Soybean plant stand establishment
Establishing a uniform, healthy soybean plant stand which closes the canopy quickly is important to maximizing yield potential. Although a perfect plant stand is not as essential for soybeans as it is for corn; higher soybean yields are often associated with more uniform stands. Attaining a good plant stand is more difficult for soybeans than other crops because the seedling must push the cotyledon leaves through the soil surface.
Each year considerable soybean acreage must be reseeded in Ontario due to poor plant stands. Although there are numerous factors that influence plant stand establishment like soil type, disease pressure, tillage practice, seed quality etc., the three important factors; planting depth, soil temperature and seed treatments are discussed in more detail below.
As water evaporates from the surface the depth at which moist soil can be found will get deeper the longer it does not rain. Due to the high water demand of soybean seed to germinate, it’s recommended to plant soybeans at least ½ inch into moist soil. This often means a total planting depth of at least 1.5 inches in early spring. Planting into the dry soil near the surface can destroy the seed as it will start to germinate but then run out of water before it can emerge. Some growers prefer to plant soybeans 2 inches or deeper even if moisture is present at a shallower depth. This idea comes from the well-known fact that corn must be seeded at a good depth to achieve proper root structure. The same concept is then transferred to soybeans. Placing the seed deeper also results in less temperature variability since soil temperature at 2 inches is less influenced by cold air than soil closer to the surface. In the spring of 2020, planting depth studies in Ontario showed that the best stands were achieved from planting less than 2 inches if adequate moisture is present. Deeper planting resulted in lower plant stands, likely because the seed ran out of energy before it could emerge. (See Table 1 below)
There is no evidence that deeper planting depth is necessary for proper root development in soybeans, therefore a planting depth of about 1.5 inches is recommended for Ontario conditions.
A 1.5 inch depth strikes a balance between placing the seed into moisture and not planting it so deep that it runs out of energy before it can emerge. It must be noted that planting ½ inch into moisture is always recommended so planting deeper than 1.5 inches in total may be necessary as soils dry out in late spring or early summer.
Old agronomy textbooks state that soybeans should not be seeded unless soil temperatures are above 10 Celsius. The difficulty with waiting for soils to warm is that rains may result in delayed planting which reduces yield potential. In the spring early planting trial of 2020, soybeans were planted at various soil temperatures on 5 dates. Soil temperatures went as low as 3 Celsius at planting and dropped to minus 3 Celsius within 12 hours after planting. The results are clear – soil temperature had little impact on final plant stands. (See Table 2 below)
The success of these plant stands shows that cold temperature alone does not destroy soybean seed vigour. It is usually a combination of factors such as soil crusting, insects and diseases, or improper planting depth that causes poor stands.
So, can soybeans be planted into cold soils if the ground is fit? The answer is yes! If soil conditions are dry enough for planting, it is not necessary to wait for warmer soil temperatures. OMAFRA has conducted early planting experiments for over 12 years and the benefits of early planting (even into cold soils) outweighs the risks of slow emergence. Newer varieties treated with a seed treatment are more resilient to the effects of cold soils than varieties seeded in the past. Long term experiments have shown a 4 bu/acre advantage to planting relatively early compared to a late May or June planting date.
Soybean seed treatments are an effective way to increase plant stands and improve yields. They are an important tool in establishing a uniform plant stand especially in no-till, clay soils or early planted fields. Good plant stands and yield response are often dependent on the weather conditions following planting and the level of disease and insect pressure. For more details on specific pests and control measures, see OMAFRA Publication 812, Field Crop Protection Guide.
Establishing a healthy uniform soybean stand can be a challenge especially if the weather does not co-operate but the following can help to improve final plant stands –
- Use a fungicide or fungicide/insecticide seed treatment
- Plant at a depth of 1.5 inches unless soil moisture is limiting
- Adequate down pressure on the drill or planter must be achieved to attain good seed-to-soil contact especially in no-till. If heavy corn residue is present spring vertical tillage will improve plant stands.
- Minimize soybeans in the crop rotation. Healthy soils with good soil structure are much less prone to crusting.