence data are needed to underpin any changes to RB209,

Phosphorus
• Recent reviews by HDC (Drost, 2013) have indicated that P nutrition is critical in years 1 and 2 to establishing productive crowns, yet there are no UK data to support current recommendations,
• Much published work is on atypical soils for the UK e.g. a clay based volcanic ash oil in New Zealand (Douglas and Follett, 1996), desert soils in Peru (Sanches and Casas, 1999; Casas 2001) or volcanic ash soils in Chile (Krarup & Krarup, 2002),
• Drost’s work indicated an optimum yield response (after 4 years) at 225 kg P2O5/ha which is above the current RB209 recommendation of 175 kg/ha at soil P index zero, and above the original recommendations in the Netherlands of ca. 100 kg P2O5/ha (Roorda van Eysinga, 1960),
• Peter Falloon in New Zealand (NZ) in personal communication reports that most of the research on asparagus nutrition has been aimed at already established asparagus crops to determine their N, P, K and pH requirements as well as a small amount of work on trace elements like boron, however it was not possible identify any research on fertilising asparagus during the establishment years i.e. years 1 and 2 after planting,
• Nevertheless it is accepted in NZ that asparagus requires high amounts of available soil P (cf Drost, 2008), and it is generally recommended that a pH of 6 or more is beneficial; therefore recommendations to growers in NZ is to apply a base dressing of TSP sufficient to raise soil test values to 45 mg/l (Olsen P) and pH to 6.0- 6.5,
• Many growers believe that placement of P within the bed would be beneficial but there has been no experimental work to support this approach,
• Moreover, asparagus plantations are established in spring in cold, damp soils and growth of the crowns at this time is slow therefore there may be an advantage in applying phosphate in a more readily available soluble form, such as di-ammonium phosphate (DAP) which has been shown to increase yield response in other vegetable crops.

Nitrogen
• While the annual offtake of N in spears is relatively low (ca. 40 kg N/ha) the amount of N stored in roots and crowns can be over 700 kg N/ha (Ledgard et al., 1993); this N reserve in productive crops is built up during the establishment phase, making early N nutrition critical,
• Much of the work in the literature reports on the early nutrition of seedlings and propagated plants (Fisher & Benson, 1984; Spiers & Nichols, 1985; Fisher et al., 1993; Nicola & Basoccu, 2000) and crown production (Krarup et al., 2002) but very little following transplanting of crowns in the field,
• Most work on nitrogen has been carried out ex-UK; for instance Hussain et al. (2006) working in Pakistan suggested that the optimum N rate for newly planted crowns was 90 kg N/ha, but their paper has some unusual experimental features e.g. carried out with plots containing only 15 plants, and on a soil with pH 9.1,
• After planting, crowns are normally top dressed in NZ, with 16 kg N/ha starting within 2 weeks of the spears emerging, and this being repeated every 4-6 weeks until early autumn (Peter Falloon personal communication); the aim being to build up as much foliage (= photosynthetic capacity) as possible in spring and summer but it is accepted that there is no advantage to the plant from continuing to put up new fern into autumn and winter, since a new fern at this time of the year is often a net drain on the carbohydrate reserves of the roots until it is fully developed, when it starts returning carbohydrates to the storage root system,
• Despite the perception of a higher N requirement in modern varieties, Drost’s report suggest that high levels of N could scorch the crowns, therefore the crop should not be over-fertilized, although this is most likely to be a problem where the N is placed in intimate contact with the roots, rather than as top dressing,
• Any changes proposed to the nitrogen management guidelines also need to take into account the fact that there is now an Nmax value set for asparagus within NVZs (180 kgN/ha).