A Values and T Values: What is That All About?

Jan 24, 2022

By Sjoerd Willem duiker and Jen Weld

While there are several types of erosion, T values and A values estimate sheet and rill erosion when soil erosion is calculated for your farm. How is soil erosion calculated for your farm, why are T values and A values important, and how do you know if your soil loss is too high?

In a previous article we reported that according to the latest USDA-NRCS estimates, as published in the last National Resource Inventory, soil erosion continues to be an area of concern in Pennsylvania. How is this reported soil erosion determined? How do I know if soil loss on my farm is too high? Soil conservation specialists use the Revised Universal Soil Loss Equation (RUSLE2) model to calculate predicted average annual soil loss (A value) and compare that with published tolerable soil loss rates (T value) unique for each soil series. When developing a soil conservation plan, planners will calculate the average annual soil loss rate over the entire length of the crop rotation (A value) and compare that with the T value. The plan is developed so that the A value is less than or equal to the T value so that soil loss does not compromise long-term soil productivity. Soil conservation personnel use RUSLE2 to calculate the A value for a field based on:

The rainfall erosion in your area based on total amount and common rainfall intensity
The erodibility of the soil based on soil texture, soil organic matter content, and soil structure
The slope length and steepness
Crop production practices such as: tillage type, crops grown in the rotation, cover crops, row spacing, contour planting
Other conservation practices such as terraces

You may wonder: but shouldn’t my soil loss be zero? In practicality, it is almost impossible to reduce soil loss to zero in crop production, and soil scientists have determined that new soil forms from parent materials all the time. Tolerable soil loss is a level of soil loss that does not exceed the rate of soil formation. While we have a poor understanding of soil formation rates, T values were established many years ago as a practical measure of an acceptable soil loss level that would not threaten long-term soil productivity. The result has been that deep soils have a higher T value than shallow soils because it takes a lot longer before the soils loss compromises the productivity of deep soils. Tolerable soil loss values for Pennsylvania soils vary from 1 T/A/yr to 5 T/A/yr.

While there are several types of erosion, T values and A values estimate sheet and rill erosion when soil erosion is calculated for your farm. Sheet erosion, also referred to as interrill erosion, results when rainfall impact disperses soil aggregates which are then lost through runoff. Rill erosion occurs when water concentrates in small rivulets that form running up and down the hill slope. Rills are different from gullies because gullies occur in concentrated-flow areas located in swales of the landscape. Other important forms of erosion that are not included when soil loss is calculated for your fields are ephemeral gully erosion, classical gully erosion, or streambank erosion. We challenge you to pull your conservation plan from the shelf and check the T values and A values on your farm.

Source : psu.edu

Video: Can Gene Editing Give Seeds a Stronger Start?

Reagan Reed has a way of describing his research that might make a plant breeder pause.

“I’m the sort of person who likes to break things to understand them,” he says.

What he breaks are genes involved in seed germination. Reed, a Ph.D. candidate at the University of California, Davis, is investigating whether changing those genes can help seeds establish healthy plants under a wider range of conditions.

Reed recently attended the National Association for Plant Breeding meeting in College Station, Texas, as an NAPB Borlaug Scholar. His work speaks to a question that matters well beyond a research lab: what does it take for a seed to get a crop off to a strong start?

A seed that germinates quickly, alongside the others planted with it, spends less time vulnerable to stresses before seedlings become established.

“If you’re able to bridge that gap between germination to stand establishment, then that’s going to give you the highest probability of having a solid yield that year,” Reed says. “There’s a bunch that can go wrong after that, but it maximises your yield from the start, at least.”

Reed works in Dr. Imtiyaz Khanday’s lab at UC Davis, where his research focuses on seed quality in tomatoes and potatoes. He brings an unusual academic background to the work: at Skidmore College, he studied both biology and Latin.

Reed believes that combination prepared him for research. Translating an ancient text means studying each word, weighing its possible meanings and using context to reach the best interpretation. There is rarely a way to know with complete certainty what an author intended. Science, he says, requires a similar willingness to work through uncertainty.

“Attention to detail and comfortability working with uncertainty,” is how he sums up what Latin taught him.

In the lab, uncertainty can produce a surprise. Reed describes changing one gene and seeing little effect. Changing a second gene also appears to do little. Put both changes in the same genetic background, though, and the result can be substantial.

“I break this gene, nothing really happens. I break this other gene, nothing really happens. I combine them into the same genetic background, and now there’s a massive effect,” he says.

Those interactions matter as researchers gain the ability to edit multiple genes at once. They also complicate any simple account of how a trait works: improving a seed may depend on understanding what several genes do together.

For Reed, the question is whether that knowledge could help seeds germinate reliably when conditions are difficult. Some of the genes he studies prevent germination under certain conditions. That response may serve a plant well in one setting while limiting its usefulness in agriculture. He wants to learn what happens when those genes are changed, and whether the result improves establishment.

Yet a discovery that works in a lab still has a long way to travel before it reaches a field. Reed argues that agricultural biotechnology faces an economic challenge as well as a scientific one. New tools can create value for farmers, he says, but developing them requires a way to support the work in an industry where margins can be thin.

His fellowship with Flagship Pioneering gave him a view across AI, agriculture-related work and human health research. It also reinforced the importance of bringing people with different expertise into the same conversation—a benefit of gatherings such as the NAPB meeting.

“You need to have a team of people with different backgrounds who are willing to work together and approach the same problem in different perspectives and different ways,” he says.

That thinking connects the different parts of Reed’s path. Latin taught him to examine details without expecting a perfect answer. Gene editing lets him test what individual genes do—and what happens when their effects combine. Working with people beyond his own field helps him ask what those findings could become.

He came to agriculture because of its reach: the possibility that work by a small group of researchers could eventually make a difference on a much larger scale. His immediate focus is smaller and more precise. He wants to understand what helps a seed get through its first days and become a healthy plant.