November 7, 2017
When it comes to growing a good crop, farmers know that moisture is the most limiting factor for plant growth. What many may not realize is that soil fertility is the second most limiting factor.
Texas AgriLife soil sampling campaigns continue this month in a number of locations across the state, an annual testing season that provides producers with a snapshot of the health of their soil.
But one of the major problems with annual or semi-annual water sampling programs is that after so many years of conducting them, farmers generally begin to take them for granted. While some soil testing programs are free to producers, more comprehensive soils tests cost money, and depending on how many acres you farm or ranch, those costs can add up in a hurry.
TAKE THE TEST
"The only way a farmer or rancher can know what nutrients may be deficient in the soil is by testing. Not testing means just guessing at the condition of the soil, which could lead to not treating the soil with needed nutrients it may need to helps forage or row crops grow; or just the opposite, we could add nutrients to the soil it may not need and run up input costs in the process," explains Dr. Vanessa Olson, Associate Professor and Texas AgriLife Extension Forage Specialist in Overton.
She says adding nutrients to the soil when they are not needed not only runs up input costs but can also be detrimental to the environment as many of these nutrients are carried by runoff to adjacent areas, including streams and rivers where aquatic species may be sensitive to agricultural runoff.
Dr. Jake Mowrer, AgriLife Extension state soil fertility specialist in College Station, agrees.
"Each year producers must determine what and how much nitrogen they will use to fertilize their crop. Soil sampling will give them an idea how many nitrates their soil already contains so they can credit that to their overall needs," he said.
Mowrer has been conducting soil testing at various locations across the state to help farmers better understand the science behind soil fertility. In one such test in 2016, Mowrer conducted a study in the Hill Country on the effect of reducing nitrogen fertilizer applications to wheat based on soil test nitrates at depths as great as three feet. It was the first of its kind on a cool-season crop.
TEST TO DEPTH
Mowrer said producers who sample their soil and understand the results can easily save money in fertilizer costs by taking advantage of existing soil nitrogen, while still achieving expected yield goals.
"As farmers we don't want to waste money by over applying nutrients into the soil that we don't actually need," Olson added. "For routine soil analysis, especially for a forage crop, we are typically testing at a depth of about six inches. But testing soils deeper, anywhere from 12 inches to several feet, would be more important for specialty crops like alfalfa, for instance. Alfalfa can be sensitive to boron, for example, plus more sensitive to pH levels at a greater depth. Soil testing should be coordinated with your local Extension specialist if there is any question about specialty crops a producer may be growing."
The geographic region, the make-up of its soils, the type of crops being planted, plus other factors will affect soil nutrient levels. For instance, in studies conducted to evaluate response of dryland corn along the upper Texas Gulf Coast to residual soil nitrate-nitrogen (NO3-N) measured to depths of 15, 30, and 61 cm, residual soil NO3-N levels ranged from 3.4 to 31.6, 7.8 to 49.3, and 9.0 to 71.7 kg ha−1, respectively, in 0 to 15, 15 to 30, and 30 to 61 cm depth increments, with cumulative NO3-N ranging from 23.5 to 114.5 kg ha−1 across site-years.
Where nitrogen fertilizer was reduced due to nitrogen crediting, yields and bushel weights at all 13 site-years showed no difference from those receiving full recommended nitrogen rates. A yield response to any level of added fertilizer nitrogen above the control was observed for only 6 of 13 site-years. These results indicate a high potential for success in crediting carryover soil NO3-N to 61 cm as a means of reducing applied nitrogen fertilizer rates.
Mowrer, who worked in a soil testing laboratory at the University of Georgia for a number of years before earning his advanced degrees, has an unusual and environmental-friendly approach to food production, which he says starts with a healthy soil. He also worked as an environmentalist and before finishing his under-graduate studies actually worked as a chef in New Orleans where he created healthy food dishes based upon the science of plant growth.
His unusual background gives him a unique perspective on how healthy growing relates to healthy eating, and he says healthy eating begins with growing food from healthy soils.
Mowrer believes the fundamental of soils is critically important for farmers to understand, and keeping soil sustainable is the key to keep us from imposing an adverse impact on adjacent environments by over-using chemicals. Knowing how much or how little of our agricultural chemicals to use is the right place to start for farmers regardless what they are growing, and that begins with sampling soils and understanding what is needed to make them and keep healthy to promote proper plant growth.
"As scientists, we generally call it soil, not dirt. But it doesn't offend me when someone, a farmer for instance, calls it dirt as long as we don't treat it like dirt. The truth is, soil is a complex matrix that includes what used to be bedrock that has broken down into boulders and then into gravel and clay and so forth. But soil is the most incredible and largest reservoir of biodiversity on the planet. It's a true natural resource," Mowrer said in a recent podcast.
UNDERSTAND THE SOIL
He said soil fertility starts with understanding the needs of soil. For instance, to protect the soil, cover crops are the best medicine. A farmer may want to build up organic matter in the field by using cereal rye that has a lot of biomass and a lot of roots, or the farmer may want to build up nitrogen in the soil and use legumes as a cover crop because they have a special symbiotic relationship with rhizobial bacteria, What you want to use as a cover crop may depend on what you expect out of a cover crop.
Allelopathic plants like cereal rye help to discourage the growth of some plant material like weeds. He says the cover crops we grow can also help with pest management. Plants like canola, cabbage and other brassica types will discourage nematode development in soils because of their inhibiting properties. In addition, a well-managed crop rotation can help to create a healthier soil, as does no-till methods of farming.
Olson and Mowrer agree the best place to start to improve and maintain soil health and fertility is with soil sampling. Testing should occur at least once a year, and farmers are encouraged to contact their extension soil specialist to determine the number and depth of testing that may be needed to meet their specific farm or ranch needs.
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