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The new plantings are carried out with densities of between 30,000 and 32,000 for large-calibre crown varieties and between 22,000 and 25,000 for the most common ones. Density increases as distances between rows widen. This trend is directly related to the reduction in pickers’ movement. In an asparagus plantation with 2-metre inter-rows, the distance travelled by a picker is 5km per ha. When rows are distanced at 3.30m, pickers cover only 3.3km per ha. After 50 days of harvest, the travel gain is 85km/ha. The picker will have walked 250km in the first plot and 165km in the second.

More soil volume for roots

But if the planting distance widens, the density must increase to keep the same number of plants per hectare. At distances of 2.50m, 5 crowns can be planted per linear metre compared to 7 crowns for an inter-row of 3.30m. Some plantations now have 4m gaps between rows. This increase requires a new arrangement of the crowns on the row. From one planting line, we move to two planting lines 20cm apart and a staggered crown layout to provide more useful space in which the claw can grow. It will therefore be necessary to open a wider planting trench. Observations made on these new asparagus designs show that they generate a 10% increase in yield compared to the smaller spread. They also extend the lifespan of asparagus plantations by 3-5 years thanks to the associated reduction in health risks. Moreover, they allow for better quality and better calibre spears thanks in particular to the large volume of earth available between rows. The widening of the inter-rows also facilitates ridging-up with a larger volume of surface soil available between the rows. It leads to better aeration of vegetation and a reduction in foliage health problems (e.g. rust and stemphylium). Working between rows is also facilitated and work times reduced (shorter rows to be covered and maintained). It reduces investment in equipment in linear metres (drip feed ducts, plastic covers, etc.). Finally, this mode of implantation allows the cultivation of green manure in the inter-rows and offers the possibility of replanting in soil that is (almost) free of asparagus.

Read also:  Establishing an asparagus plantation

Asparagus is one of the few perennial vegetable crops. Its installation is planned two years ahead and the plant can be exploited for about ten years. Nevertheless, it is the planting that is the defining moment. It’s the moment when multiple technical choices must be made. Soil and climate often represent imposed parameters, but variety, density, depth of planting, etc. are more strategic decisions. The basics of plot preparation and tillage are common to any cultivation. Notable development in asparagus cultivation has come in the form of the prior input of green manure, the addition of organic matter and the deep working of the soil. What are normally considered recommendations become obligations when replanting asparagus on asparagus. Lastly, it is the planting that determines the “economic potential” of the plot. The potential yield rests on the varietal choice, its density and the technical means implemented (irrigation, fertilisation, plastic cover, etc.). However, the planting configuration also influences yield as well as harvest speed, a determining factor in the final cost of production.

Respect the keynotes of plantation

Cultivating asparagus requires respecting certain “keynotes”. The first step is to define the inter-row spacing, taking into account the existing equipment on the farm in order to avoid the need to invest in specific materials. Then, the rows must be of adequate length to optimise the investment in mulch, drip feed systems and labour for harvesting. For example, a hectare planted with 3.30m between rows represents 3 linear kilometres of ridge. This relates to the densification of asparagus (see box). The second keynote is the volume of land available to the roots; the larger the volume, the larger the root mass. A plant’s yield is, of course, proportional to its root mass. The third keynote is to ensure that after too much rain or even flooding in winter, plants do not suffer from the presence of standing water. A solution in this regard is to incorporate high ridge cropping (see Asparagus World No. 2).

Expanding inter-row gaps

Besides respecting these keynotes, many other precautions must also be taken, starting with healthy, well-drained soil (see box). Soil preparation begins one year before planting. This is the time to sow green manure, like Ray-grass, rye, mustard, siletta, phacelia, etc., which will provide a significant mass of organic matter when incorporated into the soil. However, it also destroys perennials, meaning that the land must be free and drained at the end of the summer before planting. Modification of lime or magnesium content in the soil is carried out if necessary. Then, sub-soiling at a minimum depth of 80 cm is carried out across and in the direction of the rows. The supply of decomposed organic matter (dung, compost, etc.) is focused on the planting row. Then a spader passes along the row working at a depth of between 0.60 and 1.10m. The machine must work at very slow speeds in order to create a homogeneous, aerated, fertilised profile at all depths, which facilitates the descent of roots and thus limits the rise of the rootstock.

The orientation of the planting rows is another important element, with planting usually carried out in the direction of the slope, if one exists. The orientation of the rows can also be in the direction of the prevailing wind. However, with the widening of the distances between rows, not ridge-splitting at the end of the harvest and the grassing, the current trend is to plant across the wind as this strategy ensures better weeding efficiency. It also promotes the creation of a micro climate and avoids plastics films blowing away in high winds (see page ????).

Adapting to the current and future climate

Then comes the choice of the variety (or varieties) to plant. Asparagus World No. 1 devoted a long article to the presentation of varieties (“Varieties undergoing innovation”), while Asparagus World No. 2 contained an article on the choice of crowns (“How to achieve a good plantation”). Bear in mind, however, that the selected variety must meet market demands to ensure it can be successfully marketed. Choosing vigorous and disease-resistant varieties is vital given the new legislation focused on reducing the use of chemical crop protection. The calibre of the spears determines the crop’s profitability as it is the cost of producing a kilo of asparagus that essentially accounts for the cost of the harvest. Choose good calibres, as a 15g spear costs as much to harvest as a 45g spear, and is therefore 3 times cheaper per kilo to produce. Another key factor that now must be taken into account is how best to adapt to the current and future climatic conditions (see page 10).

