With the high cost of fertilizer, some farmers are asking if they can cut back on phosphorus. Although possible in the short term without too much noticeable effect, especially if they have been applying phosphorus (P) regularly year after year, in the long term the cumulative effect will be a drop in productivity of the soil.
“There have been lots of long term experiments where P hasn’t been applied, and P reserves in the soil just keep getting drawn down and the productivity of those soils diminishes,” says Doug Penney, a senior agri-coach with Agri-Trend Agrology. “The reduction in productivity seems to be moderate for a while and then all of a sudden it starts to really go downhill.”
In fact, the changes in cropping rotations and practices over the pasty 25 years have contributed to a problem of low P content in most soils across the Prairies. Crops such as canola and peas, which have been added to traditional cereal rotations, tend to remove more P from the soil than is being added in the spring. An average wheat crop typically removes 20 to 30 pounds of P per acre, whereas a similar-yielding canola crop removes 30 to 50 pounds per acre.
“For wheat a typical application rate is 25 pounds per acre of P, which is very close to removal,” says Penney. “Whereas if you look at canola, the rate of application would be about the same or sometimes a bit less than they would put on for wheat. Then the rate of removal for a typical yield is quite a bit higher than what had been applied. So over time that’s going to have some influence.”
Phosphorus plays a major role in plant growth, particularly in relation to the energy systems within the plant. Its main effect is to get plants off to a quicker start in the spring and encourage more vigorous growth in the seedling stage.
A number of factors contribute to P availability and uptake. A tendency to plant earlier into cooler soils, and to seed into crop residue can reduce P availability at the crucial starter stage. And the use of one fertilizer blend at seeding may help reduce operating costs, but it can reduce the amount of P available, particularly when large amounts of nitrogen are present in the blend.
“We used to put just P in the seed row by itself and the other fertilizers on as a separate operation and the effectiveness of the P in the seed row was greater than if we look at it in one blend with all the other fertilizers,” says Penney.
“And you can’t put all of that fertilizer close to the seed because you’ll get injury, so there are some systems that split that blend and put 25 or 30 per cent of it in the seed row and the rest somewhere else to avoid fertilizer injury. If you have 25 pounds per acre P in that blend and you are only using 25 to 30 per cent in the seed row — close to the seed where it’s most effective — that is a very low rate of phosphate. And it’s in there with a lot of other fertilizers so the availability is much reduced.”
MANURE USERS OFTEN HAVE HIGH P RESERVES
Thanks to the increase cost of P and nitrogen (N) fertilizer, there is growing interest in natural sources such as manure. “Manure is an excellent source of P. If you apply rates of manure to apply enough N in the soil, you are applying two or three times as much P as you need, so most soils that have received manure over the years have high levels of P in them, so that’s a huge benefit now,” says Penney.
However, with new regulations governing manure application, additional solutions will need to be explored.
Although other products are available, such as seed primers, which put a small coating of phosphate on seeds, Penney believes they do not in themselves offer a permanent fix. “They can be useful tools for improving crop yields but in the long term it doesn’t address the real issue,” he says. “It stimulates the plant to extract P from the soil where the P availability is relatively low, so that’s an advantage for that crop in that year, but it doesn’t replace the draw down of phosphate.”
MOST P IS NEAR THE SURFACE
Soil sampling work performed by Penney and other Agri-Trend experts has uncovered a trend that he believes may require a fresh look at fertilizer management if the viability of soils is to be maintained for the years ahead. “A number of our consultants sample soils by one inch increments, which has shown that both P and potassium (K) are highly stratified in a lot of fields,” says Penney. “We’ve been finding it in an awful lot of fields where you have really high concentrations in the top one or two inches and then diminishing with depth.”
Penney cites the example of the U. S. Midwest corn crop, where prior to the popularity of zero till systems, P and K fertilizers were applied in the fall and ploughed to a depth of six or eight inches. Whilst he certainly doesn’t advocate abandoning zero till, which has well-known benefits in terms of conservation and erosion issues, he does believe that ways have to be found to deep-place these nutrients cost-effectively.
And in order to tell farmers how to do that, there has to be much more research into the potential effect that this trend will eventually have on crop yields, which is something that is not currently being addressed by the scientific community. “The current situation just raises a number of questions that there aren’t clear cut answers for,” says Penney.
Penney believes that soil productivity will once again become the driving force behind agronomic research in the years ahead. “Agronomic research relative to productivity has been sadly neglected for at least 25 years because producing enough food wasn’t an issue,” he says. “Crop prices were low, productivity was high relative to consumption and production of food crops was never going to be an issue again in terms of food shortages. And of course, there’s been a reawakening all of a sudden.”
Angela Lovell writes from Manitou, Man.