Mastering phosphorus management

UNDERSTANDNIG WHAT’S IN THE SOIL

As most farmers know, plants need phosphorus very early in the growth cycle. Phosphorus is essential for all operations, including membrane development and cell division. While there might be an adequate supply of phosphorus in the soil, particularly if there is a buildup from previous applications, its availability is heavily dependent on environmental conditions. When plants are first developing, soils are often cold and wet – a time when the supply of P can be reduced. Early in the season, roots are underdeveloped, which can even further restrict the plants ability to access necessary nutrients.

“Under cold soil conditions, you may need to supply some starter to get the plant going, and to make sure that there’s P there where the plant can access it,” says Cynthia Grant of Agriculture and Agri-Food Canada.  

One problem is that phosphorus is not very mobile. Depending on soil characteristics, it ties up with calcium and magnesium or with iron. The advantages to that are that it normally won’t leach under most soil conditions. But that means that the roots have to intercept the phosphorus because it won’t move to the roots. Farmers will generally see a large response to phosphorus applications in situations where soil solution supply is restricted by weather conditions, a low supply or the inability of the plant to access those nutrients because of poor rooting conditions.

“Low soil reserves, cold soil temperatures, excessively wet or dry soils, high or low pH, fine soil texture – anything that restricts rooting or a lack of mycorrhizal fungi in the soil – can all increase your likelihood of seeing your plant yield response to starter phosphorus,” says Grant. “Again, this depends on the type of crop you’re looking at because different crops differ in their ability to take up soil and fertilizer phosphorus.”

Flax, for example, has a relatively poor ability to take up fertilizer phosphorus. It tends to rely more strongly on phosphorus that’s in the soil and less strongly on fertilizer phosphorus. Canola, on the other hand, is effective at feeding from both fertilizer and soil phosphorus.

“It can use what’s in the soil very well because it modifies its rhizosphere [the zone surrounding the roots of the plant],” says Grant. “But it can also take up the fertilizer very well because it can increase the rooting in the fertilizer reaction zone and really mine the fertilizer from the soil as well. And cereals are sort of in the middle. They’re pretty good at using both, but they’re not the star that canola is and they’re not as poor as flax.”

Interestingly, mycorrhizal associations can improve phosphorus availability making mycorrhizae important to agricultural systems. Mycorrhizae are an association between plant roots and a fungus, and they’re especially important for flax and corn, but don’t occur with canola or sugar beets. The fungal network in the soil increases the area that can be mined for nutrients. The fungi give the nutrients to the plant, the plant gives the sugars to the fungus – it’s the perfect relationship. Mycorrhizae, though, tend to be reduced by tillage and summer fallow, phosphorus fertilization, and by following a non-mycorrhizal crop like canola or sugar beet.

“In studies that we did with flax, we found that our flax seed yield is substantially higher after wheat and after canola, although tillage-effect was inconsistent, so this may reflect an effect on mycorrhizal colonization,” says Grant. “It’s something you should keep in mind when you’re growing those mycorrhizaelly-sensitive crops.”

APPLYING STARTER P
In the Prairies, farmers tend to use banded applications because banding slows the tie-up of phosphorus in soil. Bands should be placed where roots can contact them early in the season – seed-placed or side-banded. Again, the success of these applications depends on environmental conditions, so banding phosphorus near the seed is much more important with low soil phosphorus levels. If you have a well-manured soil, this may not be quite as important.

“And anything that restricts rooting – compaction, tillage pans, and cool soil conditions, because cool soil conditions restrict solubility, mobility and rooting,” says Grant. “If you’re seeding early into cold soil on relatively low phosphorus soils, that’s when you can see the big response to a starter application.” •