Have you ever opened fertilizer recommendations and asked yourself where the heck did they come up with these? After just a few weeks into “officially” working with producers one on one I have been amazed with the wide-range of fertilizer recommendations that come back from labs. This raises the question of how labs come up with fertilizer recommendations. Sometimes recommendations have varied as much as three times as much as producers are used to seeing. Most differences come from P and K, so we will focus on these. Differences arise from the fact that some labs will recommend based on a ‘crop sufficiency’ approach while others follow a ‘build/maintain’ approach. Others will simply follow a crop removal approach.
Understanding the differences between these fertilizer philosophies will explain the difference that is often observed in fertilizer recommendations. Before we look at the approaches lets define what is often referred to as the critical level, which plays an important role in the different approaches. The critical Level is the nutrient concentration that indicates the division between a crop being responsive and non-responsive to applications of fertilizer is termed the “critical level.” For example, the critical level of K is 120 ppm or 240 lb/ac. Let’s define each approach below in regards to P and K:
Crop Sufﬁciency– Sufficiency based fertility programs are intended to estimate the long-term average amount of fertilizer phosphorus required to, on the average, provide optimum economic return in the year of nutrient application. In some years greater amounts of nutrient are required for optimum yield and economic return, while in other years less than recommended amounts of nutrient would sufﬁce. There is little consideration of future soil test values and soil test values will likely stabilize in the ‘low’ to ‘medium’ crop responsive range.
Build-maintenance– Build-maintenance recommendations are intended to apply enough phosphorus or potassium to build soil test values to a target soil test value over a planned timeframe (typically 4 to 8 years). In the case of P we will build and maintain the P soil test level between 20-30 ppm (40-60 lb/ac) and for K we will build soil test levels from 120-140 ppm (240-280 lb K/ac). When the soil test values fall within these ranges you will apply fertilizer based on crop removal rates in order to maintain the soil test values in this range. If the soil test levels are greater than these “build” levels then no fertilizer will be applied or only a small amount of starter fertilizer.
Crop Removal – Fertilizer is applied based on amount of nutrients that will be removed. This takes into account yield goal and amount of P and K that is in the grain or forage.
Understanding these fertilizer recommendation philosophies should help in interpreting results. Visit with the lab and ask what their approach to fertilizer recommendations are and make sure you are comfortable with that approach before sending samples. Also, remember that the recommendations that come back are just recommendations and they need to be customized to your operation. I typically make several modifications when making fertilizer recommendations. Some of the things that alter recommendations are initial soil test levels, if land is owned or rented, no-till vs. conventional till, etc. Having a nutrient management plan is important in improving nutrient use efficiency.
Nitrogen recommendations can be even more confusing but we will cover these in a later post.