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Effects of Crop Rotation on Soil Moisture Availability and Nutrient Supply

By Dale Risula, PAg, Provincial Specialist, Special Crops; and,
Dunling Wang, PhD, PAg, Provincial Specialist, Alternative Cropping Systems, Regina

November 2020

The most influential factors for crop production in Saskatchewan are soil moisture and soil nutrients. They are often the determining factors for crop rotations. Conversely, crop sequencing impacts soil water availability and nutrient supply. Therefore, growers can use crop rotations to manage and improve water and nutrient use efficiencies.

In the semi-arid regions of Saskatchewan, crop production depends highly on available water in the root zone. Rotations must consider spring soil moisture conditions and the anticipated rainfall during the growing season as well. The precipitation in the growing season impacts crop yields more than spring soil moisture reserves. However, spring moisture conditions can be improved by conservation techniques while the precipitation cannot be changed unless you have irrigation capabilities.

Spring soil moisture reserves can be improved through managing snow distribution in the field and preventing soil moisture loss from drying winds and excessive runoff. Saskatchewan producers often leave a stubble on the field to trap snow in winter, reduce surface runoff and evaporation in spring. Generally, stubble that is six to nine inches tall can conserve half to one-inch more water over winter, compared to a field that was cultivated in the fall. This additional one-inch of water can result in 2.4 to 9.1 bushels per acre (bu/ac) more yield depending on crop variety and soil climate zone (Table 1). Furthermore, with non-tilling and direct seeding, the stubble remains standing after the new crop is sown, soil moisture loss is further reduced in the early growing season, which is crucial for crop germination and early development.

Table 1. Average moisture use efficiency (bu./ac./in.) for major crops in Saskatchewan

Soil Climate Zone Canada Western Red Spring Wheat Barley Canola Oats
Dry Brown 3.50 5.30 2.40 6.50
Brown 3.75 5.70 2.60 7.10
Dark Brown 4.00 6.20 2.80 7.75
Moist Dark Brown 4.12 6.20 3.00 7.90
Black 4.25 6.40 3.20 8.20
Moist Black 4.50 6.70 3.40 8.70
Gray 4.75 7.20 3.60 9.10

Source: Soil Climate Zones of the Canadian Prairies, Enviro Test Laboratories.

Crop rooting depth affects soil water availability and water use efficiency. Deep rooting crops often access soil moisture at lower layers while the shallow rooting crops only use the available water in the upper layer. Generally, alfalfa, safflower and sunflower roots are deeper than barley, canola, oriental mustard and wheat and in turn, the roots are deeper than field pea, flax and lentil. For each crop, rooting depth is influenced by several factors, such as soil moisture depth, precipitation amount and frequency, nutrient availability and soil temperature.

Growers can optimize water use by rotating between deep and shallow-rooted crops (Figure. 1). Shallow-rooted crops appear to better follow deep-rooted crops because water recharge is likely to occur near the soil surface and a shallow-rooted crop will utilize this moisture very efficiently. Medium or deep-rooted crops following shallow-rooted crops can take advantage of any moisture left at depth that wasn’t used previously to develop deep roots to access soil moisture at lower layers.

Unrestricted effective rooting depth of selected mature crops.
Unrestricted effective rooting depth
of selected mature crops.

It should be pointed out that some crops are more sensitive to soil moisture than others. Agriculture and Agri-Food Canada’s (AAFC) studies in Swift Current and Assiniboia have shown that sunflower and safflower yields are almost same on fallow and stubble. Field pea and lentil on stubble can achieve about 80 to 90 per cent of the yields on fallow. In contrast, wheat yields on stubble were only two-thirds to three-fourths of that on fallow. Mustard yields on stubble was less than two-thirds. Safflower or sunflower are excellent selections for stubble seeding, while wheat and mustard should be grown on fallow.

Most Saskatchewan producers usually focus on crop needs of nitrogen (N), phosphorus, sulphur and occasionally, potassium. In some soils, micronutrient availability can be extremely low and directly affect crop production.

Growing a pulse crop can reduce the need for N fertilizer. Pulses obtain it from the atmosphere through a process called biological N fixation. This can provide 50 to 90 per cent of pulse crop total N requirement, which leads to significant savings in fertilizer purchases. Additionally, crops following pulses also require less N fertilizer. The pulse residue contains high amounts of it in the chaff, straw and root material. The residue breaks down quickly returning N to the soil where it becomes available to the following crops. Pulse crops also contribute to succeeding crops by increasing productivity as crop rotation benefits, which is related to development of Arbuscular Mycorrhizal Fungi (AMF). For example, AAFC’s study in Swift Current indicated that pea crops affect the structure of the AMF community associated with the roots of the following wheat crop, resulting in increased crop yield.

Complementary cropping practice also help improve nutrient use among different crops. This is where they have different needs for, or sensitivity to, the deficiency of particular nutrient elements. One of the benefits of crop rotation between deep-rooted and shallow-rooted crops is that deep-rooted crops scavenge the nutrients, in particular the micronutrients, from lower layers to meet their growth need. These exist in the crop residues that return to the soil and then become available for the following crops. Another practice is to rotate between high and low nutrient demand crops in order to slow the process of nutrient deficiency. For example, when soil copper levels are marginal, a wheat–pea–oat–flax rotation is preferred over a wheat–canola–barley–flax rotation, because pea and flax have a low sensitivity with low levels of copper.

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