Soil Potential Index & Surveys
Soil Potential Index (SPI) and Surveys are a great opportunity to update current use information for your farmland.
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Soil Potential Index (SPI)
SPI is a calculation made at the County Conservation District office which rates soil on a scale of 0-100 based on its capability to produce crops. For example, a rating of 10 would indicate a soil poorly suited to growing crops; a rating of 95 would mean top quality soil with virtually no impediment to crop production. The Conservation District uses USDA Natural Conservation Resources Service soil mapping data to compute SPI for each field, parcel or tract of land.
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Reading and assessing the land is the first step in solving natural resource problems and sustaining our limited resources. In particular, soils provide the foundation for conservation work, and understanding the nature and properties of soils is critical to managing and conserving other natural resources.
Through its Soil Survey Program, the Natural Resources Conservation Service (NRCS) studies and inventories the soil resources of the Nation. In fact, it is a major responsibility of NRCS to conduct soil surveys on the private lands of the United States.
Soil Surveys contain information in the form of detailed soil maps, data tables, and text narratives that can be used in land-planning programs. Soil Surveys also contain predictions of soil behavior for selected land uses and highlights limitations and hazards inherent in the soil, improvements needed to overcome the limitations, and the impact of selected land uses on the environment.
The Cheshire County Soil Surveys are available in print at the CCCD office. If you are interested in obtaining one please contact us. Soil Surveys are also available online on the NRCS Web Soil Survey website.
If you need assistance navigating this website please call the CCCD office for help.
"Farmland Productivity - Comparing the Geology and Soils of New Hampshire and Vermont"
by Wendy Ward, Soil Conservation Technician, Natural Resources Conservation Service
This fall I was helping with a habitat workshop when a person asked, “Why are there more farms in Vermont, why is the farming better there?”
Simply put, it’s in the soils. What determines the agricultural productivity of the soil? Soil is more than ground up rock – it contains organic and living material such as humus and bacteria. What the soil is made of (parent material, plants and fungi) and how it develops over time (influenced by topography, climate) determines soil properties. The biggest determining factor for soil productivity is parent material – the rock and mineral deposits that make up the soil. Go grab your shovel, to explore parent material we need to dig deep – down to the bedrock.
What type of rock is your bedrock? Rocks form and reform in a cycle. Sediment deposition, wind and water erosion, volcanic activity and heat and pressure form and reform rocks in a continuous cycle. A quick review of rock types give us sedimentary, formed by deposits of material within bodies of water and at the earth's surface. Igneous, formed from magma below the earth’s surface or lava flows above the surface. Metamorphic, derived from either sedimentary or igneous rock reformed under heat and pressure. New Hampshire and Vermont have primarily igneous or metamorphic bedrocks. Here is a fun link to understanding the rock cycle.http://www.learner.org/interactives/rockcycle/diagram.html
The characteristics of each bedrock type determines the productivity of the soil above it. For example, granite is an igneous rock formed from magma. It is a hard, acidic, and resistant to weathering. The soils derived from granite are acidic and have few soluble minerals and nutrients available to plants, and therefore are not as productive for agriculture. Limestone is a sedimentary rock formed from deposits on ocean and lakebeds. It is soft, has a neutral pH, and weathers easily. It contains high amounts of calcium carbonate from the shells of marine organisms. Soils derived from limestone bedrock are close to a neutral pH, rich in soluble nutrients and very productive for agriculture. Metamorphic rocks vary in productivity based on the minerals they inherit from other rocks.
New Hampshire is the granite state so we can draw some quick assumptions on the type of bedrock and soil productivity. Vermont has three state rocks, granite, marble, and slate. Marble and slate are metamorphic rocks derived from limestone. Can we draw assumptions about Vermont? Most of Vermont’s bedrock and soils are higher in productivity than New Hampshire soils.
So why do New Hampshire and Vermont have such different bedrock types? To answer this question we need to travel back in time; say, about 520 million years, give or take a few millennium and look at plate tectonics and the geological origins of New England.
520 million years ago, a shallow sea covered what would become New England on the North American plate. Over the past 445 million years, the African/European plate advanced, collided, and separated from the North American plate. The movement of these plates created three major mountain building events (The Taconian, Acadian, and Alleghenian). As the African plate thrust into the North America plate, it created the supercontinent Pangaea. The shallow sea closed as the plates collided. 200 million years ago the plates changed direction and began to move away from each other. Pangaea broke apart leaving part of the African plate along North America - where New Hampshire is today.
These events left most of present day Vermont with metamorphic bedrock high in calcium from the ancient shallow sea. In New Hampshire…well not so much. Most of New Hampshire bedrock is igneous (granites) or metamorphic rock derived from lava or magma. Recall how the characteristics of granite vs. limestone influence nutrient availability. (Map of Geological provinces of New England).http://pubs.usgs.gov/of/2003/of03-225/Figure_2.jpg
So is that the answer, is it the bedrock? Yes, but not entirely. Another geological event influenced our soils, can you guess? Right, the glaciers! Grab your jacket and we’ll travel back to the end of the last glacial retreat 14,000 years ago.
