LESSON 1a: WEATHER EFFECTS ON CROP YIELDS

SOIL EFFECTS ON CROP YIELD

When temperature is high and soil moisture is low, we sometimes talk about heat stress of the crop. In a strict sense, heat stress is a very rare thing. Water stress is common. Water stress can be brought on by increased crop needs for water (or high atmospheric demands for water because of high temperatures). So temperature can change the demand for water, and the crop in Iowa and across the Corn Belt is usually responding to water stress rather than high temperature stresses. Consider the stress of limited moisture and the effect of moisture stress on the crop.

Begin with the seed, or with shelled corn and its equilibrium moisture with a temperature of 70^o F (21.1^oC) and a relative humidity of 50% (Table 1.2). These are not atypical values for the Midwest in the springtime. The corn just laying around in natural air at this temperature (70^o F) would come to equilibrium at about 11 or 11.5% moisture. So that is as far as the corn would dry down in those conditions, or if it is initially dryer than that, it would become increasingly moist to there.

Table 1.2 Percent moisture context for shelled corn at various temperatures and humidities.

At a humidity of 70% and 70^o F air temperature, the corn's moisture would be somewhere around 14%. It does not take a whole lot of increase in humidity to begin to increase the moisture in the corn by a considerable amount. However, even at 90% humidity, the corn is only at about 19% equilibrium moisture.

The percent of moisture in corn just when it becomes mature and before it has begun dry down is a key value. Experience would tell people it is on the order of 25-35%. Obviously, then, that would be near the amount of moisture required in the seed for it to germinate and to grow.

So even though the relative humidity may be around 90% and the temperature from 50-70^o F, there will not be enough moisture in that seed for it to germinate, emerge, and have growth. Therefore, we must assume that the humidity in the soil, if the crop is to germinate, is something above 90%. And indeed it is. Soils have to be very, very dry for the humidity in the soil to drop below 90%. When the plants have withdrawn all of the water from the soil available by the action of its roots, the soil is crumbly and dry. The relative humidity in the soil would still be at 98-99%. This is enough, then, for the seed to swell, prepare to germinate, and to emerge.

The "wilting point moisture content" of the soil is considered to be at -15 bars for agricultural crops. The relative humidity (RH) in soil at the wilting point can be approximated by the equation:

as given by Salisbury and Ross (1969, pp. 37, 66).

T is temperature in ^oK. Relative humidity in the soil is surprisingly high even when plant-available soil moisture is fully exhausted. If moisture is discernable by touch or by appearance, the soil contains at least half of its moisture capacity.

Plants are seldom in soil that is too dry for the plant to grow, to emerge, and for the things to happen that are crucial in the spring to get the plant on its way out of the ground. We always assume that there will be enough moisture for seeds to swell, but not necessarily enough moisture for seeds to grow and emerge.

One of the problems often seen when we plant in a dry spring is just enough moisture in the soil to initiate germination and emergence, but not enough to continue growth. Considerable damage from drought right at or just after planting time is occasionally reported.

Assuming that there is sufficient moisture in the soil for plants to germinate and emerge and begin their early growth, we are concerned only with temperature. The temperature effect on emergence of corn and soybeans is dramatic. The primary soil temperature or the temperature at which corn and soybeans will emerge most rapidly is near 80^o F (26.7^o C). If the temperature is 50^o F (10^o C) or less, emergence will be very slow or not occur at all (Figure 1-9). We refer to 50^o F as a base temperature, that is the temperature below which we have no development of corn or of soybean. This is the base temperature for the corn and soybean varieties that are grown in the Midwest, in the Corn Belt area. There are some varieties of corn that will grow at temperatures considerably below 50^o F, and some that the base temperature is something above 50^o F. But for most of the crop grown in the Midwest, 50^o F is the base temperature.

Fig. 1.9 Effect of soil temperature on emergence of corn and soybeans. Emergence is slow at temperatures near 50°F. Emergence is rapid near 90°F.

If the soil temperature is averaging 50^o to 55^o F (10-12.8^o C) at the time of planting, it may take 3 weeks for corn to emerge. If the temperature is averaging 60^o F (15.6^o C), it may take 10 days to 12 days. If the temperature is averaging 70^o F (21.1^o C), 5 days, and, of course, at optimal temperatures for the soil, which we almost never have at planting time in Iowa (we would hope we are planted well before that, but it may apply in other parts of the United States, particularly the South), the crop may emerge in 3-4 days with soil temperatures somewhere near 80^o F (26.7^o C).

Soybeans have about the same optimum temperature, but for the most part we like to see temperatures of the soil near 60^o F at planting time for soybeans. Soybeans will emerge with vigorous growth if temperatures are somewhat above the base temperature.

Study Question 1.7
What would be more correct base temperature than 50^oF for soybeans?

Study Question 1.8
How long would it take soybeans to emerge if temperatures averaged 70^oF?

Study Question 1.9
How much longer would it take soybeans to emerge, compared to corn, at a temperature of 65^oF?

Consider what would be the normal temperature of the soil as we go into the season. The best planting date in Iowa is considered to be the first part of May. During the first week in May, the central Iowa average soil temperature, measured at the 4-inch depth, is 60^o F. This is suitable for soybean and very good for corn (Figure 1.10). Occasionally, the soil temperatures may be 55^o F and rarely in the low 40s. The warmest at this date in central Iowa is in the upper 70s. Normally, on the first of May, the soil temperature in central Iowa is somewhere between 55^o F and 68^o F. Soils will warm to their peak in mid-July, near 80^o F. Soils in Iowa are seldom, if ever, too warm for the crop. By the time we get into the heat of summer, we usually have enough crop cover such that the soils do not heat up above that which is ideal. Click on the image below to enlarge.

Fig.1.10 Annual 4" soil temperature mean and distribution for Ames, IA (Elford and Shaw 1960).

Temperature is a very important factor in determining when the plant will emerge and when it will go through its growth stages. There is a big difference between development of the plant and growth of the plant. The chart of the effect of temperature and monthly moisture on the "growth" of the crop is not necessarily associated with the "development" of the crop (Figure 1.7). In the next section we will evaluate the effect of temperature on crop development. The growing degree day is used to integrate the influences of temperature and time on plant development.

During early stages of development, soil temperature has dramatic effects. Some controlled soil temperature studies in the field were conducted in Illinois (Bellero, Bullock and Hollinger, 1996). Greenhouse work has shown dramatic differences in plant development between heated air and under-the-bench radiant heating. Cold soil and warm air may result in abnormal coloration of young corn.