Soil Variability, Carbon Variability, Nutrient Variability

Jim Millar

Soil Scientist, USDA-NRCS

Redfield, SD

james.millar@sd.usda.gov

Soil Variability

The Natural Resources Conservation Service has mapped over 500 different soil types in South Dakota. There is a large amount of soil variability across the state, but how much soil variability exists within each field? With the advent of variable rate technology there is great interest in measuring the amount of soil variability within a field. Soil variability is the primary source of yield variability within a field. When we discuss soil variability within a field there are a large number of different soil parameters to examine. The best method to evaluate the amount of soil variability within a field is to examine what soil properties have the biggest influence on soil moisture and soil nutrients, as these are the two major soil factors controlling crop yields. The three major soil properties that influence soil moisture and soil nutrients are: 1) soil texture (the amount of sand, silt or clay); 2) soil organic matter; and 3) salt levels. If any one of these soil properties varies to a great extent in a field there is enough soil variability in the field to examine the idea of precision farming and variable rate technology.

The major limitation to crop production in South Dakota is always soil moisture, typically too little moisture and sometime too much moisture. The available water capacity of the soil is controlled by soil texture, soil organic matter and soil salt levels. A soil with a silt loam or silty clay loam texture with high levels of soil organic matter and no salts will hold up to 2.5 inches of plant available water per foot of soil. Likewise a soil with loamy sand texture with low levels of soil organic matter and no salts will only hold approximately 0.75 inches of plant available water per foot of soil. Salt can have a major impact on the amount of plant available water. As the salt levels increase in the soil, the available water capacity of the soil will decrease.

Soil texture and percent organic matter also play a large role on the cation exchange capacity of the soil. Soils with higher clay textures and higher amounts of soil organic matter will have a higher cation exchange capacity. Soils with a high cation exchange capacity will have a higher number of exchange positions to hold onto essential cations like phosphorus and potassium.

It has already been mentioned that soil organic matter has a major influence on the available water capacity and cation exchange capacity of the soil, but soil organic matter also plays a major role on the aggregate stability values. Soils with higher aggregate stability values are less prone to erosion, have higher infiltration rates, and better water and air movement into and through the soil profile.

So how much soil variability do you have in your fields or how much does the soil texture, soil organic matter or salt levels vary across the field? There are no set procedures for creating soil management zones. There have been a number of different methods created to determine the soil variability within a field. The most common methods consist of one or more of the following tools: 1) Crop yield maps; 2) NRCS soil survey maps; 3) Aerial imagery; 4) Satellite imagery; 5) Soil electrical conductivity mapping (Veris); and 6) Detailed topographic maps. All these different tools have there advantages and disadvantages. A method that may work great for one producer may not work for another producer.

Typically the more tools included in the creation of soil management zones the more accurate the zones. I would caution against using just one tool to create the soil management zones. I believe that the secret to creating accurate soil management zones is to use a multiple of different tools and to field verify or validate the data or information these tools are providing. The biggest challenge I see with this variable rate technology is not the variable rate equipment but creating accurate soil management zones. It is very difficult to make Variable Rate Technology profitable if the soil management zones are not accurate. The two major inputs that are capable of being applied at variable rates are fertilizer (mainly nitrogen and phosphorus) and seed. If the soil management zones are not accurate, the producer is applying the incorrect amount of fertilizer or seed and thus it is possible to do more harm than good with variable rate technology.

So how do you evaluate the accuracy of the soil management zones? The best indicator I have seen to indicate the accuracy of the soil management zones is to closely review the soil test results from each zone. Soil test results should tell a story about the variability of each zone. If an agronomic business sets up five management zones in the field and three of the zones have nearly identical soil test results I would speculate that there are only three different zones in the field and not five. Do not just look at the nitrogen, phosphorus and potassium numbers, but also review the pH, organic matter, and electrical conductivity data. The calcium, magnesium, sodium, zinc and sulfur data can also be a good indicator of the variability of each zone.

Carbon Variability and Nutrient Variability

The variability of the soil organic matter or soil carbon levels has a large influence on the nutrient variability in a field. Approximately 60 percent of soil organic matter is comprised of carbon. All thirteen mineral elements are released from organic matter or organic carbon decomposition. The amount of organic carbon in the soil plays a large role on the amount of plant available nitrogen, phosphorus, sulfur, and zinc in the soil. Table 1 is a good example of the impact that the soil organic carbon has on the soil fertility levels. The field was separated into four different soil management zones based on soil properties and landscape. The individual soil management zones were sampled separately for soil fertility analyses. The nitrogen and sulfur results are for 0 to 24 inch depth samples, the other analyses are from a 0 to 6 inch depth.

Table 1. Influence of Soil Organic Matter (Carbon) Levels on Soil Nutrient Levels.

Sample pH SOM EC N P K S Zn Ca

mmhos/cm lbs/ac ppm ppm lbs/ac ppm ppm

Upland 6.4 3.9 0.1 28 12 479 236 1.32 2226

Eroded 8.1 2.9 0.3 11 5 143 34 0.27 4648

Swale 6.8 4.1 0.2 37 17 551 154 1.07 2696

Saline 8.0 3.0 3.3 113 26 315 480 0.75 4972

The lowest soil organic matter (SOM) levels in the field are found on the eroded knobs, which is what would be expected. Likewise the highest soil organic matter levels are located in the swale position of the landscape. The highest pH levels are located on the eroded knobs and saline areas in the field, which is very common. The eroded knobs typically have a pH of at least 7.4 and presence of calcium carbonate salts (notice the high levels of calcium, Ca). It is typical for all soils in South Dakota to have higher pH values in the subsoil than the surface layer. If the surface layer is removed via erosion the 0 to 6 inch depth of soil on the eroded knobs that is being analyzed is actually the original subsoil and thus the reason for the higher pH values on the eroded knobs.

It was stated earlier that organic matter content has a large influence on the nitrogen, phosphorus, sulfur and zinc levels in the soil. Table 1 indicates that the lowest values of all those nutrients, including potassium, are located on the eroded landscape in the field.

The secret to crop production is soil organic matter, not only for its nutrient supplying ability, but also for its large role in the soil water holding capacity, soil aggregate stability, and soil biology functions.