Treat Water as an Input

01:04AM Aug 27, 2014
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View water supply in a new way to guide your systems approach to crop production

You can have the healthiest soil on the planet, perfectly balanced fertility, timely planting and still produce nothing if one crucial element is missing. You guessed it—water. But, you’re thinking, water is beyond my control. 

"Not so," responds Farm Journal Field Agronomist Ken Ferrie. "Rainfall is beyond your control, but you can influence the water that’s already present in the soil or available for irrigation. Water management makes the difference between maximum yield and crop failure—with environmental stewardship as a bonus."

In this new series, we’ll detail the ways you can manage water so it’s effective—in the words of an old coffee commercial—down to the very last drop. Let’s start with several basic principles and terms that will help you think about water as a manageable input for crop production.

Water, Ferrie says, is the fluid that makes life possible—including human life because the body is 60% water. In crop production, water makes life not just possible but good. 

There are 1,400 million cubic kilometers of water on the planet—if it were spread evenly over the surface, it would be 9,850' deep. So you might think moisture could take care of itself. But of course it’s not that simple. 

"More than 97% of that water is in the oceans, where it is salty and not available to crops," Ferrie says. "About 2% is tied up in glacial ice caps. A little less than 1% is in groundwater more than 2,500' deep, where it is pretty much inaccessible.

As a limited yet vital input, water demands a high level of diligence. The Water Management series details how farmers can manage earth’s most valuable resource to boost yields and profit. water_management

"Ultimately, only about 0.05% of the water on earth is available for drinking, washing and growing crops. It’s a limited resource that we need to use wisely," Ferrie says.  

Water in the plant. From a technical aspect, Ferrie explains, water provides turgor pressure, which strengthens plant cells, so the plants can stand up and grow. But its most important function might be as a raw ingredient in photosynthesis. 

"Water enables plants to use energy from sunlight to turn carbon dioxide into sugar," Ferrie says. "Without this process, life could not exist. The key element that water provides to the photosynthetic process is hydrogen."

Even though photosynthesis is critically important, it requires only 1% of the water in a plant, Ferrie notes. The other 99% is used to maintain turgor pressure, transport nutrients and sugars and cool the plant.

"One place you can manage water is in the plant," Ferrie says. "Management techniques include variety selection, row spacing, population, fertility and variable-rate planting."

Water in the soil. The second place you can manage water is in the soil. There, water provides nutrients to the plants by making root interception, mass flow and diffusion possible. 

"In root interception, roots encounter nutrients as they grow through the soil, which requires adequate moisture," Ferrie explains. "This process is especially important with immobile nutrients such as calcium and magnesium."

Mass flow occurs when water evaporates from plants’ leaves into the atmosphere. "That causes water to be pulled upward from the soil through the plant," Ferrie notes. "Soluble nutrients, such as nitrates and sulfates, are swept along as water flows to the roots."

In diffusion, nutrient ions enter the soil solution to replace ions taken up by the plant. "This process proceeds faster in soils containing high moisture content," Ferrie says. "When there is adequate water, more distant ions can rush in. Diffusion is important with phosphorus and potassium."

When you think about root interception, diffusion and mass flow, it’s easy to see how improving your soil structure—another  component of water management—will help plants take up more water and nutrients and produce higher yields, Ferrie adds.

Because water is the only medium that expands when it freezes, water improves bulk density, creating healthier soil so plants can grow better and yield more. "By managing bulk density, soil structure and organic matter content, you can improve water infiltration and storage, so the water is there when you need it," Ferrie says. 

The hydrologic cycle. As a farmer, the water you manage (known as active cycling water) is part of a global hydro-logic cycle. In that cycle, water moves off the planet into the atmosphere and later returns to the planet as rainfall. You might view your task as influencing the tiny part of the global hydrologic cycle that impacts your own fields. You want to keep water flowing without interruption, from the soil up through your plants and out into the atmosphere (from which it eventually returns). 
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If all the water on earth were spread out uniformly, it would be 9,850' deep. But only 0.05% of the planet’s water (almost too small to show up on the graph) is involved in the cycling process that makes it available for crop production and other uses.  

The parts of the hydrologic cycle you can influence are vaporization of water from the soil surface (evaporation) and from the surfaces of plants, through their stomata (transpiration). Together the two processes are called evapotranspiration. "The way you manage your soils and your crops affects the evapotranspiration rate on your farm," Ferrie says. 

In technical terms, you want the actual evapotranspiration rate (ET) to be as close to the potential evapotranspiration rate (PET) as possible. A narrow ET/PET ratio means water is moving from the soil through the plants, carrying nutrients with it, efficiently enough to meet the demands of the plant.

"The potential evaporation rate tells you how fast water will be lost in a densely vegetated plant-soil system," Ferrie says. "Dense vegetation transpires water about 65% as fast as it evaporates from an open pan. The rate of evaporation varies because of climatic factors such as temperature, relative humidity and cloud cover." 

Plant population and spacing affects the actual evaporation rate. So does the type of plant because of different leaf structure. More shading, which is expressed as leaf area index, will increase transpiration and reduce evaporation from the soil surface. "Another benefit of greater leaf area index is that more sunlight is intercepted, so plants can use it to drive photosynthesis," Ferrie explains. "Sunlight that hits the soil surface is wasted, just like water that evaporates from the soil surface." 

Because evaporation is reduced by residue cover, tillage is a water management practice. So is fertility: Nitrogen and phosphate greatly affect leaf area index, and potassium helps control water loss out of the stomata (openings) in plant leaves.

Soil conditions also affect ET. Evaporation from the soil surface at a given temperature is largely determined by the ability of the soil to replenish the surface water as it evaporates. "The top 5" to 10" supply most of that water," Ferrie says. 

You can influence the evaporation rate by helping your soil take in water so it can be stored and eventually move freely within the soil. That involves soil health improvement practices such as vertical tillage, no-till or strip-till, cover crops and drainage.

"Anything you can do to help plants grow faster improves water efficiency," Ferrie says. "A quicker canopy means more shading to reduce surface evaporation and more leaf area to increase transpiration, which drives yield."

Let’s review the tools you can use to manage water. They include variety selection, row spacing, plant spacing, population, variable-rate seeding, soil fertility, tillage (or no-till or strip-till), cover crops, drainage, irrigation and more. In other words, everything you do is part of a systems approach to managing water. 

All those variables create opportunities to improve yield and profit by preventing plants from getting thirsty. We’ll explore them in depth in future installments of this series.