UVM Ext. Agronomy Outreach Professional
While recently attending a Certified Crop Adviser Conference in NY I started doodling Venn diagrams of the information I was digesting. In the world of soil health, the ‘classic’ Venn diagram is Chemical-Biological-Physical properties all interacting and collectively leading to the ever elusive thing we call soil health. Thinking larger, we can ask the question, does soil health always lead to environmental health? Notably for us, does soil health always lead to a reduction in phosphorus loading to water bodies? And from the agricultural perspective, does soil health always lead what I am terming farm health? What I mean is agricultural productivity and sustainability, including economic realities and crop yields. If we add more organic matter, will we always get greater crop yields? If we increase infiltration, will we always get reductions in phosphorus loss? We’d like to think so, but unfortunately for us reality is complex. Along with this Venn diagram is the overlap. Things take time and teasing out these realities to make sound management recommendations can be tricky and confusing. We continue to use a combination of research and demonstration trials in an attempt to approach that perfect union where farms are building their soil quality, increasing their farm profitability and having more positive environmental impacts.
The Possible Use of Gypsum Amendments to Reduce Soluble Phosphorus
Currently on the market are a number of products being sold both for increasing soil health and better utilization of phosphorus. One demonstration project we began this fall in McKenzie Brook watershed is looking at the use of gypsum amendments to increase soil health while also reducing soluble phosphorus loss. Gypsum (calcium sulfate dehydrate) actually has a long standing history as an amendment, as a source of sulfur and calcium (without a pH change). The NRCS has a practice standard for gypsum application to improve physical and chemical properties of the soil, improve water infiltration, reduce dissolved P in surface runoff and subsurface drainage, ameliorate subsoil aluminum toxicity, and reduce potential transport of pathogens in cases of manure and biosolid application. Utilization of this practice is more common in other parts of the US and applied in bioswales. Science research thus far has primarily focused on flue gas gypsum (FGD) and results suggest there is some efficacy in improving soil health and reducing P loss, but the magnitude of effects may vary.
Sulfur is required for protein synthesis and nitrogen fixation, so in theory, additions of gypsum could increase yield potential if sulfur is limiting in the soil. Calcium is also needed in cell wall and membrane function, growth and fruit development. Perhaps even more importantly, calcium can help improve soil structure as a flocculating agent; that is, calcium can help with soil aggregation via its role as a positively charged ion (Ca2+) held by soil’s negatively charged exchange sites (CEC). It has a stronger bond than other lower charge particles like sodium (Na+), which is why gypsum amendments are used in reclaiming sodic and saline soils. This feature is also particularly relevant to our clay soils if soil aggregate stability and infiltration is poor. Gypsum can theoretically reduce phosphorus loss by two related means. The first is by increasing soil aggregation and therefore decreasing the loss of P with sediment. The second is that calcium-phosphorus complexes can form, keeping the P in a less soluble form. We have begun a demonstration project in McKenzie Brook utilizing multiple types of gypsum in contrast to a short paper fiber lime product, and hope to build upon it next year. We will have more on this topic as this project evolves.
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