Increasing Soybean Yields

Soybean field on a sunny day

There are an estimated 275,000 different plant species on earth. Each contain thousands of unique chemical compounds, however, each individual plant with its own unique genetic can also produce their own unique variations to these compounds. A plant with one thousand plant chemicals can literally combine them a million different ways. When you add diversity to a plant and soil microbial community, you can get significant changes to both the soil and the plant response with only minute changes. The changes can be dramatic.

Many companies are now experimenting with using biologicals (microbes, plant extracts, etc) to stimulate plant growth and yield. At the National No-till on the Plains conference, Wichita Kansas, a researcher (Chris Teachout) described a process he was investigating to promote higher soybean yields. Chris was using a liquid compost worm extract that he applied directly to the soybean seed. This extract is extremely high in beneficial bacteria and some plant nutrients. He also added some kelp and fish emulsion. Sea kelp adds some micronutrients and the fish emulsion stimulates beneficial fungus. Chris no-tilled soybeans into a green cover of mostly cereal rye but he also had a small plot with hairy vetch mixed in with the cereal rye. Two different plant species add diversity to the microbial mix.

What impact did this mixture have on soybean yields? In the cereal rye alone plots, he got some good results. Soybean pods ranged from 100 to 200 pods per plant. Average yields were 80 to 120 bushel. On the cereal rye plus hairy vetch plots, pods counts were as high 1000 pods per plant where the hairy vetch was the thickest. It’s thought that the hairy vetch somehow stimulated higher pod counts by either adding more beneficial microbes, possibly turning on a soybean gene for higher yield, or a combination of both. More than likely, a combination of beneficial microbes with good fertility (especially micronutrients) stimulated the soybean plant to produce more pods. Soybean roots on those plants were completely full of soybean nodules which generally translates into higher pod counts and more soybeans. 

The soil environment also played a major factor. This soil was full of soil aggregates (good soil tilth) and was well aerated. Soils that are compacted or have poor soil structure limit not only water infiltration and water storage, they also limit gas exchange. Good soil structure allows atmospheric nitrogen and oxygen to get into the soil. Soybean nodules convert the atmospheric nitrogen into protein in the nodules with the helps of Rhizobium bacteria. Plant roots also use the oxygen for respiration, just like humans, to breakdown organic substances as a food source to accelerate plant growth and yield. Adequate soil oxygen is needed to decay the existing soil humus and to weather rocks to release plant nutrients. The optimal soil oxygen content needs to be between 15 to 25 percent. With too much oxygen, the soil microbes burn up SOM and many nutrients can be lost.

Some plant nutrients are extremely mobile in plant cells. Plant mobile nutrients like nitrogen, phosphorus, potassium (N-P-K) can move easily from roots to stem, leaves and pods. Other nutrients like magnesium and micronutrients like molybdenum and sodium are also quite mobile. When plants have a deficiency of mobile nutrients, generally the deficiency symptoms show up in the older leaves. As a plant ages, often nutrients are mobilized to the newer growth, so deficiency signs show up first in mobile plant nutrients in the older leaves.

Many nutrients, however, are relatively immobile in plants. Deficiency signs generally show up first in the newer leaves. Elements like calcium, which is part of the cell wall structure, and boron which is needed to move calcium into the plant often are common plant deficiencies. Boron is a micronutrient that helps move plant sugars in the plant, is often lacking, and can easily be supplemented to improve plant health. Without adequate plant sugar, the plant roots have difficulty feeding the soil microbes which supply many other nutrients.

Other plant immobile elements include iron, manganese, zinc, and copper. All these elements are needed to activate certain plant enzymes. Iron is often lacking in cold wet soils in early spring. Iron is used to activate the enzyme that makes chlorophyll for photosynthesis and also nitrogen synthesis. Manganese is needed to increase photosynthesis and to split the water molecule (H+, OH-) to make larger plant molecules. Zinc activates 300 enzymes that improve plant growth. Copper is needed for nitrogen synthesis and to increase stalk strength. Keeping a soil healthy increases the availability of plant nutrients and optimizes plant growth and yield.