The way that this test works is it takes five key results for a nitrogen and sulfur recommendation, five key results for a phosphorus recommendation, and five key results for a potassium recommendation. Each measurement that is taken is given a star rating, this rating correlates back to the results that have influence on the nutrient efficiency rating in each category. Once we have an efficiency rating for each individual component, these are averaged to create an overall efficiency rating; with this rating and the yield goal a nutrient recommendation is created. Below we will go over the components of each section that are used to formulate the average. The factors are not listed in any order and at this point in time all the factors are weighted the same, as we get more and more research completed we will begin to weight the factors to get an overall better average.
Nitrogen and Sulfur
Organic Matter has time and time again proven that the higher it is the less commercial nitrogen and sulfur is necessary. We believe this is the case because high Organic Matter provides several positives in the soil system. The first being it houses our beneficial microbial population. The second point being it helps keep our soil structure more friable and granular to allow plants growing to have greater root mass and penetration, therefore more exploration and a greater efficiency at finding nutrients. Last but not least with higher Organic Matter soils we have increased nutrient holding capacities and the soil is more buffered to changes in pH and to a lesser extent other chemical properties.
SLAN is a measurement of organic nitrogen that is a component of amino acids or proteins found in the soil. These amounts have proven to be a slow moving number that doesn’t change quickly. This amino acid/protein N becomes slowly available over the year and when we have a high result here it gives us a larger pool of N to pull out of throughout the summer.
Solvita is a measure of biological activity, this measurement is made by drying a soil entirely and then rewetting in a sealed container with a carbon dioxide sensitive paddle that changes color based on the carbon dioxide content of the environment. The way this works is when the soil is dried down the microbial life goes dormant, when it is rewetted the microbes come out of dormancy and begin respiring and release CO2, so the amount of CO2 that is released can be correlated back to biological activity. It has been proven that the more biological activity in a soil equates to higher organic nitrogen mineralization.
VAST is a measurement of aggregate stability, this is how well the soil aggregates hold up against water (Water Stable Aggregates). This is a gauge of soil structure and can be used determine everything from susceptibility to crusting to root exploration. The higher percentage Water Stable Aggregates the less prone we are to crusting as well as being able to expect greater root mass/exploration which in turn leads us to retrieving nutrients more effectively. The amount of Water Stable Aggregates also gives us an idea on how constant the environment is for the microbial population below the ground.
C:N Ratio is a comparison of Water Extractable Organic Carbon (WEOC) to Water Extractable Nitrogen (WEN). WEOC and WEN are both necessary for microbial life but when the ratio either gets to low or to high we begin to run into some issues. When the C:N Ratio gets high >15 (meaning for every 15 parts of carbon there is 1 part nitrogen) the microbial life is going to tie up a lot of nitrogen trying to breakdown the carbon in the system, this causes our nitrogen efficiency to go down meaning it will take more nitrogen to grow a bushel of corn. When the C:N Ratio gets low <8 the microbes will not tie up very much nitrogen at all when they are breaking down the carbon which will increase nitrogen losses to leaching and denitrification, but also gives a greater ability to pick up nitrogen earlier in the season.
Olsen or Bray P1—These are both considered industry standards for the measurement of available phosphorus. While these measurements are not related to soil health they still bring a great deal of time tested information to help determine the need for phosphorus applications. The difference between the Bray extraction and the Olsen extraction is the soil pH’s that they are useful on. Bray for instance is very well calibrated on and effective on soil pH’s at 7.4 and below, this is because the Bray extract is an acidic extraction and if a soil is alkaline or contains a lot of free lime (calcium carbonate) the extract is neutralized before phosphorus is extracted. Olsen on the other hand is an alkaline extraction which makes it more effective on soil’s with a pH above 7.4 than Bray. The results returned for Bray and Olsen are similar but Olsen tends to have a slightly smaller result by a couple of ppm (unless the bray extract is neutralized by the soil pH). Due to this variance in results Bray results and Olsen results are not compared on the same scale.
