Three key factors in drought-proofing your fields

Tuesday, August 5, 2014

Improving water infiltration, retention and utilization should give you an extra week or two of freedom from moisture stress when dry weather occurs

by KEITH REID


Not every field suffers to the same degree when there is dry weather. This may be due to differences between crop varieties, but far more depends on how the soil has been treated.  With proper management, you can make your fields more "drought-proof."  The three key factors are better infiltration, retention and utilization.

Improving infiltration. Whether you irrigate or depend on rainfall, surface water runoff does not help growing crops. You want to create conditions that allow this water to soak into the soil, and that means maintaining a stable soil structure. Avoid too much cultivation, which breaks down the aggregates into powder and packs the soil surface. This can effectively seal the soil surface as soon as it starts to rain, so water cannot soak in. A diverse crop rotation, including forages or cover crops, and regular additions of organic matter, will help to stabilize soil structure.

Improving retention. Once the water is in the soil, you want to keep it there. In our climate, this is always a balancing act between getting the soil dry enough to plant in the spring, but not having it dry out completely. It may help to remember that plants cannot use water from saturated soil, so we are trying to get the water to drain freely from the larger pores (also allowing air into the soil), while retaining water in the smaller pores.  

The first step to retaining soil moisture is to reduce evaporation from the soil surface. Keeping the residue from the previous crop on the surface will aid in this, particularly until the crop canopy is established. Avoiding unnecessary tillage is also beneficial, since each time moist soil is brought to the surface, it will quickly dry out.

The second step is to increase the capacity of the soil to hold moisture. In fine-textured soils, this means creating soil structure with the right balance of large and small pores. Adding organic matter will help, but it's even better to grow plants with fine, fibrous root systems to bind the soil aggregates together. Coarse-textured soils will benefit directly from any additions of organic matter, since it is in these soils that the function of organic matter as a sponge really comes into play.

Improving utilization. More water in the soil is not going to be helpful if the plants cannot reach it. Getting more of the water back out of the soil and into the crop is a matter of increasing both the depth and the density of root growth. Soil compaction is the enemy of vigorous root growth, so keep implement weight as low as practical and minimize the number of trips over the field.

This also means that "early planting" is not a date on the calendar, but rather the first day that the soil is dry enough to work, since cultivating moist soils is extremely harmful to soil structure. Tillage pans are particularly damaging, as they stop roots from reaching water (and nutrients) in the subsoil.

Tile drainage will also increase the volume of soil that roots can access, both by reducing the susceptibility of the soil to compaction and by increasing the depth of unsaturated soil, so roots have a greater volume of soil they can utilize.

These techniques will not make a difference to your crop yields every year, but they should give you an extra week or two of freedom from moisture stress when dry weather does occur. If this period coincides with pollination or grain fill, the improvement in yields can be significant. This can be a double benefit, as those are often the years when weather scares mean that grain prices are highest. BF

Keith Reid is Soil Scientist, Agriculture and Agri-Food Canada, Guelph.