Salt Index – the Bigger Picture

Dietmar Walch, Director of Technical Services • October 14, 2019

High rates of fertilizer placed with or near the seed when planting can cause seed burn. The reason this happens is that fertilizer materials N, P, K, S etc release salt into the soil solution surrounding the seed. Water moves from zones of low to high salt concentration (osmosis). Therefore, when the salt concentration surrounding the seed zone is high, the seed is more likely not able to access the water it needs to grow a healthy root and plant. The young plant may wilt and die from lack of water. The measure of salt index is osmotic pressure, which indicates the strength of the salt solutions in competition with plant roots for water. The various fertilizer materials have different salt indexes, which indicate the likelihood of fertilizer burn.

… The lower the salt index, the less likely a fertilizer material will cause root burn …

How is salt index measured?

Although Salt Index (SI) represents the osmotic pressure that is exerted by a fertilizer product, this is very difficult to measure. Hence the fertilizer sector came with the Sodium Nitrate reference system. Sodium Nitrate has a value of 100 and all other fertilizer components are gauged accordingly. Using the Sodium Nitrate standard, the salt index for a blended fertilizer can either be calculated or measured.

Calculation method

The SI of a blended fertilizer product is the sum of the salt Index of each component per unit of plant nutrient times the number of units of that individual component. Component values are obtained from the known or published values as shown in table 1. It is important to note that these values are referenced. The two standard ways the values are referenced are either per equal weight of material or per unit of nutrients (20 lbs) as shown in the Table 1. Some companies however also reference to a unit of volume or other factors which will vary the numbers. It is also important to note that these referenced values are based on a particular source of product and as these sources change so can the actual SI change. This can lead to discrepancies between calculated and measured salt index values of a blended fertilizer product.

Measuring method

This SI method involves taking the conductivity of a 1% fertilizer solution of the product. This value is then reported as a SI value also based on the Sodium Nitrate reference system. The results of this testing can vary significantly based on how accurate the 1% solution is measured and how the instruments are calibrated. Some also report measuring the electrical conductivity of a 0.1% solution. Any other impurities in the fertilizer that is being measure will also have an effect on the electrical conductivity of the solution that is measured. Hence the source of the product can also have an impact on this measured salt Index. Table 2 shows the wide variation that can exist in values base on differing product sources or measuring methods to obtain a given salt index.

Important factors

Was the SI calculated or measured?
What unit of measure is the salt index number referenced to?
What is the concentration of fertilizer?
What is the application rate?
What is the proximity of the fertilizer to the seed?

In summary

Not all salt index values are apples to apples comparisons. Published salt indexes are guides not absolutes. Trust in history and reputation of your supplier.

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Post-Harvest Reflections

December 12, 2024

We are at the point in the year where things are slowing down around the farm and it is time to reflect on the season to remind ourselves what we learned. Over the last couple of weeks, the District Sales Managers and I have spent some time reflecting on what worked well in the field and the challenges we faced too. Here are the top 5 things we learned from 2024 growing season: #1 Split applying Nitrogen and Sulfur is a practice that should be embraced on a yearly basis when growing corn. Do you remember how the planting season started and what happened in the first 60 days after the planters started rolling? Our field conditions started out dry with some areas of the corn belt showing up on the drought map. Then mother nature blessed us with rain that never stopped in some areas making it difficult to finish planting. Growers that had split applied their Nitrogen and Sulfur were able to avoid nutrient loss from the rain but also apply the nutrients when the crop needed it most. In the end yield was better on split applied nitrogen fields compared to fields that had all the N applied in the fall or spring before planting. #2 For the past 2 seasons in the North region, we have seen the benefits of adding NACHURS Humi-Flex FA to the in-furrow starter fertilizer. NACHURS Humi-Flex FA is our fulvic acid and it is becoming a valuable component in our starter fertilizer for many reasons. We are using it to chelate the nutrients in the starter fertilizer especially phosphorus. Humi-Flex FA also buffers the soil pH to neutral in that nutrient band keeping nutrients available in fields that have less then optimal pH for crop growth. Finally, we use Humi-Flex FA to create a healthy soil microbiome that leads to more root mass and nutrient uptake. #3 Nutrient uptake charts for corn and soybeans have been a great tool for our team to help our customers visualize the crops nutrient needs based on timing during the growing season.