Assessing Soybean Plant Stands

When was the last time you accurately evaluated your soybean plant stand? It is a fact that all soybean fields look thin in the spring. Solid seeded soybean fields seeded at the currently recommended 200,000 plants/acres often look so thin that new growers sometimes question if there are enough plants to warrant investing more money into the crop and spraying a herbicide. It is important for future planning decisions that all farmers both experienced and first time growers determine how close their emerged plant stand is to the population that was targeted when seed was ordered mere weeks ago. The method used to measure soybean plant stands depends on whether it was seeded with a row crop planter or air seeder.

Assessing Soybeans Grown in Rows (15 to 36 inches apart)

The easiest way to determine soybean population is to count the number of plants in 1/1000 of an acre. Table 1 indicates how many feet of row equals 1/1000 of an acre at different row widths. If soybeans are grown in 15 inch rows, the number of soybean plants in 34 feet and 10 inches of row is counted. For uniform fields do this count in at least 6 different parts of the field. If the counts are 180, 169, 163, 175, 150 and 162 the average count is 166.5 and the average plant population is 166,500 (166.5 x 1000) plants per acre. For variable fields more counts are needed.    
Table 1: Row Lengths Equal to 1/1000 of an Acre at Different Row Widths
Row Width Length of Row
36 inches 14 feet, 6 inches
30 inches 17 feet, 5 inches
22 inches 23 feet, 9 inches
15 inches 34 feet, 10 inches

Assessing Soybeans Grown in Narrow Rows (< 12 inches) with an Air Seeder

For solid seeded soybeans, the best way to estimate plant population is to count the number of plants in an area rather than individual row lengths. The easiest way to do this is to “borrow” one of your children’s (or grandchildren’s) hula hoops. Gently toss or roll the hula hoop in the field and count the number of plants inside. As in the previous example, toss or roll the hula hoop in at least 6 spots in an even field. If using a 34 inch hula hoop and you count 38, 32, 17, 24, 28 and 25 soybean plants, the average count is 27.3 and according to Table 2 the average plant population is 188,700 (27.3 x 6,912) plants per acre. Again for variable fields more counts will be needed to accurately estimate the correct plant population.   
Table 2: Multiplication Factors to Determine Soybean Populations Using Different Sizes of Hula Hoops
Hula Hoop Diameter 30 inches 32 inches 34 inches 36 inches
Multiplication Factor 8,878 7,808 6,912 6,165

Replant Decisions

Determining soybean plant populations is usually only done when farmers are considering replanting. While the reasons for replanting can vary from poor overall field emergence (anyone seed in mid April?), to portions of fields lost from hail or excess moisture, getting an accurate determination of living plants is the first step needed to make an informed decision. In addition to the number of surviving plants, before making the decision to replant it is important to evaluate:
  • if the remaining plants are healthy and equal in maturity
  • if the remaining plants are spaced out fairly evenly or are there big gaps in the field
  • will replanting provide an economic gain or are you better off to keep the existing stand?
  • is there enough time left in the planting window to enable the reseeded crop to mature?

Evaluate your Seeding Operation for Next Year

Another reason to walk fields to determine soybean plant population is to evaluate how well you and your seeder worked this spring. It doesn’t matter whether you take digital pictures of the field or just make mental notes to yourself – the important thing is to walk your field and to see how uniform the plants are distributed.
A wide range of different sized plants could be indicative that some of the seed did not end up in the bottom of the furrow. Seeding too fast could have caused variable seed depth placement due to bouncing. If you seed with wide sweeps, maybe some of the seed placed near the tips of the sweep were seeded shallower. In normal years, the plants that developed from these shallower placed seeds are likely smaller because they sat in drier soil and took longer to germinate than seeds placed at the proper depth.
Fields with patterns of large bare patches accompanied with spots where plants are bunched tight together in the row could indicate that your seeder wasn’t delivering seeds uniformly within the row. This is a common complaint from farmers using air seeders because these types of seeders don’t accurately place individual seeds in the ground like row crop planters. Not all air seeders are created equal - some do a better job of spreading out the seed within the row than others.
Plants that are too close together often produce fewer pods per plant than plants that are properly spaced apart. I have seen many situations where three plants are within an inch of each other. One plant is normal sized with 30 plus pods while the other two plants are roughly the same height but only have 8 to 10 pods per plant. Large gaps are not good either. Nearby plants can compensate for low populations within your field by producing more branches and more pods per plant but if these extra branches come from the bottom of the plant and grow out just above the ground, a lot of the seeds from these extra branches are below the cutter bar and never make it into the combine.
When seed is cheap, having two or three plants tight together isn’t a big issue, but as seed costs continue to increase it pays to spend some time investigating the uniformity of your field. While it doesn’t make sense to buy a new seeder just to plant soybeans, maybe it is worth spending some time this winter trying to get your air seeder do a better job of seeding your soybeans next spring.