Choosing the right soil

A healthy, deep and well-drained soil is conducive to good asparagus cultivation. It is sometimes necessary to dig ditches and lay drains to improve these soil properties. But this is not about having dry soil – access to water and irrigation is essential! The ideal soil has a pH of between 6 and 7, and maximum clay content of 15% to avoid the development of fibres and bitterness in the spears. The maximum salinity tolerated by asparagus is 10 grams per litre. Soil test samples are taken at depths of 10-40 cm and 40-80 cm.

Plant in deep soil

The use of spaders and deep soil planting is a fairly recent innovation. The goal is to get as much good soil as possible deep down and to oxygenate the soil as much as possible in order to attract as many roots as possible. Some equipment can allow the soil to be worked at depths of over a metre. But, it is imperative to adapt the depth at which the soil is worked depending on the location of the bed-rock so as to avoid bringing up stones, clay, and chalk, etc. Working in the summer or autumn, rather than in the spring, will allow you to work at great depths. Before the passage of the spader, de-compaction of the plot must be carried out across the field on the entire surface followed by a second passage only on the row. “The rise of poor soil to the surface is not a problem because good soil on the surface serves no other purpose than to confine the roots horizontally. At these depths, three times more earth is mixed than before. The soil that is going to be fetched up from this depth is generally less rich than the surface soil. A new soil must therefore be made with at least 3 times more organic matter and 3 times more phosphoric acid per hectare concentrated on the row than with spader-free methods,” said Christian Befve, a consultant specialising in asparagus.

Asparagus is phytotoxic for subsequent crops

Replanting a new crop of asparagus on the same plot can be risky. Indeed, the presence of the roots of old asparagus would be phytotoxic for the new crop. In addition, phytotoxicity increases from year to year after the cessation of cultivation, especially between the 2nd and 4th year. This effect of toxins emitted by old crowns on later crops was highlighted by Ludger Aldehoff, a German researcher at BDSE Brucshal, whose work was presented at the 2017 International Asparagus Congress in Potsdam, Germany. The practical advice is to “replant as early as the year after an asparagus crop is removed or wait at least ten years.” The decrease in the root volume and absorption potential of the plant is also to be taken into account in the management of fertiliser inputs on an asparagus bed replanted on asparagus. Observations of 17 varieties of asparagus in virgin soil or replanted show that they react differently. Cumulus, Tallems, Gijnlim and Vitalim are found to retain their root potential in the replanting condition, while the root potential of Steiniva, Bacchus, Fortems and Ramon is reduced by about 30%. The other varieties show a decrease of about 20%.

Read also:  Asparagus is becoming denser

Over the last few years several pot-based fertilising trials were conducted to investigate different aspects of fertiliser application in asparagus. One of these trials was designed with seedlings in sandy soil with different nutritional contents.

Trial design 

The trial contained several groups of 5 randomly positioned 5.5l pots, each group having just one nutrient withheld (all macro minerals and most microminerals). As base material sandy subsoil of an existing asparagus field was analysed and then fertilised according to the trial design.

Seedlings were produced in grow plates filled with upper sandy soil from the same field. Once seedlings had developed at least one stalk, two plants were transferred into each pot. Overhead irrigation occurred at -200hPa and an additional small amount of the macro minerals and boron was applied once in the summer. In autumn, the crowns were weighed, and the mineral status of the roots analysed. This was repeated over three consecutive years.

Results and discussion

The induced lack of nutrients was evident in the amount of crown mass in autumn. Some treatments did not differ from the control (e.g. without giving Mg or Mo), whereas other treatments showed significant reduction in root mass (e.g. without any fertiliser or P).

When a mineral was withheld from the plants, it translated to a reduced content in the root. For example, without giving nitrogen (“-N”), this group only contained 61 % of nitrogen compared to the average amount of all of the samples. The control group that received the whole spectrum of nutrients (“+all) was showing nearly 100% of each nutrient in the analysis. This indicates a close relationship between the soil and the root content, too – at least in the young stage of the asparagus plant.

Zinc was the only exception. Although lack of this mineral caused a significant reduction in root mass there was only a small decrease in root content. There could be two explanations. Firstly, plants that lacked zinc in the soil showed the biggest differences between root mass in this trial (data not presented). Individual plants

either grew normally or had significantly reduced growth (picture plants with lack Zn). Secondly, the data showed all groups had an overall low zinc content compared to other trials and high amounts of data from crop fields in praxis.

Additionally, calcium appears to be an especially important nutrient in asparagus. The relationship between fertilization and root content was closest in this trial with very young plants. Lack of calcium also caused a lack of many other minerals. Closer inspection of the roots revealed a reduction in fibrous roots which may decrease absorption of some other micronutrients (picture plants with lack Ca).

In conclusion

Although there were significant differences in the growth of the crowns and mineral content of the roots there were no visible symptoms in fern. Other trials with plugs or crown also showed similar results. This indicates that root tissue samples may be more useful for improving fertilisation strategies than fern.

The substantial differences in crown weight without visible differences in fern appearance in these various fertilisation strategies show that asparagus can have a significant “hidden hunger”. This also indicates that checking the nutrient status of asparagus roots during winter can be a useful tool for improving plant growth.

As other crops, asparagus shows some antagonism in the absorption of minerals. For example, manganese and calcium show that lack of one of these leads to better absorption of the other. This could be used to check the basic absorption potential of the immobile calcium in older plants.