Manure and Soybeans - How many soybean are using manure and what's the right approach?

 

The other day, I got the question, how much manure is soybeans using, and what type is it? Right when I got it, I asked – do you mean manure applied directly to soybean, or since this was a lifecycle/greenhouse gas question, how much mined and manufactured commercial fertilizer is being offset because of how we use manure in our crop rotations? That may seem like a slight distinction to some, but it me that is a big difference. Why? Because soybean is a legume and, as such, is capable of fixing much of its nitrogen needs, applying manure to it, especially nitrogen-rich manures, may not be the best use of our manure resources (though this depends, in some cases, applying low nitrogen manures that have high P and K may still make sense). However, using soybean in a rotation, with, for example, corn, can be a great way to better match crop nutrient removal to the amount of P and K applied.

Within this post, I'll try to answer a few points

1.      Benefits and challenges of applying manure to soybean.

2.      How banking P and K from manure application within a rotation supports fertility for soybeans.

3.      How many acres of soybean are receiving manure, and an Iowa-centric estimate of how manure supports soybeans within the state.

Manure Application to Soybean: Pros and Cons

Pros of Applying Manure to Soybeans:

 Nutrient Supply: Manure, especially from livestock like swine and cattle, provides essential nutrients such as phosphorus (P) and potassium (K), vital for soybean growth. While soybeans don't require as much nitrogen (N), the P and K in manure can boost soil fertility and improve crop yield.

Soil Health: Manure applications can improve soil organic matter, structure, and microbial activity. Manure improves soil health and water retention, benefiting the cropping system, including soybeans.

Cost-Effective Fertility: Manure is a cost-effective alternative to commercial fertilizers, especially for farms with ready access to livestock manure. Farmers can reduce reliance on purchased fertilizers by incorporating manure into their nutrient management plans.

Improved Soil Fertility Over Time: Manure can provide slow-release nutrients that benefit subsequent crops. When manure is applied to corn in a corn-soybean rotation, excess nutrients not utilized by the corn can become available to soybeans in the following season.

Cons of Applying Manure to Soybeans:

Nitrogen Misalignment: As a legume, soybeans can fix nitrogen through nodulation. Applying nitrogen-rich manure directly to soybeans can lead to inefficient nitrogen use, potentially increasing nitrogen losses through leaching or denitrification, as the soybeans won't need as much of it as they are fixing their own, or it means reducing fixation as a result of high nitrogen levels in soil when that nitrogen could have been used to replace nitrogen in other, non-leguminous production systems.

Soil Compaction: If manure is applied under wet conditions or if heavy machinery is used during application, soil compaction can occur, detrimental to root development and water infiltration in soybeans.

Manure Application in Corn-Soybean Rotations: "Banking" of Phosphorus and Potassium

Although applying manure directly to soybeans may only sometimes be the most efficient use of its nitrogen content, manure is often applied in corn-soybean rotations with the specific intent of providing phosphorus (P) and potassium (K) for both crops.

Manure Application Before Corn:

In a typical corn-soybean rotation, manure is commonly applied to corn because corn requires higher nitrogen. When manure is applied before corn, it supplies nitrogen and deposits significant amounts of phosphorus and potassium into the soil. Since corn may not utilize all of the P and K, these nutrients remain in the soil, available for the following soybean crop.

Phosphorus and potassium are less mobile in the soil compared to nitrogen. As a result, these nutrients persist and become accessible to soybeans in the year following manure application. This method effectively "banks" nutrients, supporting soybean growth without additional fertilizer applications.

By applying manure to corn with the understanding that residual P and K will benefit soybeans, farmers can maximize the use of manure nutrients across the two-year (or longer) crop rotation. This approach helps balance nutrient levels and avoid over-applicating phosphorus and potassium, which could lead to environmental concerns like eutrophication.

Farmers can reduce or even eliminate the need for synthetic P and K fertilizers for soybeans when they take advantage of the nutrients from manure applied to corn. Doing so not only cuts production costs but also promotes more sustainable nutrient cycling within the cropping system and can lower the carbon footprint of soybean as both the energy associated with mining P and K fertilizers are eliminated as are application passes of these fertilizers.

While direct manure application to soybeans is not suggested due to the legume's nitrogen-fixing ability, it can still offer benefits in terms of phosphorus and potassium supply and soil health improvement. However, the common practice in corn-soybean rotations is to apply manure to the corn crop to use the excess P and K for the following soybean crop. This strategic approach ensures that nutrients are efficiently utilized over the rotation, benefiting both crops while minimizing the environmental impact and fertilizer costs.

Acres of Soybean Receiving Manure

The USDA ARMS survey estimates acres of different crops receiving manure. While not all done in the same year, it does provide a reasonable approximation of which crops are receiving manure. In 2020, they estimate that 2.3% of acres receive manure, or about 1.9 million acres of soybeans. It is ranked as the second most popular crop to receive manure, following corn, though corn receives about 80% of all manure produced, and soybeans receive only 10% of the manure. However, given the above conversation, it must be recognized that this is acres receiving manure in a given year and not accounting for carryover phosphorus and potassium from manure used to support soybean production in the following year.

 Table 1. USDA ARMS data on manure application rates by field crop across the US.

The second figure is also from a UDSA ARMS survey and estimates where the source of manure was for crop production. Of the manures applied to soybean, only about 7% was from swine, or about 130,000 acres across the US. At first, this may seem strange, but swine manure is typically the highest available nitrogen among manure sources and especially has much nutrient value (50%) related to nitrogen content. In contrast, other manures, cattle, and poultry generally only have 10-20% of their nutrient value tied to nitrogen with most value coming from phosphorus and potassium. Because of this, swine manure is probably the least likely to be applied to a legume like soybean. Beef and poultry manure also tend to be solid manures, which are more likely to be surface applied, especially in minimal tillage systems; timing the application after a corn crop can offer more residue and protection from runoff losses than applying after soybean (and before corn).

Figure 1. Manure source (by animal species) for the major crops receiving livestock manures.

However, that doesn't mean that swine manure isn't an essential source of nutrients for soybean production, just that it is being used as a multi-year fertilizer for soybeans in a corn-soybean rotation rather than the direct crop receiving manure. In Iowa, approximately 25% of all corn land receives manure, which is almost exclusively applied to fertilizer. However, even though continuous corn is more prevalent around livestock and swine farms than in other locations, most of the land (~80-85%) is in a corn-soybean rotation where manure is supplying P and K for the soybean crop). In Iowa, there are about 12.4 million corn acres, and assuming that 25% receive manure and 80% are in a corn-soybean rotation, there would be 2.5 million acres of soybean being fertilized by manure alone in Iowa. In Iowa, 80% of this would be from swine manure, so two million acres. The two million acres in a single state need to be reported on or captured in how USDA ARMS surveys farmers and collects manure application data. However, it represents a substantial amount of Iowa soybean acres (20%). While Iowa is a leader in acreage impacted in this way, similar patterns would be seen in Illinois, Minnesota, and Indiana, though with lower amounts of manure.

Soybean producers must proactively claim how this circular use of nutrients occurs across multiple years of a crop production system. It impacts the sustainability of soybean production in terms of both greenhouse gas scoring as related to reduced demand for mined phosphorus and potassium fertilizers, savings of energy use for a field pass specific to supply nutrients to soybean production, and a message of improved efficiency and reducing greenhouse gas footprints by more fully considering how manure is used.