Uniform Cereal Variety Testing Program

Grain Analysis: A Fertilizer Management Tool

Greg Schwab, WSU Extension Soil Fertility Specialist (gschwab@wsu.edu), 3/5/02

One of the speakers at the Direct Seed Conference suggested that grain nutrient analysis could substitute for soil sampling.  This person was advocating what is commonly referred to as a maintenance approach to fertilizer recommendations.  Using this approach, a grower would analyze the nutrient content of the grain and then apply the removed nutrients as fertilizer the following year.  In Table 1, I have listed the “average” nutrient content of the harvested portion of several crops (actual nutrient content can vary as much as 30% depending on nutrient status of the soil, weather condition, weed and disease pressure, and variety grown).  The maintenance approach to fertilizer recommendations is more commonly used in regions where soils are highly weathered and thus have a relatively low supply of essential plant nutrients.  In eastern WA, we are blessed with fertile soils; a typical soil in the Palouse will have approximately 8000 lbs (or more) of Ca in the surface 6 inches.  This is enough Ca for 500 years of 100 bu/a wheat (assuming no more Ca is released through the weathering process and no Ca is lost), and is the reason why Ca fertilizers are not needed in the Palouse.  Consider, for example, an 80 bu/acre winter wheat crop followed by hard red spring wheat.  The winter wheat crop would remove on average 88, 40, 28, 10, 13, and 6.5  lbs of N-P₂O₅-K₂O-S-Ca-Mg per acre plus many other micronutrients.  If the removal rates are used as the fertilizer recommendation for the following hard red spring wheat crop, then N and S would have been supplied at sub optimum rates while higher than recommended rates of P, K, Ca, and Mg would have been applied.  If, on the other hand, the following crop was lentils, all of the nutrients would be applied at rates higher than normally recommended.  With this example it is easy to see that using a maintenance fertilizer program in eastern WA would not only be very expensive for the grower, but could also have a negative environmental impact in the case of nitrogen and phosphorus. 

 The other approach of nutrient management is called the sufficiency approach, and  is commonly used (and recommended) for regions with fertile soils (eastern WA).  Using this method, plant nutrients are only applied at the rate required to obtain maximum economic yield.  This approach maximizes profit while minimizing potential environmental impacts.  In order to accurately predict the fertilizer rate, we must account for nutrients already in the soil (soil test).  In some cases, especially nitrogen, the sufficiency rate will exceed the rate removed by the previous crop.  Higher rates are required because some of the applied nitrogen is lost by denitrification (occurring in wet soils), nitrogen volatilization from the soil and plant (occurring during evaporation and transpiration), or nitrate leaching.  Of course, nitrogen is also applied at higher rates in order to achieve grain protein goals required by millers and bakers. 

 Although grain sampling is much easier than soil sampling, adding nutrients at removal rates will be much more costly than traditional soil sampling, and grain sampling does not provide the soil moisture data needed to accurately predict the yield potential of the following crop.  With that said, I believe that grain analysis in combination with soil analysis can be a very useful tool: to access previous nutrient management decisions, and then used to adjust fertilizer application rates for future crops.  Many hard read spring wheat growers are already doing this by adjusting their N application rates based on the grain protein of the previous crop.

 Table 1.  Concentration of plant nutrients in the harvested portion of the crop. 

            1Legumes get most of their nitrogen from N fixation. 
            2Not available.