The majority of farmers and ranchers are a lot more cognizant of soil fertility in their crop fields than in their pasture or hay ground. Maintaining soil pH and adequate nitrogen (N), phosphorus (P), and potassium (K) status is necessary to develop a nutrient plan to help us achieve desired grain yields.
While it’s fairly standard to sample crop ground every two to four years, many forget when the last time pasture ground was sampled. Although it’s not practical to soil sample pastures every year or go to the extent of grid sampling pastures, it is a good management tool to sample every three to five years. This ensures that soil fertility and forage productivity are being maintained or improved.
A well-known “superpower” of a beef cow is her ability to turn grass into steak. To accomplish this, she actually only utilizes roughly 10-30 percent of the N, P, and K ingested through the forage, and the remaining 70-90 percent of the nutrients are excreted in urine and manure. While recycling of these nutrients may be sufficient to maintain adequate forage productivity in the short term, manure distribution is inconsistent across pastures and will steadily take a toll on productivity. Continuous overgrazing year over year can also further decrease nutrient availability.
Although the cow herd does retain a small portion of the nutrients each year, removal of the forage for stored feeds has a larger impact. In well-managed pastures, a cutting of hay is often taken in early summer to capitalize on the extra spring forage. Research from the University of Missouri has shown that harvesting 3 tons of hay per acre per year can result in removal of 80 percent of the N, P, and K nutrients. This represents a large nutrient loss that is often overlooked. In order to maintain forage yield, these nutrients need to be replaced either in the form of commercial fertilizer or manure hauled in from a drylot area or feedlot.
Collecting soil samples
In a perfect world, we could identify 20-acre blocks within a pasture to pull 10-20 core soil samples from that would contain a consistent soil type, equal topography, and similar previous management (whether it’s been farmed or continuous pasture). However, in reality, a large majority of our pastures are odd-shaped, contain some form of a stream or pond, and have a wide variety of elevation. Thus, the best option may be to take soil samples from an area that is most representative of the pasture. For instance, if the majority of the pasture has a north-facing slope, collect the soil samples from the side hill. It’s still advised to collect a minimum of 10 cores from a zigzag pattern in that representative area. The more samples collected, the more accurate representation of the soil we can get. However, one good soil sample that is representative of the pasture is better than knowing nothing about the soil fertility of the pasture.
Keep in mind that variability across pastures can be tremendous and effected by previous management, manure distribution, feeding areas, and dwelling areas. Generally, in a rotational grazing system, manure distribution is much more consistent opposed to a continuously grazed pasture. Soil fertility is often elevated in feeding areas due to feed waste, especially if we are consistent in where we provide supplemental feeds. Nutrient buildup will also occur in common loafing areas such as near water sources or in shaded areas and should not be sampled. Avoid sampling within 150 feet from a gravel road as lime from the dust can raise soil pH and not be representative of the pasture.
Generally, a soil probe is used to collect a core that is 6-8 inches in depth. A core less than 6 inches in depth may overestimate soil fertility. Discard any organic material above ground as well as root structure directly below ground, and place 6 inches of the core into a clean, plastic bucket. Once all the cores are combined in the bucket, break up the cores and thoroughly mix the soil samples. A minimum of 1.5 cups of soil can then be placed into a bag and mailed to a lab for analysis. It’s very important to make sure that the soil samples are being sent to a certified lab.
Using the data
If farmers and ranchers go through the effort of collecting soil data, it’s important to utilize the information to make informed decisions to improve pasture productivity and quality. Fall is often a good time to soil-sample pasture ground and provides an opportunity to evaluate results and develop a nutrient plan for application the following spring.
Most forages experience optimal growth when the soil pH is in the 6.0-6.5 range as some nutrients become unavailable to the plant when the pH drops below this range. Nitrogen fertilization is often a higher priority compared to P and K as grass yields are quickly boosted in response to N; however, if P and K supplies are inadequate in pastures, productivity is suppressed. Depending on your operation and forage needs, N fertilization in late summer or early fall to boost fall forage production may be more valuable than spring applied N. Keep in mind that N fertilization reduces the presence of legume species due to grass competition and thus has potential to increase ergovaline levels in infected fescue and can increase fescue toxicosis concerns if not managed properly.
Local beef extension specialists or extension agronomists should be able to address questions about interpreting soil sample results and provide assistance in developing a nutrient plan to optimize forage production. — Erika Lundy, beef extension specialist, Iowa Beef Center. This article first appeared in the November 2017 Iowa Cattleman magazine. Reprinted with permission.
