Manipulating Soil Salt Content

Keeping salt levels in soil at reasonable levels is of utmost importance. Too much salt and trees shut down. Too little salt and not enough water is taken up (photo courtesy Rex Dufour, NCAT.)

Harvest winds down and many of us love to celebrate with an ice-cold beer. If that beer is at a favorite watering hole, you’ll likely reach for a few peanuts, pretzels or bar mix in the bowl in front of you. Have another handful of snacks, and you’re certainly going to need another frosty long neck.

Ever wonder why they give you that free salty snack on the bar? It makes you thirsty. As a former starving college student who got wrapped up for a time in the restaurant business, I thought, “That was a pretty cool trick. Make your customers thirsty when they come in, and they’ll drink more.” But if you overdo it, you have to deal with unruly customers and sober up your clients. Now imagine your trees are your customers and you are the barkeep. How do you keep them drinking but not overdo it? We have to manipulate the salt. Too much and they simply shut down. Too little and they don’t drink enough.


Salts, Not Just Sodium

When we talk about salt in our soils, most farmers are usually referring specifically to sodium. We often forget to realize most fertilizers are a salt. It’s not just sodium that matters, but that can wreak the most havoc in many soils. The salinity index of a fertilizer is more important than just calling a fertilizer a salt. Salinity index measures the increase in ECs in a soil solution. We compare the number created from a salt solution made with a particular fertilizer to a similar solution made with sodium nitrate. Sodium nitrate is 100% soluble and was chosen as the standard when the index was created.

A fertilizer with a high salinity index will create a stronger osmotic pressure in soil solution and pull harder on water than the roots can. A lower index will allow a fertilizer to flow more easily into the plant tissue. It’s all about creating concentrations. Salinity concentrations are dynamically induced to flow from high to low to create an equilibrium. When we add salts to our soils, we can very easily manipulate the ground to pull harder on the water. This is NOT an ideal situation. It becomes way too hard for trees to properly “drink” in that hydration. When we fertigate, we are adding salt to our soils. Think about the old-school way of feeding our plants; (three) 50-unit shots of nitrogen fertilizers!? Remember the salt used to standardized the index at 100 is a nitrate salt!

Let’s take another look at the soil solution. So much of our water is already saline and many times sodic. We are constantly rinsing our roots with salt water. If we have treated our water for pH, we are able to disassociate the ions from each other in the salt solution. When the hydrogen reacts, the off gas is CO2. We can be left with ions like calcium, magnesium and potassium. However, most times, unfortunately, we are left with the more readily soluble sodium ion. It’s small, the same charge as potassium and very mobile. A high percentage of sodium in the solution will block the cation absorption of the other nutrients. So, how do we get it out? Leach it.


Keep Soil Moist

As we roll into fall, don’t let the soil dry out. Even if it’s only pulsing small shots every week to only 12 to 24 inches, keep the soil moist. When Mother Nature decides to play nice, she will bring clean and below-neutral pH rain water. This lower pH will break apart more of the salts in our soil. Since sodium is more easily disassociated, it will be free first. As calcium is solubilized, it will have a double positive charge. As it adsorbs onto the soil colloid, it will displace two single charged sodium ions. Same as magnesium. Potassium is a single charge, so it’s one-to-one with sodium in leaching. Out they go with gravity as water moves down through the profile. Get it out of the root zone. If you need to add more soluble calcium, magnesium or potassium to your soil, do it right before a rain if you can. Smaller shots work better. Over-applications will create another imbalance.

I make the argument often that over-applications can create more havoc than the benefits we were trying to achieve. If you look at a soil with a perfect 68% base saturation and 2,000 ppm of measured calcium, you may not need to add that nutrient. Two thousand ppm is 4,000 pounds of calcium every six inches. Take it to two feet and we have 16,000 pounds. And that is just what etched off with the acetate extraction that day.

That’s not the total load of calcium in the soil! A typical permanent crop will use the same amount of calcium in a season as nitrogen. If you applied 150 units of N, a tree will utilize 150 units of calcium. A typical fall soil amendment application of 4,000 pounds of dry mix at 23% calcium calculates into 960 pounds of calcium. By my calculations, you’d be good for six years in a perfect world. And that doesn’t take into account the 16,000 pounds we already had there!

What have you just done to your K balance? Let’s look at potassium. Often, we apply 400 pounds of SOP in the fall (the 200 pounds we calculate a typical almond crop to use.) But, we did it after the demand has dropped for K. Left on top of the soil, how much actually got into the tree before dormancy? Now the rains come December through March, pushing a small ion (same size and charge as sodium) right past the roots. As potassium demand picks up in the spring, we have rinsed whatever became soluble through our root zone at a time the tree couldn’t drink it in. Now, it’s not there in the spring. We also increased the ECs of our soil right on top as the water flowed right on by.

Let’s rethink this. Before we overdo many of our applications all at one shot, let’s time them to meet the demands of the customers. Serve them when they are thirsty. Add just enough salt to keep them drinking. Don’t overdo it, or they stop drinking. As the rains come, get the ground wet so Mother Nature can rinse our soils clean with better water. When we overserve our customers, it’s our responsibility to clean up the mess.