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.
