Don’t let phosphate deficiency limit wheat potential – Arable Farming – Tim Kerr, George Baxter
Lack of phosphate could be limiting yield potential in many wheat crops, even some of the best. Paul Spackman examines what can be done about it this autumn. ...
Phosphate is essential for many plant functions, from root development to DNA formation, yet research suggests a worryingly large proportion of wheat crops are deficient.
Analysis of more than 900 grain samples collected over four years (2016-2019) through the Yield Enhancement Network (YEN) shows three-quarters were deficient in at least one nutrient, the most common being phosphate.
The finding is a concern especially as YEN samples are inherently skewed towards better crops selected for yield optimisation.
Decline
The situation is not entirely surprising though, considering Defra’s survey of fertiliser practice shows a long term decline in phosphate use, with just 42% of winter wheat fields receiving P fertilisers in 2018, at an average rate of 26kg/ha; less than half of the offtake for a typical 8t/ha crop.
Other research by the Professional Agricultural Analysis Group suggests almost a quarter (22%) of arable soils surveyed in 2018/2019 were below the target index of 2.
“Phosphate use has been falling for 25 years, driven by cost savings, and it could be one of the biggest factors holding back wheat yields,” says Hutchinsons crop nutrition manager Tim Kerr.
Tim Kerr, crop nutrition manager
Available
“We are depleting P reserves by removing more than is put back. Even if soils have a reasonable P supply, it may not be available when and where crops need it, as only a small proportion of plant-available P is held in the soil solution and that has to be replenished regularly.”
Soil phosphate is inherently variable, so standard soil tests only provide a guideline when assessing nutrient status and crop needs, says Mr Kerr.
Soil type, structure, moisture and bacterial activity all influence how much soluble phosphorus from fertiliser or natural processes, such as weathering of rocks or breakdown of organic materials, is available to plants. Furthermore, soluble P can easily react with clay particles, calcium and metals such as iron and aluminium in the soil and covert to less available inorganic forms through phosphorus fixation.
This process is also influenced by pH, so high (alkaline) or low (acidic) pH soils will be more likely to show low levels of available P, potentially putting crops at greater deficiency risk.
“If soil testing reveals a low P index, first check whether it could be due to pH. If soil is too acidic, liming will help by bringing soils closer to the optimum of pH 6.5-7,” says Mr Kerr.
If soil is genuinely deficient (i.e. index 0-1), growers should develop a long-term plan to build levels with regular additions of organic phosphate sources, such as sewage sludge, composts, FYM, or digestate, and not rely on artificial fertilisers, he adds.
Early root development is among many critical plant functions reliant on having adequate phosphate, so any deficiency will limit crops establishing strong foundations.
“The first 60 days after drilling is crucial, as wheat requires about 15% of total phosphate during this time to develop new roots,” explains Mr Kerr.
Establish
“The germ inside seed can only keep plants going for a week or so, therefore roots must establish quickly to take up what they need from soil.”
Starter fertilisers applied with seed at drilling could therefore make a difference, he says.
“Roots need to touch phosphate to take it in, so it must be close by, especially in the first 60 days when they can only explore a small area. Placed fertiliser is easily accessible by roots, so is very efficient.
“If an excellent seedbed can be created and you get sufficient moisture, then seedlings may be able to access all they need from soil, but that is never guaranteed, so starter fertiliser is a good risk management technique to overcome any potential shortfall during establishment and help manage in-field variability.”
Starter fertilisers are not intended to meet crop requirements for the entire season and should be seen as a way of setting crops up to efficiently ‘mine’ nutrients and water through the year, he adds. Unlike other nutrients, phosphate demand is fairly consistent through the season.
Synergy
Additionally, there is synergy between phosphate and plant utilisation of other nutrients, notably nitrogen, partly because more root mass improves nutrient scavenging ability, allowing plants to take up applied fertilisers more efficiently.
