Sustainable fertiliser use to benefit both crops and environment

How can growers use modern analysis and application technology to improve fertiliser use, both to make it more sustainable and improve yields and profitability? That was the key question posed at a recent Sustainability Conference on the Future of Fertilisation held at Stockbridge Technology Centre (STC), which also presented details of Innovate UK’s Tru-Nject project. Richard Crowhurst reports.

In his introduction, STC chief executive Graham Ward OBE addressed both Brexit and the future challenges facing agricultural production. “We ought to look at the future of agriculture in a way that improves the product and the overall environment,” he stressed. “We are talking about the better agronomic management of outdoor crops: how do you get a crop in the field which maximises its growth and repeats itself year after year?”

That question was discussed, if not answered, by independent soils expert Neil Fuller who pointed out that while the addition of 64 million hungry people each year to the global population presented huge challenges, it also creates opportunities for farmers and growers. “Farming needs to increase in productivity by two per cent per year,” he warned. “But it’s not just about quantity, it’s also about quality.” Highlighting a drastic decline in the nutritional quality of food since the start of the twentieth century, he urged the entire food chain to work together to address the issue.

He also pointed out that changing climatic conditions would also change the way in which fertilisers work and plants’ requirements for nutrients at different stages of growth. “We have very little resilience in our farming systems to cope with changes in climate, and when we drop fertilisers into the soil they are subject to all sorts of different factors,” he said.

Soil temperature, pH and moisture deficit all affect the availability of key nutrients. Pointing out that increasing soil organic matter by one per cent can increase soil water holding capacity by 150-200 tonnes per hectare (equivalent to 15-20 mm of rain), he continued: “Managing soil structure and carbon can have a huge impact on fertiliser efficiency. These factors are all manageable so we can make changes, and by increasing soil carbon we could mitigate man-made climate change in one or two generations. That’s a hugely positive story.”

Improving soil health and biology, such as using under-sown cover crops to increase nitrogen fixing organisms such as Mycorrhiza and rhizobacteria, has the potential to reduce agriculture’s carbon footprint by improving the efficiency of nitrogen fertilisers, as well as reducing other environmental effects such as runoff and pollution. “By reducing artificial N we could transform agriculture and its environmental impact. It is about driving carbon and energy back into the soil and providing nutrition back to the plant,” he concluded.

New RB209

Since 1973 ‘RB209’ has been the key source of advice and guidance for crop nutrition requirements and applications, but with the last edition of the Fertiliser Manual published in 2010, and increasing use of fertiliser placement and variable rate application, James Holmes of ADHB said that it “had fallen out of favour.”

He explained the background to the latest version which was published by AHDB in the summer of 2017. Reiterating Neil’s comments, he said, “Soil is complex, but we can break things down and understand it quite well. Soil structure is important, but it is how things come together in the environment that’s vital.”

Among other things, he stressed the importance of soil pH in making nutrients available, warning that for many soils pH levels were now above optimal values. “Rates of liming are below what we expect them to be,” he warned.

James also said that part of the redesign of RB209 included a field assessment method which could be carried out in the office, although it was up to growers whether they felt actual soil nitrogen measurements were necessary in addition to the calculated values. “It is important to get the soil type and soil category correct, and you also need to consider cover crops as the last thing you want to do is to over apply nitrogen,” he explained. With this in mind the new version of RB209 has reintroduced the concept of yield adjustments for nitrogen recommendations.

“High yielding crops require a greater quantity of N,” he pointed out. “Take the previous five yields for that crop and discard the highest and lowest values, then average the remaining three values.” However, he also stressed that growers and agronomists should trust their own knowledge and experience or particular soils and crops, “[RB209] is a guide. You know that better than anyone else.”

Tru-Nject project

Funded by Innovate UK and run by a consortium including Cranfield University, STC and precision farming specialists Manterra Limited, the Tru-Nject project aims to provide high-resolution data on soil fertility and then couple this with the accurate and consistent placement of nutrients.

Three seasons of field testing of the system at commercial field scale has combined the use of N placement technologies together with a vis-NIRS soil sensor from Cranfield. This sensor, which is attached to a subsoiler, provides up to 1,500 readings per ha, which means that using the spoke-wheel below ground injector supplied by Manterra, liquid nitrogen applications can be varied meter-by-meter and bed-by-bed to maximise the efficiency and consistency of nitrogen applications.

“Cranfield’s online scanning system is very much an R&D model, but the final stage of this project can be a commercial prospect,” stressed Dave George of STC. “It is continuously mapping the soil to build soil fertility maps.”

The data from a single pass of the scanner was then compared to physical soil samples from the field. “We needed to know how accurate these maps are,” pointed out Dave. “This is where the maths bit comes in, but the result was more than 95 per cent accuracy. The system models soil fertility metrics with a high degree of accuracy and can layer other data on top. The more data, the more accurate we can become.”

While the Tru-Nject project had combined the scanner data with fertiliser injection in order to deliver the nitrogen directly to the moist root zone, Dave stressed that the benefits from variable rate application of other forms of fertiliser would also be beneficial. Different trials with both surface application and injection showed that in some situations the biggest yields (of cereal crops) were obtained by evening out overall N (by applying more N in the areas with the lowest fertility), while in others boosting N in the most fertile parts of the soil provided the biggest margin. The trials also supported the use of liquid products over solid fertilisers.

“The best returns all used the scanning system,” explained Dave. “However, it’s very important to get the modelling right. This is a tool for agronomists and the fertility map needs to be interpreted to turn it into an application map. The reduced variability in application is also a benefit and we have seen similar benefits field vegetables during the first two years of trials.”

Despite the positives, a note of caution was sounded by technology provider and farmer Andrew Manfield of Manterra Limited. “It is very easy to get carried away with the progress of big technology. There are still quite a few unknowns despite what we’ve heard,” he warned.

He also pointed out that what appeared to be the optimal application rate from a scientific or agronomic point of view might not always be practicable. “RB209 is state of the art, but there are limitations we face in practice. For example, restrictions in NVZs and other factors, such as pesticides, will affect yield and therefore application rate. Intense rainfall events followed by dry periods are a challenge to application as soil water is vital to the uptake of all nutrients and we can’t know for certain the shape of the yield response to fertiliser. We have to make money and if we get the decision wrong the profit penalty can be severe.”

Despite these caveats, he stressed that both the form of fertiliser and the way it is applied affect the uptake of fertiliser by the plant. “The balance of the consensus is that liquid stands a better chance as there is less impact from a lack of soil moisture,” he explained. “In the past government data was quite critical of urea and I think the volatilisation losses were overplayed; having said that, you don’t want to be surface applying urea in an un-stabilised form. It is extremely difficult to get ammonium fertiliser into the plant in a field situation as it is often broken down into nitrate, but more ammonium occurs in better healthier soils.”

Placement and injection

Placement and injection can help to overcome some of these problems by putting the fertiliser directly in the soil and close to the root zone. Andrew pointed out that while many vegetable crops are already grown with fertiliser placement, there is a challenge in using the technique in arable crops.

His solution is the use of a point injection system, which can inject at depths between 30 and 75 mm. “You end up with a deposit of fertiliser and as it is concentrated, ammonium is less likely to be broken down too quickly depending on other factors,” he explained. The system also causes minimal damage in cereal crops and you can adjust the spacing and width of the application wheels to work with precision farming and controlled traffic systems.

Trials with onions at G’s Produce have also shown that using narrower machine widths is not a problem, although more work is required to find the optimal application time. “The aim is to put all these elements together to minimise the gap between what we intend to give to the plant and what we actually put in,” he concluded.