Here are three tips on land and field assessment that will help you better understand how a particular site might act when put into production:
Above all nothing beats a site visit to verify both what’s on the maps and what is not. Online mapping tools and print soil surveys only give you a rough idea or introduction to what exists.
Bring a shovel or a soil probe or both! Be sure to ask the landowner’s permission to dig, assuring them you will backfill any holes as best as you can. Remember these tips for preliminary farmland assessment:
How do soil type and water features vary with field and forest topography?
Topography is one of the five factors of soil formation that give a soil its character and determine its agricultural potential (along with climate, original formation material, time and organisms). Steeper slopes erode and will usually have less topsoil than soils directly downslope where sediments deposit. Low spots seep groundwater from higher spots. Further contributing to their ability to hold water and nutrients, low spots have more organic matter than high spots where vegetation is less dense and soils are perpetually aerated and dry. Convex landforms generally shed water, are more exposed and dry, and have less inherent fertility. Concave spots tend to gather water and can be more fertile assuming they are not excessively wet.
Soil maps indicate percent slope. This is a broad generalization. Visit the site to determine if there is one consistent section of ground sloping evenly in one direction, or a hodgepodge of dips and climbs sloping all over the place. The soils in both scenarios would be mapped as having the same average degree of slope, but act totally differently in terms of ground and surface water movement through the field, soil fertility status, and other physical characteristics.
Look around in the immediate vicinity or in the entire field to identify these areas of contrast: concave and convex, up-slope and down-slope, high spot and low spot. Soils will vary with the contrasts. You can anticipate how varying soil types might call for varying management strategies. For example, a convex area at the top of the slope in a field might need more compost or manure application than a convex low spot. Depending on how low and wet it is, you might not even be able to plant in certain spots, or you might need to adjust the timing of your operations to reflect seasonal fluctuations in soil moisture. For the same crop, some sections of the field might require raised beds or planting on ridges to keep soils aerated, while drier spots might require crops to be planted at grade.
This all might seem obvious. Just remember, the slightest difference in topography might have significant implications for yield, cost of production and adaptation strategies. Just about the only thing constant in soils is variation! NRCS soil surveys (upon which most other soil mapping is based) acknowledge there are typically pockets of soil that differ from the predominant soil type on the landscape. Walk the entire landscape and look for potential problem spots or even the slightest variation that might affect yield or require adaptive management.
What does the soil look like below surface? Any indicators that reveal what is happening year round?
You can only know by sampling or digging. Use a soil probe or augur to minimize the size of your holes to be refilled. Soil probes or augurs give you a lot of information and can go deep with relatively little effort. But augurs are expensive and sometimes hard to get a hold of. A shovel disturbs more earth with more work, but a shovel hole is a simple, low-cost way to do the trick.
Dig or augur down about two feet to get a sense of what is happening in the topsoil and just below it. Get to know the predominant soils by digging samples or auguring in spots that represent what likely exists across the landscape. One sample can represent a broader area of the same elevation. Once you have characterized the predominant soil or soils, look for inconsistent high/low spots, convex/concave spots or inconsistencies in vegetation types. Sample the soil in these spots to gauge differences in organic matter levels, topsoil depth, soil texture and structure, soil color, moisture holding capacity or limiting features such as excess stones, compacted layers, bedrock or even standing water.
Soil colors are excellent indicators of the seasonally high water table. Known as “depletions,” greyish/whitish colors in contrast with the dominant brownish soil color are giveaways that the water table has reached the level in the soil where the greyish/whitish colors are found. In soils these colors usually coexist with reddish-brown or rusty colors. Together these contrasting colors are commonly known as “mottles.” When digging sample holes, use a knife or other small tool to chip away at the sides of the hole to expose the natural soil layering. This will give you an idea of exactly where the greyish mottles occur and the water table was highest. You might find just a tiny fraction of the soil layer is made up of the greyish/whitish colors, in which case the frequency of the saturation period was low. Larger or more numerous mottles indicate a longer period of saturation or more regular saturation period year over year.
Digging sample holes or auguring might reveal other red flags, such as a limiting layer. It is common for one soil type to overlay another, for example a loamy soil over an extremely gravelly soil, so be sure to dig down a couple feet to make sure what you find close to the surface extends down deep.
Depending on what you find down under, you might be able to improve the situation with management practices. You can change fertility status if you deem soils test low in nutrients. In some cases, you can drain away wetness with swales or tile drainage. You can add organic matter over time through manuring, cover cropping or other conservation strategies. You can improve aeration or soil “tilth” using a combination of moderate tillage and cover cropping.
You can not alter the sand, silt or clay content. In most cases with the exception of installing drainage or grassed waterways it will be cost prohibitive to alter the topography and drainage patterns of the landscape. You can’t remove bedrock, unless you grew up mining? You also might have a tough time removing hillsides or mountains up-slope that are creating natural watersheds and wetness problems for your land.
How does groundwater move throughout the entire landscape?
Some fields look like they would be perfect for pasture or crop production at any time of year. Nice and high on the landscape, no wetland vegetation whatsoever, no suspicious wetland features visible for miles and not much variation in topography that would create wet spots. But the farmer or landowner knows that the field is chronically wet. A sampling of the soils confirms it- the seasonally high water table is high, and the soil is wet, maybe too wet to get on early in the year… What is going on?!? Take a look up and beyond to the greater area landscape… There are hills far off in the distance climbing in elevation surrounding the entire farm property!
The simple explanation here is groundwater moves not just from field to ditch or stream, but throughout the entire landscape. Keep in mind that soils are only about two feet thick on average, and everything underneath is heavily compacted soil in transition to bedrock. Groundwater infiltrates soil everywhere, but has a much harder time infiltrating the geology underneath. If you had x-ray vision and could see what was happening two feet under, you might see groundwater slowly moving laterally across miles and miles of the landscape and down-slope. Sometimes the topography is such that groundwater will move towards a particular field. That field’s underlying geology might be such that there is limited volume for the groundwater to occupy, thus creating a condition of locally saturated soils or groundwater moving back up towards the surface.
At times it might actually be two different types of soils, one on top of the other that creates this kind of “perched water table” condition. This happens when a much finer material such as a silt loam overlays a much coarser material such as a loamy sand. It may be counter intuitive, but it’s a well-known concept of soil hydrology that groundwater moving through the silt loam (finer layer) will not enter into the sandy soil (coarser layer) below unless the silt loam above is fully saturated. This creates a problematic condition of continuous near saturation that can be just as limiting as total saturation.
Perched water tables and excessive groundwater can be addressed by installing drainage ditches or swales or tile drainage beneath the surface. Contact a specialist with UVM Extension or NRCS for more information.
Look beyond the fields in question to see if there are any hillsides or higher elevations that might create a watershed for your field.
Remember the three starting points for land assessment: Look around! Look beyond! Look under!