Planting soybean into sod, 2011

Chad Lee and Jim Herbek, University of Kentucky
email: Chad Lee or Jim Herbek

Planting soybean into sod is an option for producers looking to increase acres.  Soybean could generate a gross return of  $500 per acre, or more, depending on yield and marketing. Soybean is also a good option for producers who need to renovate pasture and hayfields. Some producers have a lot of experience with soybean while others may be looking at the crop for the first time. The following guidelines attempt to be applicable to both groups of producers.

Consider the following when converting sod to soybean:

1. Inoculation is a must. Any field that has been out of soybeans for more than three years should be inoculated with Bradyrhizobium japonicum. The Bradyrhizobium are living organisms that will form nodules on the soybean roots and provide elemental nitrogen to the soybean plant. Soybeans require over 300 pounds of nitrogen per acre, so getting proper inoculation is absolutely critical. Liquid formulations may be preferred in this case simply because they have a better chance of getting good coverage of each seed. Since the risk of failure can be very expensive, use the highest rate of inoculant.
2. Scout fields when soybeans are have about three to four trifoliate leaves. Nodulation should occur by then. If nodulation has not occurred, there are some options for rescue treatments.  None of the rescue treatments are as cheap as the inoculant.
3. The vast majority of hay and pasture fields are potassium (K) deficient. Hay removes a lot of K₂O from the soil, while pasture fields tend to remove a little less. Without even conducting a soil test, odds are very good that the sod will need about 60 pounds of K₂O per acre.
4. Soil test now, or as soon as the soil allows. A soil test represents 20 acres and will cost about $5 to $10 per sample. The fertilizer bill will be well over $1000 for those same 20 acres. A soil test will tell you how much fertilizer you need, making sure you are spending money where it is needed.  Pull about 10 to 20 cores, each 4 inches deep, for a 20-acre area. Mix all of the cores together and from that mix, send in a sample for testing.
5. Apply potassium and phosphorus fertilizer according to the soil test recommendations. These applications can be made anytime before planting.
6. It’s too late for lime this year. If your soil pH comes back low, you can apply agriculture lime, but it will not help much until the 2012 growing season. If you could have applied lime last fall, that would have been ideal. Pelletized lime is marketed as reacting faster with the soil, but it will not react fast enough to help with this season. Save your money and stay with commercial agriculture lime. If your pH is really low (5.5 or less), lower your expectations for yield. Unless you receive an inch of rain every week during the growing season, yields will be reduced.   If soil pH is below 6.2, molybdenum deficiency can occur.  You can apply sodium molybdate as a seed treatment; however, this can greatly lower the numbers of live rhizobia for seed that is inoculated, so the seed should be planted immediately.  A better option for inoculated seed would be to broadcast the molybdenum on the soil. In AGR-1: Lime and Nutrient Recommendations, the suggestion is to “dissolve one pound of sodium molybdenate (6.4 oz of molybdenum) in 20 to 40 gallons of water and spray uniformly ahead of final seedbed preparation. Not more than 2 pounds of sodium molybdenate (13 oz molybdenum) per acre should be used in any five-year period.” In reality, no one is going to apply 40 gallons of water to the acre. Some of you will try to cheat and get away with 10. If you do, make sure that the sodium molybdenate will stay in solution.
7. Control the weeds early and stay aggressive. If at all possible, burndown the sod before you plant soybean. Ideally, soybean should be planted into “brown” remnants of weeds. Either gramoxone or glyphosate are good options. Gramoxone tends to work a little better than glyphosate at cooler temperatures.  Glyphosate is preferred if a high percentage of the grass is orchardgrass. Don’t be surprised if it takes two passes to completely kill the perennial plants in the sod. 
8.  Try to stay no-till if possible. Water-holding capacity is maximized with no-tillage.
9. Whether no-tillage or tillage is used, there may be low areas in the field that are suitable for sod waterways. Keep those areas of the sod. They will help slow surface run-off and reduce erosion.
10.  Test for compaction. Many hay and pasture fields have some surface compaction. The next time the fields are saturated with water, walk them with a penetrometer to test for compaction.  (Most county extension offices have a penetrometer.) If that compaction is 3 to 4 inches deep, then you may need to do some surface tillage to break up the compaction. A field cultivator or chisel plow is the preferred tillage tool, if tillage is necessary. If the compaction is an inch or less, most no-till planters with sufficient weight can break through that compaction.
11. Plant a slightly higher seeding rate. Planting into sod, means planting into grubs, wireworms, voles, field mice and other critters. Expect a little more seedling loss. A final stand of 100,000 plants per acre is sufficient for maximum soybean yields in most scenarios.  However, a final stand 70% of the original seeding rate is a reasonable expectation for sod. An initial seeding rate of 150,000 seeds per acre, or more is a good starting point.
12. Place seeds about 1.5 inches deep. Get the seeding depth correct. You will pay for it greatly if you do not. If you are using a no-till drill, take some time to make sure seeding depth is correct. Planters generally do a better job of seed placement than drills, but both must be set properly.
13. Soybean in row widths of 15 inches or less usually yields a little better than soybean in 30-inch rows. However, soybean in 30-inch rows can produce acceptable yields. If the planter will do a better job of cutting through residue and placing the seed, then use the planter. 
14.  Select a good soybean variety. Based on the University of Kentucky trials, there is a huge swing in yield potential from commercial varieties. Selecting a variety with a good track record in the state improves your odds of having a good variety and getting good yields. Never ask the seed salesman to “Give me whatever you have.” Do your homework.
15. Consider using a seed insecticide. Seed treatments may help some with control of grubs and wireworms. If the season is cool and cloudy during germination, a foliar insecticide applied to young plants (1 to 3 fully emerged leaves) could be warranted. The cool, cloudy conditions slow crop growth and favor insect damage. Insect damage is typically more severe in fields with a lot of residue. Bright, sunny conditions favor quicker growth of the young plants and insect damage is less severe most of the time.
16. Spreading about one bushel of cracked corn per acre is a method for slowing down damage from voles, field mice and other varmints. The animals will eat the cracked corn, first, and are less likely to dig up seeds. There are some baits that can be spread on field borders.
17. Assuming that the burndown application worked well, there will be some perennial and annual weeds that need to be controlled when the soybean is emerging. In general, when weeds emerge with the soybean crop, they should be sprayed before they reach 6 inches tall. In warm conditions, weeds can grow very quickly. A weed that was 3 inches today may be 6 inches in only a couple of days. Finding small weeds in residue requires walking some fields. Small weeds are difficult to see from the windshield of a pick-up.
18. Scout the fields on a regular basis to identify problems quickly. In some cases, quick recognition of a problem might allow you time to resolve it.

These are general guidelines and specific fields might require additional management. For more information on soybean production, consult your local county extension agent.