LESSON 2a: WEATHER EFFECTS ON CROPS

SOIL MOISTURE EFFECTS ON CROP YIELD

This section considers soil water and its influence on crop stress and crop yield. The first section discussed mainly the variability of crop yield, especially the variability over time. The second section showed the direct effects of weather on crops, particularly monthly average weather, and reviewed how warm temperatures could reduce yield in July or August, and a slightly cooler than usual month could result in a contribution to yield for that month.

Soil water is the amount of water available to a crop from the soil. Not all water in the soil is crop-available water. Across Iowa, there is variability in the amount of water typically available to the crop. Measurements of crop-available moisture in the state of Iowa are taken in the month of November and also in April. The data from these measurements are summarized in Fig. 2.2.

Fig. 2.2 Average inches of water available in the 5-foot soil profile for plant growth (Shaw 1982).

In mid-April, central Iowa typically has 8 inches of plant-available water in the top 5 feet of soil. This assumes that crops have a 5-foot rooting depth and the water in the top 5 feet of soil is the water available to the crop. Some years, of course, both corn and soybeans may root deeper than 5 feet and, on occasion, the roots may not reach the 5-foot depth. But basically that is the rooting depth. Eight inches of plant-available water typically available to the crop provides about one-third of the total requirement for the growing season.

Across the state there is some variability in plant-available water. About 7 inches is the typical amount of water on April 15 in northwest Iowa and about 10 inches, or perhaps a little above, in east-central Iowa. The water-holding field capacity of the soil is about 10 inches. Some soils may have 11 inches of water-holding capacity, and some few close to 12 inches, but for the most part, a rule of thumb is that our soils hold 10 or 11 inches of plant-available water in the top 5 feet.

Water at the beginning of the year is somewhat of a crop insurance plan. The bank of water that is built up in the soils can carry a crop through a brief period of heat stress, or water stress, by providing moisture to the roots. Not all areas of the world have the excellent plant-available water reservoir that exists in Iowa.

Some years the season begins with less than half of the normal plant-available water; 4 or 5 inches of water available in the soil would not be uncommon in a drier year. During the '90s, however, it has been typical to have the plant-available water close to the field capacity, sometimes exceeding field capacity (that is, having water sitting in the fields or having water-logged conditions or saturated conditions as they are sometimes called).

Plants use water according to the withdrawal of the soil moisture by the roots and evaporation of that water from pores in the leaves. The water lost through the plant tissue is known as transpiration, and the water lost directly from the soil surface is evaporation. The combined total loss of water from the soil and the plant to the atmosphere is known as evapotranspiration.

Potential evapotranspiration is the amount of water a plant could be using according to the growth stage of the plant (Fig. 2.3). If there are no plants in the field, we assume that after an initial day or two following rain the crops would be using about 0.37 of the water that would evaporate from a puddle or from a pan. This introduces the concept of pan evaporation. The standard is a large pan of water in or near a field. The amount of water that evaporates from the National Weather Service Evaporation Pan is measured. The water that would evaporate from a bare field would be 0.37 of that, after the surface has had time to become something less than muddy.

Corn Evapotranspiration Factor

Fig. 2.3 Variation of corn evapotranspiration factor over the growing season (Shaw 1982). For example, on June 7 evapotranpiration from a corn field would be 40% of measured pan evaporation. On August 1 it would be 82%.

When the crop begins to get some size to it, about the 1st of July, the evapotranspiration would be more than half of the amount that is evaporating from a pan. So if the pan used 0.2 of an inch of water during a 24-hour period, about 0.1 of an inch of water is withdrawn from the soil by the crop and by the evaporation from the soil. The maximum amount of water drawn from the soil by corn comes somewhere around the 1st of August, just after tasseling and silking, and is about 0.82 of the pan evaporation. The corn crop would be using 82% of the water that would evaporate from a pond in an open field, or from a pan of water.

Study Question 2.1
If the amount of evaporation measured from a pan on June 22 was 0.33 in., how much water did the corn actually use?

With soybeans, it is a little bit different. A soybean plant uses water, similarly to a corn plant early in development. But at 60 days after planting, soybeans will be using the full amount of water that would evaporate from a pan and, in fact, peak out at about 1.02, or 1.05 of pan evaporation. A few percent, maybe up to 5% more water is potentially used by a soybean field than would evaporate from a pond of water (Fig. 2.4).< P>

Soybean Evapotranspiration Factor

Fig. 2.4 Variation of soybean evapotranspiration factor over the growing season.

The total amount of water used by a corn crop and a soybean crop would normally be the same. However, the soybean tends to use water a little bit later in the season than does the corn crop. For a full yield, the total water consumption is approximately the same. Some years the soybeans may actually use more water. Some years the corn may have a greater actual water use.

Records are kept of the amount of water that evaporates from a pan. For the state of Iowa in July the pan evaporation is something on the order of 7.75 to more than 8.5 inches of water evaporated for the month (Fig. 2.5). We can relate that to the amount of water that the crop could potentially use. As mentioned earlier, it amounts to near 24 inches of water that the crop would potentially use for its entire growing season.

May	June	July	August	September	October

Fig. 2.5 Month average pan evaporation for Iowa 1954-1980. Click on figure to display (Shaw 1982).

If the crop is somewhat short of water, that is, if the soil moisture is not at field capacity, or if for some reason the water is not completely available to the plant, then the amount of water used is less than the potential. The corn does not use 80% of the amount of water evaporated from the pan if the soil is drier than ideal.

We can use some rules of thumb on this potential water use. If we use a rule of thumb and say that corn at emergence would have a factor of about 0.3 of the pan evaporation, then about 0.6 would be the value on the 1st of July. From 15 July through silk, we would say the corn is using ideally about 0.75 of the water that would evaporate from a pan, and after pollination until the corn begins to mature and the leaves senesce, it would use about 0.83, until the end of the season (Table 2.1).

Table 2.1 Crop stage and relative water use (portion of pan evaporation)

Study Question 2.2
Assume a planting date of June 1 for soybeans. For the month of August, how much water would soybeans use?

This gives an estimate of what is going on in the field. Now consider more closely the composition of the field. If we make a diagram of how a field of soil may be composed, perhaps 5% organic matter, 45% of mineral materials, and water, 25% of the soil volume, and air about 25% of the soil volume (Fig. 2.6), we see that we do have soil particles and air spaces in a representative slice of the soil. This would be about 25% of the volume of a well-aerated soil (Fig. 2.6).

Fig. 2.6 Percentage composition by volume of a moist soil.

The water in the soil sometimes fills very small fissures in the soil particles, depending on the type of particles, but most of the water, at least that available to the plant, exists as a film on the soil particles (Fig. 2.7). A very thin film would be tightly held to a soil particle by physical bonding, often known as hydrogen bonding.

Fig. 2.7 Detailed structure of soil water adhering to the soil structure.

Further away from the particle and into the air space, we come closer and closer to free water conditions. A water-logged soil or a soil that is at saturation would have the air spaces completely filled with water. If the soil is well drained, over a short period of time gravity effects on the water would remove water until air spaces appeared again and left just a film of water on the soil particles. That film of water is the water that is available to the crop roots.

When the gravity-affected water has drained out of the field, we say the field is at field capacity. From then on, the only way to remove water from the soil (except at the very top few inches) is by plant roots. The roots withdraw the film of water, and there is no other way for it to leave the soil. In the top few inches the water can be withdrawn from the soil by direct heating. The sun and wind cause drying by evaporation because of heating and movement of air and wind at the soil surface. Down a few inches into the soil, roots are the mechanism by which the water is removed. Other water movement processes are seldom significant.