Moisture Stress Biggest Limiting Factor in Crop Production

July 11, 2005

COLUMBUS, Ohio — The biggest element in the world of agricultural production that makes or breaks a crop is water, and potential rains from Tropical Depression Dennis may bring much needed relief from the abnormally dry conditions Ohio is currently experiencing.

While most areas throughout Ohio are near normal with regards to soil moisture, some areas are two to three three inches below normal in rainfall, and spotty heavy rain showers have done little to alleviate crop stress.

Ohio State University Extension's Agronomic Crops Team has re-launched its drought Web site, which was created in 2002 — the last time the state was in a drought situation. The site can be accessed by logging on to http://corn.osu.edu/drought02/.

As it stands now, remnants from Tropical Depression Dennis are heading northwest, dumping heavy rains in Tennessee, Kentucky and parts of Indiana and Illinois. The storm is expected to stall over the Ohio Valley and Ohio may see some of that rainfall later in the week.

"The biggest risk factor for crop production in terms of yield in Ohio is moisture stress," said Brian Slater, an Ohio State University natural resources researcher with the Ohio Agricultural Research and Development Center. "We've had some good rainfalls, but those plants are soon going to be desperate for more water to keep things moving. The ideal situation for maximum growth is when it rains sufficiently for the soil to stay moist enough to meet the crop's evapo-transpiration demand (the amount of water being lost from the leaves of the plant)."

Soil is intricately structured and the amount of water that is available for a plant's use is dependent partly upon the characteristics of the soil and how much water it can hold at any given time.

Slater equates soil to a bucket with defined layers of water availability: oven dried, wilting point, field capacity and saturation.

There is always a layer at the bottom of the bucket between oven dried and wilting point that holds "unavailable" water. No matter how much the bucket is filled, plants are unable to access water in this area.

As the bucket fills, water takes up a layer known as field capacity. This is the maximum amount of water that is available for plants to use and is largely dependent upon the soil texture, whether clay, sandy, loam or a combination of the three.

Clay soils are made up of fine particles that hold onto water very strongly, but make it difficult for plants to access. The large surface area of the soil particles and fine pores hold onto water very tightly, resulting in less available water capacity for plants. Clay soils can hold roughly over 4 inches of water per foot, but due to their texture, less than 1.5 inches of water per foot may be accessible by the plant.

Sandy soils also don't hold on to water very well. They are made up of very large particles that tend to drain water quickly. Sandy soils, at best, can hold about 2 inches of water per foot, and a little over an inch of that is available to plants.

Slater said the ideal soils are medium textured, such as silt loam, a mixture of fine and large particles that offers a range of water holding capacity. Silt loam can hold as much as 5 inches of water per foot, with half of that being available to plants.

"Ohio is fortunate in that medium textured soils are common in the state, and many Ohio soils have high water holding capacity," said Slater.

Available water consists of a lower limit, which marks the wilting point (the point in which the plant starts to die) and an upper limit, field capacity, the amount of water in the soil after a period of drainage. Saturation occurs when the soil pores are filled and no more water can be absorbed into the soil. The excess water then either acts as run-off or "ponds" on the soil, or drains through the soil, also becoming unavailable for plant use.

"Generally speaking 70 percent of a plant's roots are located in the top 20 inches of the soil. In situations of excess rainfall, saturation actually suffocates the roots and prevents them from getting the water they need," said Slater. "Saturation is more evident after heavy rain where such things as compaction, crusting and low infiltration rates restrict water transmission."

Corn is one Ohio crop that can suffer greatly from lack of available water, especially during its most critical stage of development: pollination and grain fill. The crop can lose up to 50 percent of its yields due to stunted development during this time period.

Average water use of a corn crop during pollination and grain fill is about 1/3 inch per day. So, for example, with a water storage capacity of 1.8 inches per foot, a fully charged silt clay loam soil might carry corn with a three foot rooting depth up to 18 days during silking and early grain fill stages.

However, corn development this year may not be taking advantage of such a best-case scenario. Consistent springtime soil moisture failed to promote root growth to its fullest water-exploring capacity.

"With moisture constantly being available to plants, there was little incentive for the roots to grow outwards and downwards to find more water. The result is a compact root system, which in subsequent dry conditions is the worst possible situation we can have," said Slater. "In sunny, hot and windy situations, the rate the plant can get water to the roots is slower than the plants can transpire (or transmit moisture through the leaves). This is when plants begin to wilt."

Cloudy, cool days are more advantageous to the plant under such environmental conditions, as transpiration occurs more slowly, so that the soil can more easily supply sufficient moisture to the plant roots.

Author(s): 
Candace Pollock
Source(s): 
Brian Slater