NEWS FLASH! Water Makes Grass Grow!
By Jim Gerrish
Reprinted from the Stockman GrassFarmer
MAY, Idaho: When we look at the basics of pasture productivity, creating a more effective water cycle is up there right next to capturing more solar energy on every acre.
Sure, the title on this column might seem a little silly. Everybody already knows water makes grass grow. If that's the case, why do most of us let so much water go to waste?
What happens when it rains on your farm or ranch? Where does the water go? Infiltration or runoff? Evaporation or transpiration? If it doesn't rain for a couple of weeks are you ready to say there's a drought?
What we decide to do every day with our grazing management will likely affect how you answer those questions.
The first rule for building a healthier water cycle is keep the ground covered. Part of that cover will be green, living plants and part of it will be plant litter or dead and decaying plant material. The worst enemy of a healthy water cycle in your pasture is grazing too short. The more frequently the pasture is grazed short, the more water loss there will be from your pasture.
Let's walk through the ripple effect of grazing pastures too short. The first thing to think about is there is just less physical protection against the impact of falling rain drops hitting the soil. The impact of a rain drop is far greater than most people realize. With a velocity between 10 to 20 miles per hour for an individual drop and millions of drops falling in a 1" rainfall, the amount of soil displaced from the surface is astounding.
The worst situation is bare soil. Iowa State researchers have found up to 90 tons of soil/acre can be displaced in a heavy downpour. No, that does not mean that all of that soil leaves that acre. What it means is moving soil particles clog and seal soil pores. That is what reduces infiltration potential and accelerates runoff loss.
Fortunately pastures with their perennial cover don't suffer the same fate as fresh tilled crop soils. Or so we like to believe. I have been in far too many pastures that have no more than 50 percent of the ground covered by living plants and litter.
No, I'm not talking about eight inch rainfall rangeland in Idaho. I am talking about seeded pastures in the Midwest, the Northeast, and the Upper South. All places that should have the most productive pastures in America. There is also too much bare soil in most irrigated pastures here in the West.
We really notice the runoff from our pastures when the big rains come and the ditches run full and the. ponds fill overnight. Too many people think having your ponds fill overnight is a good thing. All that really means is not enough water went into your so it.
We also tend to overlook the smaller runoff events that follow somewhat smaller rainfall events. Those losses disturb me even more because our soils should have the capacity to take in rainfall in those lesser amounts.
We often dream about deep soils with the capacity to hold water to ten feet down and roots growing effortlessly through all that soil to capture that deep water. The sad truth is it is the infiltration capacity of the surface foot that defines the health of our water cycle more than gross water holding capacity.
How fast can the surface few inches absorb water? That will determine how quickly the upper soil becomes saturated to the point that all additional rainfall is lost as runoff.
I remember one of those embarrassingly dumb moments early in my university career when a soil scientist colleague asked me when I thought the difference in runoff between a continuously grazed pasture and an intensively managed pasture would occur. In my naivete, I said when it was saturated. He politely informed me that, when the soil is saturated, everything will runoff. Duh...
The real value to managing the soil surface cover and building organic matter and soil structure is in how much longer it extends the rate and time of infiltration between when the first rain drop falls and when the soil becomes saturated. The infiltration rate on overgrazed and abused pastures with silt loam soils may be as slow as half an inch per hour while on healthy, biologically active soils the rate can easily exceed 2-3 inches per hour on silt loam soils.
A quick reminder about soil types, sandy soils typically infiltrate water at the fastest rate while clay soils have the slowest infiltration rates. Loams and silt loams are intermediate. While a soil type sets the base level for infiltration potential, the grazing management imposed will be the final determinant of infiltration rate.
Once we get the water into the soil, the next concern is how much water is retained in the rooting zone for plant use. We can lose water both out the top through evaporation and out the bottom through deep percolation. Our management choices can affect both of these avenues of water loss.
The key to minimizing evaporative loss is keeping the soil covered. Difference in soil temperature under a healthy pasture sward and bare soil can be measured to depths of over one foot. Differences between a pasture with at least 6-8 inches of cover vs. one grazed down to 2-4 inches is measurable to several inches of depth.
Elevated soil temperature accelerates evaporation. The temperature differential at midday between the well covered soil and a soil that is 50 percent bare can exceed 20 degrees F at the four inch depth. That makes a huge difference in evaporation losses. Remember, evaporated water does not grow pasture!
The key strategy for increasing water holding capacity by slowing water leaking out of the rooting zone to deeper depths is increasing the organic matter (SOM) content of the soil. Organic matter has a roughly 20: 1 water weight to SOM weight ratio. This creates a sponge effect that allows the soil to hold water in place in that part of the soil profile where 80M resides. The deeper in the soil profile that you can create additional organic matter, the greater the beneficial effect.
You can make a difference in the drought resistance of your pastures by keeping these two key points in mind: Keep the ground covered and build deep organic matter in the soil.