By: The OMAFRA Field Crop Team
Dry conditions and insufficient rainfall such as much of the province has experienced this year can make for a stressful season. Rather than casting an envious eye at weather reports from rain-rich regions, we should instead ensure that our own crops have full access to the moisture that is available throughout the soil profile, and that any rainfall we do receive makes it down to crop roots where it is needed. In other words, increasing or maintaining water infiltration, and maximizing root access to soil water by facilitating root growth and exploration.
The first step in providing crops with adequate moisture is to capture as much as possible by allowing it to infiltrate into the soil. The critical zone here is right at the soil surface.The same soil, in the same field, but with clearly different surface structure. There was no crop residue at the first site, allowing raindrops to pound surface aggregates into individual particles which then filled surface pores, creating a crust as they dried. Soil cover – dead or alive – protects the soil from crusting and keeps it open for water infiltration and air exchange. Residue cover feeds soil life that make this possible by continuously aggregates soil particles, as well as deep-burrowing earthworms whose burrows help channel water deeper into the soil. Aim to keep at least 30% of the field covered after planting to achieve these benefits.
Root access to soil water
Moving down from the surface, the critical factors are how much plant-available water the soil can hold, and how well crop roots can access it. Both are related to the porosity of the soil, as well as interrelated factors such as texture
two soils with the same problem. Compaction in the 5-30cm depth range has created a layer of high density and corresponding low porosity. These photos were taken in early July when dry conditions were starting to show consequences. In both cases compaction caused by tillage and traffic created a barrier to roots getting to deeper layers of the profile that contained plenty of moisture.
In soils with higher clay content the compacted layers can become very hard and impede root growth more easily. Heavier soils also hold on to water more tightly, and as compaction reduces the number of larger pores only small pores are left that hold water too tightly for roots to take up.
Increased impedance also changes the way roots grow. They become thicker and can appear flattened as they try to force their way through dense soil layers (fig. 7). In these compacted layers, the proportion of fine roots decreases, further reducing the ability of plants to access water in small pores. The root water uptake rate can be over 60% lower in a heavily compacted layer than a moderately compacted layer. Root growth will be mostly limited to above this layer, exactly where moisture runs out first.
Having cereals in rotation provides the best opportunity to make amends. Once cereals have been harvested, there is plenty of time to plant a cover crop whichcan go a long way to fixing the problem while also providing nutrient credits. If the problem is with crusting at the surface, focus on covers with plenty of fine, fibrous roots (such as grasses and cereals), but make sure to add a legume, as these will provide biomass that is more available to soil microbes responsible for aggregation in addition to possibly providing nitrogen for the following crop. Adding organic amendments at the same time can supply some or all of the phosphorus and potassium needs of the next crop, as well as increasing organic matter levels in the surface to improve infiltration.
If the problem is compaction, the goal is to pierce channels through this layer that will serve as conduits for water and air, which in turn will allow roots to access the subsoil, its moisture and nutrients. Radishes are an example of a cover crop that has seen steady increase in popularity for this purpose, though it is not the only one. In addition to creating large and deep “biopores”, research in Maryland by Dr. Ray Weil indicates that deep-rooted cover crops including forage radish can access subsoil nutrients including nitrogen and phosphorus that are then potentially available for the subsequent crop. However, Ontario research has shown that some of these are lost during freeze-thaw cycles, and investigations are ongoing as to whether the addition of a winter-hardy cereal such as rye can mitigate this.
Lastly, some situations require mechanical methods (i.e tillage) to break up dense compacted soil layers completely. These situations would benefit greatly by getting roots growing as soon as possible - while limiting recompaction from traffic - to stabilize this new structure. Best would be subsoiling a summer cover crop after planting, using disks in front of the shanks to minimize surface disturbance. If you do insist on pulling out the deep ripper to handle your compaction problems, I will insist that you please refer to the excellent “Guide to successful subsoiling” prepared by the CETAB+ that will help you make sure you are actually achieving your goals and not making matters worse.Source : fieldcropnews