1) Planting clovers on soils with a pH below 6.0. You might see some initial germination but your growth will stunt and you won't see a vigorous thick stand.
2) Not using enough fertilizer. So many people just put an initial application of fertilizer down but they overlook the important early fall application of potassium that is needed. A huge amount of potassium is mined from the soil by legumes. A good indicator of nutrient use of forages is looking at the nutrient content of those forages. Clovers and alfalfa run 2-3% potassium. You need 200-300 units of potassium every year on your legumes.
3) planting UNINOCULATED clovers. How many of you know if the legumes in the seed your buying is inoculated. When i ask this question in person, i typically get the same kind of look from people. Ask your supplier, don't assume. You need all of your legumes to be inoculated with the CORRECT inoculant.
4) Not having enough PURE LIVE SEED spread or drilled on the plot. Know the species your planting and know the correct seeding rate. If your broadcasting instead of drilling, you want 20-30% higher seeding rate than if your drilling. If your using coated seeds, look to see if 35-50% of the seed is coated. I always tell people to plant at least 20% higher if your using coated seeds.
5) Not planting a companion crop. In the mass builder mix i sell, i use berseem clover. Berseem is fast growing and helps keep weeds at bay. Also the deer prefer it over many forages and they will consume berseem first, allowing the slower growing clovers time to establish. Some people use oats or pea/oat blend as a nurse crop.
6) Poor seed to soil contact. I always talk about a first hand account of this out on Grandpa Ray's farm. My uncle didn't beat a rain and didn't roll some of an experimental plot for me. I got 1/2 it done myself and there was very little weed pressure and a more vigorous growth on the rolled plot. The plot unrolled had much slower germination and as a result, weed issues that i had to deal with.
7) Not overseeding every year. Most farmers in the grazing community know the value of frost seeding or spinning seed every year to fill in the thinning spots in a field. You might only need 1-3 lbs of clover seed per acre every year to overseed. You might have a clover plot last 10-15 years if you continuously do all you can to keep a plot thick.
8)Not clipping. Clipping your plots right before your weeds produce seed heads, will reduce new weed growth in the future. Very few graziers spray for weeds and there is no reason to have to spray for grasses if you follow all the steps i have posted above.
9) Poor preplant prep work. You want to spray with glyphosate using ammonium sulfate. Spray with 1.5-2 quarts per acre instead of 1. Some grasses like quack grass will not have effective kill at the standard level. You want to spray when the weeds are actively growing and at a decent air temperature. Then work the ground and you might need to roll the soil before broadcasting the seed. Then rolling a second time. If you bury the seed at depths of 1/2 or more, you will get slow germination or poor germination, which will lead to weeds getting ahead of the legumes.
10) Germination count and weed seed counts on the tag of seed you buy. So many people don't look close at tags. If your buying seed with a low germination count, you won't have a thick stand. If the seed contains a higher amount of weeds, your going to have more weeds to deal with in your plots. There are very few growers in the US and each grower will have various lots of seed and there is a discount for seed that isn't as high of quality. I am not one of those companies that buy up poor quality seed.
My 2 cents...
Simple....Do not put any nitrogen fertilize on it. Nitrogen makes things grow. Legumes make their own nitrogen. More nitrogen equals more weeds and grasses.
Yes, you would get some nitrogen down but it's the sulfur benefit you will see a huge response from.
Second point, not all fertilizer suppliers people out there buy from have fertilizer with no nitrogen. 5-20-20 or 8-13-28 might be all they have that has lower nitrogen. It won't harm you. You might need to mix a 5-20-20 with 0-0-60 if that meets what your soil test says, but that low amount of units of nitrogen applied will have no harm.
Yes, you really shouldn't feed the weeds with high amounts of nitrogen and potassium is your biggest focus on the legumes.
My soybeans have been lacking recently. The first two seasons they grew well but not the last two. Should I suspect a lack of postassium?
Elk....
Even if people put down fertilizer, there are issues that come into play as to the response to any fertilizer. Soil organic matter levels, heavy rain falls, soil microbial activities. But the biggest issue people have is low soil pH. This is why people need your pH to be around 6.5. If people plant soybeans on ground that is 5.0 pH you will only get around 50% of the use of the potassium fertilizer you put down. If your pH is 5.5 that number is slighly over 72%.
Even if people soil sample your fields, an issue can also be how people sample your fields. A sample is only as good as the one taken.