“We are going to be talking about some alternative fertility for forage systems that are outside of the usual fertilizer additives that we think of when we think about forage fertility,” said Dr. Amanda Grev during a MidAtlantic Women in Agriculture meeting. Grev is a forage and pasture specialist at University of Maryland Extension.
While fertilizer costs have started to level off, overall prices remain higher than in previous years. Fertilizer prices have risen about 80% since the end of 2020. Forage producers, however, run the risk of mining the soil for nutrients if they do not meet the demands of forage crops.
Assume one ton of hay contains 45 lbs. of nitrogen (N), 15 lbs. of phosphate and 44 lbs. of potash. The cost of those nutrients (as of June 2023) were $0.68/lb. for N, $0.64/lb. for phosphate and $0.51/lb. for potash. According to Grev’s calculations, this means for every ton of hay harvested, $68 worth of nutrients are removed. A producer can reapply nutrients as fertilizer – or consider some of the options presented by Grev.
While it may be the obvious first step, some producers still aren’t collecting soil tests and using the information to their advantage.
“Figure out what fields or pastures might be deficient or adequate in terms of forage nutrients,” Grev said. She also said to pay close attention to pH level because pH affects the utilization of minerals. The ideal pH range for forage crops is between 6 and 7.
As soils become more acidic, some of the essential nutrients – phosphorus, calcium and magnesium, among others – become less available to plants. At the same time, under acidic conditions, other nutrients – like copper, aluminum and iron – tend to become more available to plants. Forage crops high in copper, aluminum and iron can be toxic to livestock.
As an example to illustrate the effects of soil pH, assume that a producer is spreading 200 lbs. of N, 50 lbs. of phosphate and 150 lbs. of potash per acre. The cost for this fertilizer is about $245/acre using the numbers from June 2023. If the soil that fertilizer is spread on is more acidic (in this example, Grev used a pH of 5.6), there is a decrease in availability of those purchased nutrients to the plants.
“So we’re applying nutrients, but they’re not necessarily available to the plants, and we can assign a dollar value to that decrease in efficiency. For this example, it’s about $72/acre that we’re essentially losing,” Grev said.
Soil pH also impacts the root growth and development of plants. In more acidic conditions, root growth is more limited and roots cannot optimally extract nutrients and water from the soil. Low pH also means less soil mineralization is occurring and less N is being fixed by legumes.
Manure can be a valuable source of nutrients for forages. If a producer is spreading 10 tons/acre of solid cow manure, that amounts to, on average, 154 tons of N/acre, 84 tons of phosphate/acre and 180 tons of potash/acre.
Assigning the same nutrient costs described above, the total fertilizer value from that application could be up to $250.28/acre. “It is a significant savings, and is a significant source of nutrients for forages,” Grev said.
When spreading manure, Grev advocated for injecting it rather than broadcasting. She cited a study where there was a huge reduction in nitrate losses (also known as nitrification) for injected manure.
“I realize it’s not always an option, but if it’s possible, either incorporate or inject manures rather than spreading them on the soil surface. We can minimize losses that way. We can also minimize run-off,” she said.
Grev also suggested spreading manure when soil temperatures are lower. As soil temperatures increase, there is an increase in nitrification.
In terms of pasture management, the more animals are moved (i.e., rotationally grazed), the more evenly their manure and urine is distributed across the landscape. One study showed that in a continuously grazed pasture system, where animals are never moved, it takes an astounding 27 years to get one pile of manure per square yard. If that pasture was subdivided into a 14-day rotation, that number decreases to seven to eight years. In a four-day rotation it would further decrease to around four to five years, and in a daily rotation it might be as little as one to two years.
Grev said, “Usually when we look at animals on a pasture, we think of the animals themselves as the dollar value from that pasture. I also encourage you to consider that the manure from those animals also contains a high dollar value that can be applied to the pasture. The question I ask is: How uniformly are those nutrients distributed?”
Legumes like clover and alfalfa have the ability to fix atmospheric N because of their symbiotic relationship with rhizobium bacteria in the soil. Different legume species, different productivity levels and different growing conditions can affect the amount of N fixed, but on average productive legumes are able to fix 50 to 300 lbs. of N an acre every year.
Only a very small amount of this N is shared directly from legume to non-legume plants via mycelial networks in the soil. However, there are also dead roots, dead nodules and sloughed cells from legumes decomposing in the soil and adding N to it.
The majority of N fixed by legumes, however, is found in the top growth of the plants above the soil surface. This N can be made available to other forages through plant decomposition or consumption of the top growth (and subsequent defecation) by livestock.
“What we need if we’re going to rely on legumes for fixing nitrogen is a good productive stand that’s producing yield and is actively fixing nitrogen,” Grev said.
Typically, legumes need to make up greater than 30% of a stand on a dry matter basis to have a substantial contribution to the N fixation pool.
Fertilizer applications can have an impact on legume-to-grass ratios in a stand. Because they can’t produce their own N, addition of N fertilizer shifts the advantage to the grasses, while adding phosphorus and potassium typically favors legumes.
“Among other things, you can use your fertility strategy and your fertilization to help maintain a larger grass component or a larger legume component within a given stand,” Grev said.
by Sonja Heyck-Merlin