Many farmers across Western Canada have been struggling with wetter than normal conditions over the past few years. This spring many will again be facing soils that are close to the saturation point and puzzling over how to deal with them. Some farmers may turn to tillage to help dry them out, or at the very least to smooth our ruts. While it might get them back on the fields faster, they could be inadvertently setting their newly planted crops back by making nutrients harder to access.
Tillage causes physical disruption to soil organisms, especially larger ones like earthworms, and those that have large networks like fungi. “Tillage can physically destroy the networks of hyphae (long, thread like filaments) that fungi create, and for the larger bodied organisms like earthworms tillage could cause death due to crushing, removal of their tunnels and burrows, and so on,” says Dr. Mario Tenuta, professor of applied ecology at the University of Manitoba. “The other thing that tillage does is redistribute materials, and that’s one of the reasons why farmers do till, to mix in surface crop residue below the surface. Once residue is in contact with the soil, it’s also in contact with active soil organisms, which can speed up decomposition of the material.”
Disrupting fungal networks limits P
But the most critical issue from a plant productivity and crop production standpoint is the effect of tillage on the networks of mycorrhizae fungi, which relay nutrients and compounds to plants. “Primarily for crop plants, the network transfers phosphorus to plants, which allows them to photosynthesize for better yields and feed the fungi to keep that network surviving and active,” says Tenuta.
“Tillage breaks the network, which has to be completely re-synthesized and that requires a re-deployment of the whole network,” says Tenuta. “The reconstruction process will take energy from the fungus, and that takes energy away from the process of acquiring phosphorus and translocating it to the plant, because the fungi are trying to repair itself instead. It’s a loss of efficiency for the fungus and the plant.”
Because commonly, farmers till in the fall after harvest and/or the spring before the crop is planted, there is a delay in re-establishing the fungal network. “The early season uptake of nutrients for a crop can be compromised because of that,” says Tenuta. “In that period of time where it’s getting rebuilt, it’s not as efficient in transferring the phosphorus and that can be a big thing for a plant or a crop because it’s often the early season uptake of phosphorus that is really critical for plant establishment and growth.”
Farmers can’t compensate by simply putting on more phosphorus fertilizer. “Farmers can do a couple of things, they can increase soil test phosphorus, or the background level of phosphorus that’s available for plants, but researchers have found that even if they do that, some crops highly dependent on mycorrhizal fungi such as flax, are still compromised for early treatment P uptake,” says Tenuta.
“To get over that negative hit on the mycorrhizal fungus, and its inability to help with phosphorus acquisition, it’s necessary to add phosphorus in and/or near the seed row in a band, not just broadcasting it, so that the phosphorus is more available to the emerging seedling in the early spring. It makes things more complicated, and more input intensive and expensive in management of the phosphorus.”
As well, there is a limit to the amount of phosphorus that can safely be placed with the seed in the furrow without causing seedling injury. The safe amount of in-row phosphorus is well documented and available on provincial websites. Safe levels depend on the soil type, the crop, the source of phosphorus and moisture. Tenuta recommends farmers band phosphorus to the side of the seed-row if unsure of safe in-row limits.
Wet soils tempt tillage
The good news is that the fungal networks do rebuild themselves within a year, but not necessarily fast enough to make a different to the new crop. “If a farmer has done a fall and spring tillage, then plants a crop like wheat or corn, if we compared it to a no-till crop, early on in the season there would be more presence of fungi in the roots of the plants in the no-till situation,” says Tenuta.
“If we went back later in the growing season, colonization of the roots by the fungi would be about the same. They will rebuild the network but it’s delayed, and eventually it’ll catch up and establish with the crop. However, it may not be as efficient later in the season. A lot of our crops need phosphorus earlier on in the season so that’s the problem. If it catches up later on it might be too late.”
Rarely do no-till farmers use tillage unless they have a major issue that is hampering their ability to achieve their yield potential. But do the costs in terms of losing the benefits provided by soil organisms outweigh the benefit of a tillage operation in these cases?
“Often there is a problem related to establishing the crop, in getting a stand emergence that’s decent to get good yields, so if there is something that is near catastrophic happening in their situation, that’s compromising their yield, they are playing a balancing act,” says Tenuta. “That balancing act is, they may compromise their early phosphorus acquisition by doing tillage, but they may not get a decent crop unless they do till. They may be able to compensate with good phosphorus management but it becomes a pragmatic decision for the grower in terms of doing tillage.”
If producers really want to try and get moisture levels down in their soil and decide to till, they need to think carefully about their phosphorus needs, says Tenuta, especially in crops that are very sensitive to early-season phosphorus that require mycorrhizae. “If they have a crop like flax, corn or sunflower that requires early-season phosphorus and are very mycorrhizal dependent, I would really recommend them to think about making sure they have good phosphorus as a band when they plant,” he adds.