At this time, deposits of sand, gravels, stones, and rocks from the glacier covered most of our bedrock. Dense unsorted glacial till covered much of the landscape. Sorted materials from melt waters deposited coarse, excessively drained sands along rivers. The glacier strewed rocks and boulders on the surface of the landscape. These glacial deposits share characteristics such as texture and chemistry derived from the underlying bedrock. Ancient glacial lakes, such as the Champlain Sea and Lake Hitchcock, covered about one third of Vermont but only about one seventh of New Hampshire. Fine textured silts and clays deposited on these lake bottoms, an ideal foundation for productive soil. Again, Vermont seems to have received the better deal in terms of productive soils for agriculture due to its greater number of glacial lakes. Vermont also ended up with fewer rocks and stones on the surface.
Now we know why Vermont has a higher proportion of soils ideal for agricultural production than New Hampshire. You may ask “How can this information be useful for me on my farm, or in my town?” Time to get our shovels again and look at soils and soil mapping.
Soils have a taxonomical system to describe them. Just as plants and animals are organized by taxonomical systems and defined to individual species, a “soil series” can thought of as a soil species. The USDA publishes maps and interpretive tables describing the properties and limitations of the soils. Since we are talking about agricultural productivity of the soils, let’s look at the prime farmland classification of soils.
USDA-NRCS defines prime farmland soils of national, statewide, and local importance. I have not considered farmland of statewide or local importance as definitions vary by state and are not directly comparable. Prime farmland is land that has the best combination of physical and chemical characteristics for producing food, feed, forage, fiber. Prime farmland does not directly consider bedrock in the classification, but incorporates the inherited bedrock characteristics such as ph, texture, and available nutrients.
Some of the characteristics of prime farmland soils are moderate pH, fine textured partials, infrequent flooding during the growing season, deep to bedrock (40” or more), gentle slopes, and a minimal amount of surface stones. You can locate and access soils information for most areas of the United States in the USDA-NRCS Websoil survey website.http://websoilsurvey.nrcs.usda.gov/app/
New Hampshire has less than one-third the amount of prime farmland soils than Vermont, less than half the land in active agriculture and less than one-fourth the land in crop production.
Here are some numbers that illustrate the disparity between these two states.
USDA-NRCS soils data.
Vermont total prime farmland soils - 382,000 acres.
New Hampshire total prime farmland soils 125,000 acres.
The National Agricultural Statistic Service 2007 census
Vermont total land in farms 1,233,313 acres
New Hampshire total land in farms 471,991 acres
Vermont total land in cropland 516,924 acres
New Hampshire land total cropland 128,938 acres
I am not suggesting if you want to farm to pack it up and move to Vermont. There are many areas with productive soils here in New Hampshire that can and do provide abundant harvests. The Major river valleys of New Hampshire boast lands with high agricultural productivity. If you are looking for land to farm, use the soils and bedrock maps to search for productive soils and site limitations. If you are already farming, these maps can help you make informed management decisions on your farm.
The smallest delineation on a soil map is three acres. It is important to understand there may be inclusions of different soils or wetlands within this map unit. The soil description will contain what to expect for inclusions. Walk the landscape to evaluate where these inclusions may be. Is that depression a wetland that I need to avoid? Does that swale indicate spring flooding? If so it is not the best place for the greenhouses. Does that knoll indicate some excessively drained soils? I may need to add irrigation if I plant crops there. There is granite bedrock here - I may have fertility and pH challenges if I buy this land. You can talk to your local Conservation District about understanding and interpreting soil maps. http://www.nhacd.org/districts.htm
Unfortunately, a common crop I see on New Hampshire farmland today is development. You can’t grow much if it is covered with asphalt and shingles. There are several sources of funding and organizations to help people keep land in the family using easements. Here is a link for New Hampshire organizations. http://www.directorynh.com/NHAssociations-Organizations/NHConservation.html
Ask your local conservation district about funding for soil conservation practices and land protection easements using USDA Farm Bill Programs. Keep farmlands growing food instead of shopping malls?
Thanks for exploring the bedrock and soil of New Hampshire and Vermont with me. And just to be safe we best fill that hole back in.
Thanks to NRCS New Hampshire and Vermont Soil Scientists for their input and information on prime farmland soils.
Resources and further reading:
Roadside Geology of Vermont and New Hampshire by Bradford B. VanDiver.
Where the Great River Risesby The Connecticut River Joint Commissions
Generalized Lithology and Lithogeochemical Character of Near-Surface Bedrock in the New England Region by G.R. Robinson, Jr. and K.E. Kapo, 2003 USGS Digital Open-File Report 03-225.http://pubs.usgs.gov/of/2003/of03-225/of03-225.pdf
Generalized Geology Map - New Hampshire http://des.nh.gov/organization/commissioner/pip/publications/geologic/documents/generalized_bedrock.pdf
Generalized Geology Map - Vermont http://www.anr.state.vt.us/dec/geo/images/geo5.JPG
Land Use Maps of New England. http://nercrd.psu.edu/land_use/maps/index.html
New England Agricultural Statistics Table 1. State Summary Highlights: 2007 http://www.agcensus.usda.gov/Publications/2007/Full_Report/Volume_1,_Chapter_2_US_State_Level/st99_2_001_001.pdf