Top Crop Summit
Top Crop Summit speaker series: 4R phosphorus fertilizer management on the Prairies
By Bruce Barker
Dr. Don Flaten recently retired from the University of Manitoba, where he was a professor in soil fertility, crop nutrition and nutrient management. Don discussed best management practices for phosphorus fertilization at the Top Crop Summit Feb. 22.
A historical review of nitrogen (N), phosphorus (P), potassium (K) and sulphur (S) management was conducted in 1993 at the University of Saskatchewan, and commonly became known as the Red Book. It is archived at Canadian Agronomist at canadianagronomist.ca/resource/the-red-book/, where much good information can still be found.
The sequel to the Red Book covering P management was completed in 2019 by Dr. Cindy Grant and Flaten. It is titled ‘4R Management of Phosphorus Fertilizer in the Northern Great Plains: A Review of the Scientific Literature’ and can be found at Fertilizer Canada’s website fertilizercanada.ca/news-events/news/4r-management-of-phosphorus-fertilizer-in-the-northern-great-plains-a-review-of-the-scientific-literature/.
The 2019 review covered 4R principles and practices of right rate, source, placement, and timing, as well as the role of P in crop nutrition, P behaviour in the soil and environmental and sustainability concerns.
Between 1993 and 2019, many changes in cropping systems occurred, such as reduced tillage, movement toward diversified rotations and higher-yielding cultivars. Currently at play, high crop prices increase rewards for high yields and increase penalties for under-fertilizing. Conversely, high fertilizer prices increase the cost of fertilization and penalties for over-fertilization. These increased risks highlight the importance of using 4R nutrient management principles.
Recent research by the former International Plant Nutrition Institute in 2015 found 59 per cent of fields in Alberta were deficient in P, 64 per cent in Manitoba and 81 per cent in Saskatchewan. Flaten summarized several research projects as they related to rate, crop response to fertilizer P, and management strategies for short-term sufficiency and long-term sustainability.
Environmentally, small amounts of P loss can cause large problems with water quality. Most P loss from cropland to the environment is dissolved P during snowmelt. A research study by Liu et al (2021) found that P loss from cropland in Manitoba is linearly related to Olsen soil test P concentrations. Therefore, it’s important to avoid excessive accumulations of P, such as in situations where livestock manure is applied too heavily or frequently on the same fields.
Avoiding excess P depletion or accumulation over the long term is a cornerstone of right rate. As mentioned, surpluses can increase risk of P loss and water quality problems. However, deficits can reduce P fertility and long term productivity. To minimize both types of problems, farmers should aim for Olsen soil test P levels of 10 to 20 ppm.
The overall strategy for long term sustainable P management uses three approaches, depending on soil test P values. If soil test P is low, build levels in cereal years, with side- or mid-row band applications, or with manure. Consider a maintenance strategy of applying P fertilizer to match crop removal when target soil test P levels are attained. If soil test P exceeds target levels, add only low rates of starter P.
Monoammonium phosphate (MAP; 11-52-0) is the most common source of P fertilizer. It is a granular formulation, easy to handle and relatively inexpensive. Research has found that MAP is equally as effective as APP (10-34-0) liquid fertilizer on the Prairies, and that there was no difference between orthophosphate and polyphosphate fertilizers.
Struvite is another P fertilizer source and is recovered from municipal wastewater plants. It represents a step toward sustainable use of recycled P. It is safer to apply in the seedrow than MAP, but often takes longer to become plant available resulting in lower yield response in the year of application.
Phosphorus nutrition is particularly important for the first three to six weeks of spring plant growth. Additionally, P movement is limited in the soil, and typically moves only one inch after application. As a result, adequate P needs to be placed near or in the seedrow, especially in cold soils when root growth is slow.
Almost all P fertilizer is banded under the soil surface in or near the seedrow on the Prairies. This is both agronomically and environmentally beneficial. Research by Wagar et al (1986) found that broadcast fertilizer P cannot fully compensate for the benefits of starter P in the seedrow.
Excessive seed-placed P can cause seedling damage to canola and other sensitive crops.
Availability of P can be delayed when P fertilizer is banded with high rates of N in either a side- or mid-row band. The high fertilizer band toxicity can delay P uptake by several weeks, so some starter P is recommended in or near the seedrow.
Overall, 4R P management strategies can be combined in a cohesive combination suited to the crop, economics and environment.
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