Soybean plants that grow poorly or produce poor quality seed represent potential problems. Measuring quality of soybean plants and seeds is essential.
Sometimes the reason for poor quality soybeans is a low rate of photosynthesis, possibly due to cloudy, cool weather — or the opposite extreme, heat and drought. Nutrient deficiencies may contribute. Unfavorable soil conditions — waterlogging and poor aeration — can also disrupt normal plant functions.
Healthy plants will produce more nutritious food and seeds with more protein and oil. High quality seed will have a higher test weight.
Tools for measuring soybean quality
A handy tool to help you monitor the health of your growing plants is a refractometer. This is a precision instrument which quickly measures the percent sugars in a plant’s sap, while you are standing in the field. In general, the higher the sugar content, the higher the protein and oil content, since sugars are later turned into protein and oil. Fairly inexpensive, refractometers are routinely used in the food industry, by canneries, wineries and breweries, for example, to measure the quality of the fruits and vegetables they buy from the farmer or of the foods and drinks they manufacture.
Using a refractometer is easy. Just squeeze a drop or two of juice from the stem or leaves of the plant onto the glass prism of the refractometer, close the “lid” and look through the eyepiece. The sugar content is read on a numbered scale in units called brix (same as percent).
By comparing with standard readings or with past readings you have made, you can see how your crops measure up that day. Be careful to always take a reading from the same part of a plant (upper mature leaves and stems are good) each time and at the same time of day. The sugar content will vary in different parts of the plant, at different times and in different weather conditions (higher on warm, sunny days in the afternoon). Sometimes a sick plant will have a high sugar reading in the early morning, whereas a healthy plant should not.
Standard refractometer readings for soybeans and peas:
Find out what’s wrong
You can’t really tell what is going on in your fields while driving by at 50 miles per hour. You need to stop and walk through your fields and carefully observe what is happening. Look for telltale signs of insect damage or mineral deficiency in your plants. Check their sugar content with a refractometer. Are there isolated problem areas in a field?
Most important, check your soil. Carry a shovel and occasionally dig down and turn up a shovelful of soil. Is it loose and friable, or tight and hard? Is there a hardpan six inches under? Are there earthworms, and fungi in last year’s residues? Does the soil have that good rich smell of humus? Is there a weed problem?
Dig up the root system of a plant. Are the roots a healthy white color or discolored with decaying spots? Are there plenty of nitrogen-fixing nodules on the upper roots? Are the nodules a healthy pink inside?
These are all things you should notice and questions you should be able to answer about your fields. If you spot a problem, figure out its cause and work to overcome it. The basic place to begin is the soil.
Source: How to Grow Super Soybeans by Dr. Harold Willis