Managing Manure Salts: Is it an issue, and when?

 When applying manure, there's more to consider than nutrients and hydrology. While they generally occupy most of my comments, salts in manure can significantly impact your soil and crops, particularly when combined with the wrong conditions. You must consider several key factors to ensure your fields stay productive and your soil remains healthy.

 

How Salts Affect Crop Growth

Crops need a balanced environment to thrive. When the salt concentration in the soil becomes too high, it can interfere with the plant's ability to absorb water. Water moves from low salt concentration to areas of high salt concentration—a process called diffusion. Plants use this process to their advantage, maintaining higher ion concentrations in the plant than the surrounding area and wanting to draw water in – a process called osmosis. When the soil around the roots is salty, plants struggle to take up water, even if there's plenty of moisture in the soil (saline). Such conditions can lead to water stress, reduced growth, and lower yields.

 

Crop Tolerance to Salts

Different crops have varying levels of tolerance to salts. For example, corn has a moderate tolerance, with a threshold of about 1,700 microS/cm in the soil water. Once the salt concentration exceeds this level, you'll likely see a drop in yield. Other crops, like soybeans, are even less tolerant (1,000-1,500 microS/cm) and can be affected at lower salt levels. Understanding the salt tolerance of the growing crops can help you make better manure and water management decisions.

 

Impact on Soil Structure

High salt levels can also damage soil structure. Sodium, a common component of salts, can cause soil particles to disperse, leading to poor water infiltration and drainage (sodic). Such soil dispersion can create a hard crust, making it harder for crops to establish roots and access nutrients. Over time, this can reduce soil fertility and make it more challenging to manage your fields.

 

Understanding Salt Leaching in Your Fields

In Iowa, we average about 26 to 38 inches of rain annually. Around 4-8 inches typically drain away through tile or natural soil drainage. This drainage helps remove salts from the soil as they leach from the root zone and, for the most part, prevents them from building up to harmful levels. However, adding irrigation water higher in salt or applying manure with high salt content can change this. In particular, the leaching fraction is often used to help control salt levels within a field as the soluble salts will move with the water.

 

To calculate the Leaching Fraction, Use the formula

Leaching Fraction = (Drainage Water) / (Total Water Applied)

 

Let's start to think about these balances throughout Iowa. In that case, we'll average around 4 inches of drainage to 26 inches of water in Northwest Iowa and 8 inches of drainage to 38 inches in southeast Iowa. The leaching fractions are 15-21%

Suppose you rely only on rainfall; about 15-21% of your water leaches and removes salts. When you add irrigation, this fraction can change. However, even with more water applied, your soil might not drain any better, especially if it's naturally slow-draining or you are using more water to meet the transpiration demand of your crop, i.e., the water is evaporated or transpired. Even though you added water, the leached volume didn't change.

 

Balancing Salt Application with Crop Tolerance

When applying manure or irrigation water, limiting the salt levels to what your crops can tolerate is essential. For example, corn has a salt tolerance level of around 1,700 microS/cm. Soybeans are less salt tolerant at around 1,000 – 1,500 microS/cm. To understand what salt levels you'd expect, you need to know the salt added to the soil through irrigation or manure and estimate what leaching removes.

Think of it as a balance: the amount of salt you add should equal or lower than the amount you could expect to leach out. If you're leaching 4 inches of water with a salt level of 1,000 microS/cm, you can safely apply an equivalent amount of salt. But if you add more, especially during dry years, you could set yourself up for trouble. It's often a good idea to be conservative with your salt applications—cutting them in half gives you some breathing room in case of a dry spell.

 

Timing is Everything

Timing your applications is crucial if you're trying to manage salty wastewater. You'll want to avoid applying salty water during critical growth stages, like when your corn is tasseling or forming ears. Instead, aim for the spring or fall, when rainfall is more likely to help dilute and leach those salts. There's also a difference between using water for irrigation and disposing of salty water. When irrigating, the goal is to supplement the crop's water needs, which means you're adding water when the soil is dry. But with salty wastewater, it's better to apply when there's more water in the soil to help with dilution.

 

How Much Salt is in Manure?

The range of salt in manure is highly variable, but a typical range from deep pit swine manure may be 5,000 to 15,000 microS/cm, while dairy manure may range from 2,000 to 10,000 microS/cm. Regarding application rates, deep pit swine manure rates are often 3,000-6,000 gallons per acre (0.11 to 0.22 inches), while dairy manure is often 10,000 to 15,000 gallons per acre (0.37 to 0.55 inches). At first glance, this can look concerning as these concentrations are much larger than what we said plans could handle, but as they get mixed in with rainwater and flushed with leaching, so where do we end up?

For the examples, we will assume dairy manure at 10,000 microS/cm at 0.55 inches as it has the highest salt loading. I'll also work with the Northwest Iowa data and take 4 inches of drainage to 26 inches of water. We'll work with a 5,000 microS/cm limit to be safe.

Drainage: 4 inches (5,000 microS/cm) = 20,000 (microS/cm)(inches)

Added: 0.55 inches (10,000 microS/cm) = 5,500 (microS/cm)(inches)

Essentially, we are checking that potential salt removal would be higher than salt additions, and if that is the case, we are in good shape, at least over the long term.

If you are willing to look over shorter periods, things can change. For example, from 2021-2023, parts of Iowa were about 15 inches below average on moisture, or about 5 inches a year. As a result, drainage water fell drastically, in the worst places averaging about 1 inch of drainage. If you take that change, it would have only leached 5,000 (microS/cm)(inches) per year, and that means we'd have added to our salt pool and maybe had an issue in some parts of fields.

All this illustrates that if you live in an area where less drainage occurs or have poorly draining fields, the issues of salts from added manures can become an important consideration in your management plan.

 

A Few Practical Considerations for Farmers

Monitor Your Salt Levels: Keep an eye on the salt content of your manure, especially during drier years.

Calculate Your Leaching Fraction: Use the formula Leaching Fraction = (Drainage Water) / (Total Water Applied) to estimate how much water—and salts—are being leached out. It is more coincident that it may be an issue if dry years start stacking together.

Consider Soil Type: Remember that poorly drained soils may not leach salts as effectively, increasing the risk of salt buildup. You may need to adjust your application rates or frequency in these cases.

While for the most part, salt buildup in most Iowa soils won't be a concern with manures, as you think about other practices like irrigation, poorly drained soils, or working with different industrial by-products that may have

 

By understanding the balance between salt application and leaching, you can manage manure salts more effectively, ensuring healthy crops and long-term soil productivity, as this is a case where an ounce of prevention is worth a pound of cure.