Solvita—Much in the same way Solvita influences nitrogen rates it influences phosphorus rates. As Organic Matter is consumed by microbes , organic phosphorus is converted into an inorganic plant available form. Another aspect of microbial life that influences phosphorus uptake is mycorrhizal fungi. Mycorrhizal fungi are a long filament type fungus that creates a huge network throughout the soil profile and when they come in contact with the root of a plant.
P-Saturation stands for phosphorus saturation of the soil. Phosphorus can be tied up in many ways within the soil. P-Sat is a result that is comparing the P flushed from the soil using a very simple and weak extract (H3A) to that amount of Iron and Aluminum flushed from the soil. This is important in understanding the true availability of the phosphorous in your soils system.
pH—The ideal pH for most nutrient availability is neutral 6.7-7.0. Nitrogen and sulfur are nutrients that are effected the least by pH. Phosphorus is however effected the most. The higher pH soils (more alkaline) tend to tie up phosphorus and make it un available to the plant. The phosphate ions tie up with calcium and magnesium to make up less soluble compounds. However, when a soils pH is significantly lower (acidic) iron and aluminum tie up phosphorous making less soluble compounds.
VAST is a measurement of aggregate stability, this is how well the soil aggregates hold up against water (Water Stable Aggregates). This is a gauge of soil structure and can be used determine everything from susceptibility to crusting to root exploration. The higher percentage Water Stable Aggregates the less prone we are to crusting as well as being able to expect greater root mass/exploration which in turn leads us to retrieving Nutrients more effectively. The amount of Water Stable Aggregates also gives us an idea on how constant the environment is for the microbial population below the ground. If we think about run off this is more prone when VAST results are low, Phosphorus moves less in the soil then most nutrients making P very prone to be moved by heavy rainfall or erosion. VAST also gives us some indication (not always directly) if the soils bulk density is high or low. The more air space in the soil the better our roots will explore the soil cavities. Since P is not very mobile this equates to better P utilization.
Ammonium Acetate ppm—This test result has been widely used as the most important way of evaluating yours soils supply of potassium. Many labs extract with other extractions as well, we use ammonium acetate as we feel this is the most validated method of extraction.
K% Base Saturation—This result explains what percent of the soils exchanges sites are occupied by K. We utilize this result before we make any potassium fertility recommendations. This helps us understand the K availability, especially in excessively dry years. Some Universities stand by the fact that Base Sat K should not be involved with a fertilizer recommendation, but we find in South Dakota soils that this does have value. The higher the CEC your soil has the more available K is needed. This may be somewhat related to the fact that root exploration is generally less in high CEC soils and on our hill tops etc. We don’t over value this result, but we feel it needs to be part of the equation.
Na% Base Saturation is calculated the same way that all other base saturations are calculated. We look at this result and compare it vs the K%. The reason we feel this result can affect K uptake and should affect our K fertility program is associated with the following reasons. 1) in the soil Na and K compete for plant uptake and can easily be interchangeable as the plant has a tough time differentiating the two. The second reason is due to how Na reacts within the soil, Na destroys soil aggregates and makes soils very tight and poorly drained, which both inhibits root growth and K uptake.
Mg% Base Saturation—Higher % Mg in your soils limit soil aeration and oxygen levels within the soil profile. These are both needed for root respiration and K uptake. Higher Mg% soils also tend to be tighter in structure which limits root exploration of the soil.
VAST is a measurement of aggregate stability, this is how well the soil aggregates hold up against water (Water Stable Aggregates). This is a gauge of soil structure and can be used determine everything from susceptibility to crusting to root exploration. The higher percentage Water Stable Aggregates the less prone we are to crusting as well as being able to expect greater root mass/exploration which in turn leads us to retrieving Nutrients more effectively. The amount of Water Stable Aggregates also gives us an idea on how constant the environment is for the microbial population below the ground. Another reason why VAST can directly affect potassium uptake is the fact that better aggregate stability also leads to better moisture infiltration at the soils surface. Potassium availability and moisture availability are directly related with one another.