Phosphorus benefits to plants:
- Stronger establishment
- Boosts root development (particularly lateral and fibrous roots)
- Improved yield – used in flowering
- Even crop maturation – counters effects of excess N
- Straw strength – reduces lodging by strengthening cell walls
- Improved resilience to stress and disease
P deficiency risk factors:
- Acidic or very alkaline soils reduce P availability
- Low organic matter means less organic P for mineralisation into available forms
- Cold or wet conditions (poor aeration) reduce P absorption
- Poorly developed root systems cannot access soil P
- Low P reserves (below index 2)
- Fine-textured soils, such as clay loam, have a greater phosphorus-fixing capacity than sandy, coarse-textured soils
- Iron-rich soils are also more likely to lock up P
Starter fertilisers on test
Equipment costs and logistics make the decision to use starter fertilisers far from straightforward, however they are increasingly a part of many drilling routines.
Growers will rightly want to see strong agronomic reasons for making any investment, which is one reason Hutchinsons is trialling starter phosphate fertilisers to assess the benefits in wheat and other crops across different soils.
Many products use micro-granular technology which offers a much greater surface area – typically larger by a factor of 300 – compared to standard granular fertiliser, improving uptake efficiency. Slow-release forms are said to ensure available P can be used by crops rather than getting locked up in soil.
Tim says too much focus is sometimes placed on using water-soluble P which, although very plant-available, is likely to become unavailable soon after application.
Efficient
“Ironically, P fertilisers with less water-soluble phosphate could be more efficient at supplying P to the growing crop as they release P more slowly as the crop requires it.”
Hutchinsons trials are comparing several new starter fertilisers from recycled sources and established options, such as Primary-P.
One product is Crystal Green, a combination of phosphorus, nitrogen and magnesium, derived from waste water treatment. It has been used in the amenity sector for several years, but has not been cost-effective for agriculture, says Mr Kerr.
Another under investigation in cereals, maize and vegetable crops is re-granulated meat and bonemeal ash, a by-product from the meat industry. Although research is ongoing, initial results look very encouraging, he says.
Liquid starter fertilisers are another option to consider, possibly offering a slightly easier solution in terms of application equipment and logistics.
Growers are reminded many starter fertilisers contain a small amount of nitrogen, but authorities confirm microgranular products containing N can be applied to winter cereals in autumn, provided the rate is below 3.5 kg/ha.
In the field
Starter fertilisers are an important part of the strategy to address low phosphate on one Cambridgeshire County Council farm.
Phosphate indices on the deep-bodied, organic clay loam (10-12% organic matter) are typically one or below, yet detailed Healthy Soils analysis shows a relatively large soil P reserve that is unavailable to crops.
“The soil is naturally high in iron and aluminium which is locking up phosphate,” says Hutchinsons agronomist George Baxter.
“There’s an estimated five tonnes per hectare of P in the soil, with less than 0.1t/ha (100kg/ha) available.”
The farm started using Primary-P starter fertiliser on wheat, spring barley and peas, four years ago to kick-start early root growth.
“It’s much more efficient than broadcasting TSP fertiliser and we’ve seen a real benefit for establishment and crop growth,” says Mr Baxter.
This year, all 40ha of winter wheat and a similar area of spring barley received Primary-P, which is applied with seed using a Stocks applicator mounted on a Weaving GD drill.
George Baxter, agronomist, Cambridgeshire
Soil health
This is part of a wider strategy to improve soil nutrition and health. Five years ago the farm moved to shallower tillage, cultivating the top 5-7.5cm, and also uses multi-species catch and cover cropping.
“Stopping deep tillage was originally for black-grass control, but we’ve seen big improvements in soil biology, which is key to a healthy productive soil, and for converting phosphate into available forms,” says Mr Baxter.
Furthermore, catch crops before autumn drilling, and winter cover crops ahead of spring drilling provide an effective way of scavenging phosphate from the available pool at a time when no other crop is growing, preventing it becoming locked up. Both are in the ground for eight weeks before being terminated, when phosphate is then released slowly in plant-available organic form as residues breakdown.
“A healthy soil that works for us is vital to promote phosphate cycling, plus the many other benefits,” Mr Baxter adds.
TSP is still applied on a rotational basis, targeted at areas and crops where it is less likely to become locked up quickly.
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