The removal of phosphorus from the soil, by the removal of crop biomass from a site.
Vegetative mining is a long-term strategy for decreasing the soil test phosphorus values of a soil, typically in heavily manured sites. Crops that take up and accumulate phosphorus in the above ground biomass are grown, harvested and removed from the site. With time the export of phosphorus with the crop harvest, will reduce the soil test phosphorus values of the soil and thus reduce the potential impact on water quality.
How Does This Practice Work?
The potential for soil phosphorus losses from a site is related to the amount of phosphorus bound to the soil. In general, soils that test higher for phosphorus have a greater potential to impact surface water quality from runoff than soils that have a lower soil test phosphorus value. Reducing the soil test phosphorus by removing phosphorus from the site with vegetation will reduce the potential impact on surface waters.
On sites where high or very high soil test phosphorus values limit or restrict the continued application of fertilizer or manure phosphorus, a strategy of vegetative mining can help to draw down the soil test phosphorus.
In general, there is not a large difference in the concentration of phosphorus found in the aboveground harvestable portion of most crops. With the removal of greater quantities of a crop from a field, more phosphorus is removed increasing the impact of vegetative mining on the soil test phosphorus. Forage crops such as silage and hay crops (where most of the above ground biomass is harvested) are recommended over grain crops where a smaller percentage of the crop is removed from the field.
In its simplest form this practice may involve switching the management of a field from a forage pasture to a hay crop, or from grain to silage corn. Changing from a lower yielding to a higher yielding variety will increase the potential for vegetative mining of phosphorus.
Where This Practice Applies and Its Limitations
The vegetative mining of phosphorus can be conducted on all sites where crops can be grown and harvested. It is not recommended for sites where soil phosphorus might be limiting. Vegetative mining is a practice that is best suited to sites where the soils test high or very high for phosphorus. On soils with extremely high levels of high soil phosphorus it may take many years, or even decades to detect a change in soil phosphorus levels. Vegetative mining will be most effective on soils that do not receive additional phosphorus additions. The effects will be more rapid on soils with low soil phosphorus reserves. For example, on lighter or sandier soils that tend to have a lower nutrient holding or cation exchange capacity than heavier or soils with more clay or higher soil organic matter contents.
Vegetative mining will have little if any effect on pasture systems, or with low yielding grain crops.
Vegetative mining is one tool that can enable phosphorus additions to sites where the addition of phosphorus is limited or restricted (for example, by results of a site risk analysis). The results of vegetative mining will not become evident in the short-term and depending on the soil-type and the amount of soil phosphorus may not be effective in significantly reducing the risk to nearby surface waters for many years. As with practices with similar objectives, vegetative mining is most effective when combined with other practices designed to prevent the movement of phosphorus from agricultural fields.
Cost of Implementing the Practice
The cost of implementing a program of vegetative mining on a site is usually minimal. Switching production from one crop type to another, and changing harvesting techniques may require the use of additional equipment, and changes in management.
Operation and Maintenance
Once a strategy of vegetative mining has been implemented on a site, additional operation and maintenance costs should be minimal. As with all crops, attention should be paid to maintaining the optimum levels of all soil nutrients and ensuring the soil pH is within the recommended range for the crop.
Details of soil phosphorus removal rates can be found at the following databases:
USDA Crop Nutrient Tool. USDA Plants Database: https://plantsorig.sc.egov.usda.gov/npk/main
International Plant Nutrition Institute (IPNI) materials on Nutrient Uptake and Removal http://www.ipni.net/article/IPNI-3295
Ludemann, Cameron I. et al. (2022), Global data on crop nutrient concentration and harvest indices, Dryad, Dataset, https://doi.org/10.5061/dryad.n2z34tn0x
For more accurate estimates in your state contact your Cooperative Extension Service or NRCS office.
For Further Information
Contact your local soil and water conservation district, USDA-NRCS or Cooperative Extension Service office. To find your local USDA Service Center, visit https://www.nrcs.usda.gov/contact/find-a-service-center.
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Walker, F. 2023. Vegetation Mining. SERA17 Phosphorus Conservation Practices Fact Sheets. https://sera17.wordpress.ncsu.edu/vegetative-mining/