This blog is a continuation of a series of other blog posts entitled, Basic Cation Saturation Ratios (BCSR), Soil Balancing, Desired Values and Ratios as well as What is Cation Exchange Capacity (CEC)?

 SLAN or Sufficient Level of Available Nutrients is a philosophy for Phosphorus (P) and Potassium (K) fertility that focuses on applying the minimum amount of fertilizer needed to maximize profitability in the year of application, with no concern for future soil test values or fertilizer requirements. Generally, recommendations based on nutrient sufficiency will provide 90% to 95% of maximum yield and a high rate of return per unit of fertilizer applied. In more simple terms, SLAN is fertilizing the crop, with rates based on likelihood of achieving a yield response.



Build and Maintain fertility programs are different from the SLAN approach in that they are not intended to maximize economic returns in any given year. Rather, they are designed to provide flexibility and consistent economic returns over the long-term by removing P and K as yield-limiting factors. Fertilizer applications are designed to raise the soil test values to the “optimal” soil test range where they will provide 100% of maximum yield, with low risk of yield loss due to insufficient fertility. This requires applying “build up” levels of nutrients as well as crop removal rates (or maintenance rates) of fertilizer to get to the desired range and stay there. A simple definition of Build Maintain is fertilizing the soil with rates based on increasing soil test values to defined level, then adjusting the rate to maintain soil test values at the critical level.

It can be argued that SLAN and Build/Maintain can be similar in some ways, both making an application of fertilizer to keep soil test levels where they are and feed the crop to be grown. However, SLAN is geared more towards achieving a fertility level where maximum yield can be obtained in that year where Build/Maintain is trying to keep the soil test levels from dropping.

SLAN can be more attractive to farmers when fertilizer prices are higher compared to grain prices, when resources are limited, in cash rent or short-term lease agreements and in soils where it is difficult to build soil test levels or soils that have a high capacity to fix P and K. The result of a SLAN approach is a lower fertilizer bill in the short term at the potential risk of limiting long term productivity.

Build and maintain is more attractive to farmers when grain prices are high compared to fertilizer prices, in times when resources are readily available, when fertilizer prices are expected to increase in the future and on ground that is owned or has a long-term lease. The result of Build and Maintain is a soil that is very unlikely to be yield limited by P and K. However, fertilizer cost can be quite high in the build phase, especially in soil testing very low in P and K and in sub economic returns in yield limited years.

Both the SLAN and Build and Maintain philosophies rely on fertility values derived from research specific to a crop to be grown and soil nutrient inventories. These values have been identified by most all the major land grant universities. While the numbers do vary slightly from university to university, P and K rate recommendations provided by most university extension services incorporate elements of both SLAN and Build Maintain strategies. Some have strict interoperations of each, while others blend the two philosophies.

The SLAN critical values for phosphate on corn and soybeans is around 20 to 40 lbs. of P. For potash it varies between 200 and 240 lbs. of K depending on soil type. For Build Maintain, the optimum values (from Chapter 8 Illinois Agronomy Handbook) are to build to a soil test level of 40-50 lbs. of P (for most of S. IL) and maintain between 40-70 lbs. of P. For K, to build to 260 or 300 lbs., depending on soil type and to maintain between 260 and 400 depending on soil type.

So, which one is right? Both, neither one is wrong.

Is one better than the other? It will depend on your situation and management based on the factors listed above.

How should I approach P and K fertility on my farm? Here are a few thoughts on how I would approach spending my fertility dollar.

  • Take soil tests. You can’t decide on using SLAN or Build Maintain without a soil test.
  • You also need to resample your soil on a regular basis to evaluate how the strategy you adopt is working.
  • Always fertilize when soil test levels fall below the optimal range. The risk of yield loss is high, and the ROI fertilizer is greatest for very low- and low-testing soils.
  • Avoid application on high-testing soils and never apply on soils that test in the very, high range. There is no ROI on fertilizer application in these situations.
  • Use your yield monitor to apply removal rates in fields that test below the critical value. Put that fertilizer back where it came from.
  • If you don’t have a yield monitor, use removal rates to apply fertilizer on fields that are not excessive in soil test levels.
  • Adapt and utilize the 4R Nutrient Stewardship principles of fertilizer management

What is the 4R approach?

The 4R nutrient stewardship principles are the same globally, but how they are used locally varies depending on field and site-specific characteristics such as soil, cropping system, management techniques and climate. The scientific principles of the 4R framework include:

  • RIGHT SOURCE – Ensure a balanced supply of essential nutrients, considering both naturally, available sources and the characteristics of specific products, in plant available forms.
  • RIGHT RATE – Assess and make decisions based on soil nutrient supply and plant demand.
  • RIGHT TIME – Assess and make decisions based on the dynamics of crop uptake, soil supply, nutrient loss risks, and field operation logistics.
  • RIGHT PLACE – Address root-soil dynamics, nutrient movement, and manage spatial variability within the field to meet site-specific crop needs and limit potential losses from the field.

In the end you can’t go wrong with either SLAN or Build Maintain. Both are proven soil fertility methods that incorporate both economics and yield response into their recommendations as well as being the basis for a good solid 4R approach to nutrient stewardship.



(Parts of this blog were adapted from the Fertilizer Institute, 4R Stewardship website, OSU publication Fertilization Based on Sufficiency, Build-up and Maintenance Concept and Pioneer Crop Focus Phosphorus and Potassium Fertility for Corn and Soybean.)

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About the Author: Kelly Robertson

Kelly Robertson has been a soil fertility agronomist and precision agriculture consultant since 1989 and also spends time in farm/agronomy management roles for farms in Southern Illinois. In 2012, Kelly and his wife Lori started Precision Crop Services in Benton where they provide agronomic services for their customers including soil testing, crop scouting, data analysis, GPS/GIS services including variable rate seeding and fertility recommendations as well as farm and agronomy management for their customers. He is a Certified Professional Agronomist, Certified Crop Advisor, Certified 4R Nutrient Management Specialist, 2015 Illinois Soybean Association Double-Crop Specialist, 2016 Illinois CCA of the Year and the 2021 Illinois Soybean Assoc. Dave Rahe Excellence in Soils Consulting Award winner.

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