AGRONOMY

The same question always arises this time of year for a summerfallow wheat grower. Is there enough moisture to seed? We all would like to seed into moisture and have the crop come up in 5-7 days. A standard comment is, “we sure could use an inch of rain over a couple of days”.
The Pendleton Experiment Station has summarized their weather records in an attempt to answer this question. It is written up in the 2006 Dryland Annual Report 1068, page 78-85. I draw on this research for this column.
We look for that optimum seeding date; that optimum day when there is adequate seed-zone moisture for germination following summerfallow. Seed placed in dry soil will not germinate. Soil moisture must be at least 11 percent by weight for the seed to be able to imbibe water and to begin the germination process. Soil moisture at 13 percent results in more consistent and rapid germination resulting in a more uniform stand.
The soil surface following fallow is air dry and there is insufficient moisture for seed to germinate. The depth of the “dust mulch” layer on the soil surface varies depending on the depth of rod-weeding in a tillage system. It is usually from 3 to 5 inches deep. The soil in the dust mulch has about 4 percent moisture by weight. The depth to moist soil in a chem. fallow system is more variable both from field to field and within a given field but typically ranges from 6 to 9 inches.
The amount of moisture needed to wet the dry soil to the minimum moisture content necessary to bring about germination is determined by the moisture content of the air dry soil, the minimum moisture content necessary for seed germination, the depth to which the soil must be moistened and the weather conditions prior to and immediately following the rainfall.
Pendleton Experiment Station has studied how much moisture it takes to wet the dust mulch to 13 percent. They assumed that tillage had “fluffed up” the dust mulch layer so bulk density is less than normal. Based on these assumptions, a minimum of 0.45 inches of moisture is required to wet the dust mulch layer from 4 to 13 percent. In chem. fallow the same amount of moisture will wet deeper where the soil capillaries have not been severed by tillage “fluffing”.
We must note that the amount of rainfall need to get the above amounts of moisture into the soil is greater than the 0.45 because of evaporative losses. Conditions that are cool, overcast and the wind is calm make rainfall more efficient. There are weather records showing cases where the potential evaporative loss the day after a rain exceeded the amount of the rainfall.
The objective of this work was to estimate the likelihood of receiving sufficient rainfall to bring about germination. They selected September and October data. They broke data into three rainfall amounts. They used 0.4-0.6, 0.6-0.8, and more than .08. inches of rainfall. They then determined how often each amount fell within a three time frames. These are a 3 day period, a 5 day or a 7 day period.
They found in September, 0.8 inches of rainfall within three days occurred 9 times in 77 years. 0.8 inches of rain fell within 7 days in 17 years. This record pretty much eliminates doing the easy way out, seeding in September and not worrying about putting it into moisture.
Pendleton Experiment Station determined rainfall of 0.6-0.8 over 3 days is adequate to wet the soil surface and bring about good germination. They also determined 0.4-0.6 over 7 days was unlikely to be sufficient in most weather conditions.
I personally feel every one week delay of emergence during October results in a five bushel reduction in yield. The yield reduction with later emergence is because plants accumulate fewer growing degree days before they mature, and they mature later in the following summer when they are more likely to experience water stress.
The conclusion is: during September there is almost never sufficient rainfall to wet the soil surface enough to cause acceptable germination. We must in our systems control the moisture line so we can seed into moisture during September and into October.