Soybeans in the field wet and cold By Bill Casady

The year 2004 will be remembered as a year when moisture was both a blessing and a curse. Timely rains produced an especially abundant corn crop, leaving little room to store everything we grew. Then the rains kept coming and there were still crops in the field. Some of the remaining crops were “mudded out,” leaving fields bruised and abraded with scars that will return to haunt future crop canopies, ironically and especially, when there’s too little moisture during the growing season.

The soybean is a relatively hygroscopic seed, capable of drying and rewetting quickly with the weather. It’s not uncommon for soybean moisture contents to move several points in a day. It’s easy to start harvesting at the correct moisture content in the morning and to be harvesting a very dry soybean crop at the end of the day. Now that the cold has settled in, though, soybean moisture content in the field will remain relatively high. Equilibrium moisture content for the soybean under cool wet fall and winter conditions tends to remain well above 13 percent.

Table 1. Allowable storage time for soybeans before significant spoilage occurs (days).
Temp	Soybean moisture content
	14	16	18	20	22
40	230	140	75	40	12
45	175	95	40	19	9
50	134	65	23	12	7
55	100	40	15	9	4
60	76	27	11	7	3
70	41	14	6	3	2
80	25	8	2	1	0

Recent reports of soybean moisture contents above 18 percent are cause for concern. When the time is right, these crops need to come out of the field, but they’re going to need to be dried and stored with extra care. Threshing damage will be high above 18 percent. Likewise, the risk of spoilage is high when moisture content is above 18 percent (Table 1). For example, spoilage will occur within a week for stored soybeans at 22 percent moisture content and 50 degrees. The cold helps improve storage, but not enough to protect the beans from an inevitable attack from storage molds.

Drying must be done with extreme care to avoid both overdrying soybeans at the bottom of the bin and initiating conditions at the top of the bin that will promote spoilage. As is always the case, the top layer of seed will not dry until the bottom layers are dry. As air moves through the lower layers, it picks up moisture until it becomes saturated, which, of course, is known as relative humidity of 100 percent. This saturated air continues through the upper layers with no ability to cause drying.

Heat improves the ability of the air to hold moisture, so drying occurs more quickly, but it doesn’t take much heat to overdry soybeans at the bottom of the bin. Research has shown that exposing soybeans to relative humidity values of less than 40 percent can cause excessive splitting. As a rule of thumb, a temperature rise of no more than about 3 to 5 degrees will reduce relative humidity enough to dry soybeans efficiently without serious overdrying.

Table 2. Equilibrium moisture content for soybeans.
Temp	Relative humidity (%)
	50	60	70	80	90
32	10.0	11.8	13.7	16.7	19.8
40	9.8	11.5	13.5	16.0	19.6
50	9.5	11.2	13.2	15.7	19.4
60	9.2	11.0	13.0	15.4	19.1
70	8.9	10.7	12.7	15.2	18.9
80	8.6	10.4	12.5	15.0	18.7

Table 2 provides equilibrium moisture contents for soybeans under various conditions. For example, suppose the air temperature is 40 degrees with 80 percent relative humidity. For these conditions, the air will not be able to reduce moisture content below 16 percent.

A 5-degree rise in temperature will increase the temperature to 45 degrees, of course, and with the use of a psychrometric chart, we can predict that relative humidity will drop to 65 percent. At these conditions, we can estimate from Table 2 that soybean equilibrium moisture content would be approximately 12 percent, so drying can occur. Again, with the help of a psychrometric chart, we would predict that saturated air would leave the top layer at about 40 degrees. Using Table 1, we can predict that we have as much as 40 days to dry the top layer before it spoils. The take-home message is that it doesn’t take much heat to dry, or even to overdry, a soybean.

Meanwhile, with a temperature rise of only 5 degrees (to 45 degrees), we haven’t increased the temperature so much that the top layer is in immediate danger of spoiling. But, suppose we increase the air temperature to 80 degrees. With the help of a psychrometric chart, we would predict that saturated air would leave the top layer at about 57 degrees. Again, using Table 1 we would predict that the soybeans at the top would have only about 1 week before spoilage occurs, so be careful with supplemental heat.

The key to safely drying and storing a late and wet soybean crop is to use plenty of air. It doesn’t hurt anything to have as much as 2 or 3 cubic feet per minute per bushel to quickly dry the crop with little or no heat at all. Translated, this means dry shallow layers. Once a shallow layer of about 4 feet is dry, either move it to permanent storage somewhere else or add another layer on top. The second 4-foot layer will cut back on the amount of air that can flow through the bin, but at least it will begin to dry immediately because the layer below is dry.

Grain drying calculations are not straightforward. Each situation is unique, and it is difficult if not almost impossible to enumerate all the possible conditions that could be encountered. But as a rule of thumb for soybeans, use no more than about 5 degrees on a typical day and absolutely no more than about 20 degrees supplemental heat on a rainy day when the relative humidity is 100 percent to safely dry soybeans without severe overdrying and risk of spoilage.

Bill Casady, Agricultural Engineering
(573) 882-4370