There was a time when fertiliser recommendations were expressed in "bags per acre" and if the spreader hopper was completely empty after applying the desired number of bags on the correct acreage, the job was deemed to be "accurate".
Today that's not enough. If the aim of the grower is to get a uniform and consistent sample to the packing house, every plant must have the correct level of nutrients delivered at the right time. Simply averaging applications out over a given area won't do.
There are, of course, a number of ways to achieve this: using nutrients in a solution or suspension through a sprayer, as fertigation through irrigation, as solid broadcast through a spreader, or placed in situ by planter or drill.
Broadcasting by spreader remains a highly popular option, perhaps because buying nutrients in solid fertiliser form gives growers flexibility and more freedom to shop around. A wide variety of nutrient types, sources and ratios is available and storage needs no capital expenditure.
However, can fertiliser spreaders match the accuracy claimed by sprayers and meet the variable-rate application challenges of precision-growing?
Spreader manufacturers specifically, and the science and practice of spreading in general, have met the challenges of the past decade, and today's grower has sufficient choice to enable nutrients to be applied accurately to almost any crop.
Accuracy is much more than rate per acre. It is essential that nutrients are spread evenly across the crop at any given bout width. Typically, a spreader deposits fertiliser more heavily immediately behind the tractor, tailing off gradually on either side. On the return trip down the neighbouring tramline, or bout, the pass overlaps the former and the spread patterns are evened out.
To achieve this, spreader settings, fertiliser flow and height above the crop must be specific for the product being spread and the machine must be in good condition. Even slight faults in spreader plates or vanes can spoil accuracy. As a result of this, a whole new profession of freelance and in-house spreader calibration specialists has emerged.
Even so, accurate spreading is a marriage between the product being spread and the machine doing the job. According to independent calibration expert SCS Spreader & Sprayer Testing's Rob Foxall, the overwhelming majority of new machines on the UK market can be set up to accurately apply "quality" fertiliser over moderate bout widths. However, more sophisticated machines may be needed for extreme bout widths or for fertiliser of unknown quality.
Calibration is considered essential by many because fertilisers do vary considerably and the physical characteristics of the product will significantly affect the spread.
This is not just the difference between compounds and nitrogen, but a complex interaction between particle size and density: the variation of size about a mean, from large particles at one end to dust at the other; particle shape, angularity and hardness; coatings and moisture content; and solidity or hollowness.
At one time, some imported fertilisers had a reputation for poor quality. Growers were faced with a dilemma. Should one invest in a sophisticated spreader that claims the ability to spread anything and thus save by buying cheap fertiliser or invest less on the machine and use more consistent product?
Fortunately, fertiliser quality is now improving across the globe but there are significant differences in the physical attributes between manufacturers, between grades and sometimes even depending on which factory produced the product.
Calibration involves spreading fertiliser across an array of special trays and measuring to ensure the amount of fertiliser that is applied to each section of ground is as even as possible. The level of variance is known as the coefficient of variation (CV) and for effective spreading this should be no more than 10 per cent. Any more, and loss of both yield and quality is the result.
A final caveat on fertiliser quality concerns "compounds". Technically, a compound is a fertiliser that contains more than one nutrient, typically nitrogen, phosphate, potash and/or sulphur, with or without other minor or trace elements. Unfortunately, the technical vocabulary associated with fertilisers is large, obscure and used very loosely.
Many growers have consequently come to associate the term "compound" with products that are strictly speaking "complex", which means that all the nutrients are typically combined in the correct proportions within each individual granule. Thus, all complexes are compounds, but most certainly, not all compounds are complex. Some compounds are blends of individual nutrients; some are blends of two or more different complexes, and so on.
While British blenders go to great lengths to match the size and density of individual particles, a mismatch in an inferior blended product can result in nutrients segregating in the fertiliser hopper. Sadly, no amount of calibration can overcome this.
When buying a new spreader, the market is well served by a substantial range of machines suitable for field vegetables, salads and potatoes. KRM, Kuhn, Amazone and Vicon (Kvernland) share the honours as major players, while Lely (Tulip), Sulky and Teagle continue to supply specialist, local or niche markets.
Vicon's rear-mounted pendulum spreaders are said to be the most popular and numerous across the globe, but today are best suited for smaller farms and narrower bout widths. Like most leading manufacturers, Vicon's latest machines operate through rear-mounted twin discs. These rely on alterations in the length of the vanes to allow for calibration. The range, distributed by Kvernland Group of St Helens, includes machines spreading up to 45m and a dynamic weighing system, allowing constant calibration on the move.
Kuhn spreaders also employ twin-disc technology and are designed to adapt to all kinds of work: spreading fertiliser across open fields, along field edges and marginal zones, late fertilisation and application to special crops (wine-growing, tree cultivation, market-gardening etc). MDS models (500-1,800 litres) have working widths of 10-24m and incorporate localised spreading systems (over one or two strips, or seven rows). AXIS models (1,000-3,000 litres) have working widths of 12-42m. UKS models are said to distribute all kinds of products including mineral or organic fertiliser, gravel, salt and lime.
Amazone offers a comprehensive range of mounted spreaders and trailed bulk-material large-area spreaders in working widths of 10-52m. For specialist growers, the ZA-XW features an especially narrow hopper shape of only 1m for special crops. Its hopper capacities range from 500 to 700 litres with working widths of 10-18m, or 2-5m with row-spreading attachment. Bearing in mind the importance of the physical characteristics of the spread material, Amazone maintains a comprehensive database of fertilisers linked by spreader model to give optimum rates, speeds and bout widths for all machines.
KRM offers two spreader ranges. Its own KRM-mounted machines, also twin discs, operate a unique spreading system, called the Trend. With the two discs counter-rotating towards each other, the final result is a "double-double" overlap, which gives a high degree of tolerance when spreading.
KRM also imports the Danish Bredal-trailed and mounted spreaders with their ground-driven floor belt delivering the material to the discs. This ensures that the spread rate remains constant regardless of forward speed and flow rate. Coupled with "rate independent of forward speed", this gives - according to the company - an extremely accurate fertiliser spreader that is simple to set.
Foxall confirms that all these machines are capable of doing a good job at broadcasting fertiliser evenly. But are they up to the challenges of the future?
Variable-rate application is well established on cereals and oilseed rape, and is becoming more widespread on potatoes. It will almost inevitably follow a much wider range of field vegetables and salad crops before too long.
Already the manufacturers have demonstrated their ability to produce systems to apply fertiliser at variable rates through electronic control and GPS technology. The most sophisticated machines from each manufacturer are even fitted with - or have installable options for - built-in weight sensors and a significant choice of electronics.
The challenge lies not so much in kitting out the machines for variable application, but rather in developing application maps and algorithms to give accurate crop application rates to the spreader electronics.
As with calibration, this has given rise to a network of regional freelance specialists such as Robin Thompson of Precise Crop Nutrition in Stamford, in Lincolnshire.
"It's still early days for this technology in vegetables," says Thompson. "While yield maps related to P and K based on hectare grid sampling have been around for some time, the real challenge is localised nitrogen recommendations."
Thompson is utilising satellite and aero imagery to evaluate the colour and vigour of crops and alternative methods such as soil conductivity to give variable-rate nitrogen maps. Yara's N sensor is also up and running for potatoes, giving variable-rate recommendations.
The days when the spreader was the Cinderella of the farmyard are well and truly gone.