• Soil chemistry, types and amounts of clay, soil temperature and soil moisture content can all affect nutrient availability.
• Soil phosphorus availability is driven by soil pH 6.0-7.0.
• The three states of soil-P and their dynamics are complex, with the solution-P replenished throughout the growing season.
• The amount of plant available soil-P at any given moment is determined by several different parameters ultimately defining the benefits obtained from P-based starter
• Soil-K availability is governed by types and amounts of clays present, CEC, and soil pH.
• Low K soil test levels, low CEC and pH < 6 represent some situations where K based started fertilizers may be of benefit.

The specific field conditions when starters provide positive yield responses is largely governed by the soil chemistry, types and amounts of clays present, soil temperature at emergence and soil moisture content. The soil cycling of crop nutrients are both chemical and biological in nature.Chemical and biological processes are driven by three factors: soil temperature (determines speed of process), soil moisture and soil aeration. These factors will affect plant uptake, in addition to oxygen for respiration and oxidation of nutrients to available plant forms.

Soil phosphorous (soil-P) is found in one of three forms: solution-P, adsorbed-P and precipitated-P.Soil replenishes solution-P continually throughout the growing season.The amount of total soil P held in these forms is relative to soil pH, soil temperature and soil P-test level within the field.Soil pH < 6 or > 7 results in more P- precipitates out of solution, forming compounds of iron and aluminum at lower pH and calcium and magnesium at high pH.Soil-P movement to the plant is via diffusion which is the movement of ions from an area of higher concentration to an area of lower concentration.As P concentrations near the plant root surface decreases from plant uptake, higher soil-P concentrations further away from the roots move closer to the plant roots. In the spring as soil temperatures increase, soil pH governs the amount of soil-P in solution and available to the plant. True soil-P-deficiencies rarely occur under soil temperatures of 60⁰F+.An equilibrium does exist between soil solution-P, adsorbed-P and precipitated-P and varies with soil-pH, soil temperature and soil moisture content.Soil pH for maximum plant availability of P is between pH 6.6-6.8.The amount of plant available soil-P at any given moment is determined by several different parameters ultimately defining the benefits obtained from P-based starter:

• Low soil P-test level (most states <35-40 lb./acre)
• Cold soil conditions (<60⁰F) at planting that extend through VE to V2
• Soil pH’s < 6.0 or > 7.0
• Low clay content soils and/or low % Organic Matter soils
• No-till or strip till practices leading to cooler and more moist soils
  • Low K-soil test level (most states < 250-300 lb./acre)
  • Low CEC soils incapable of holding K⁺ cations
  • Soils with pH < 6.0 where H⁺ cations dominate CEC
  • 1:1 clayey soil where attempts at building soil-K test levels have failed

Soil potassium (soil-K) can be found in one of three forms: solution-K, adsorbed-K attached to CEC, and fixed-K between clay platelets. Soil cycling of solution-K and adsorbed-K is mostly a seasonal process requiring shrinking and swelling of clays via the addition and loss of soil water and/or freezing and thawing. Clay structure determines the amount of fixed-K in a soil and how easily fixed-K is released to one of the two other soil forms.Older weathered clays (1:1 clays) shrink and swell to a lesser degree due to hydrogen bonds, resulting in greater ability to fix-K.In soils made of these clays addition of potassium fertilizer often results in little or no building of the soil test level.Like soil P, solution-K reaches the plant roots by diffusion. In the absence of a crop to draw up solution-K, K can leach downward in the soil profile until it adheres to a cation exchange site.Likewise, soils with low CEC’s (low clay and/or low % organic matter) have limited ability to retain applied K.Cations have a defined order of attraction to the CEC varying in order of strength of adherence.This is called the lyotropic series and follows the order: H⁺>Al⁺³>Ca²⁺>Mg⁺²>NH₄⁺=K⁺>Na⁺.As soil pH decreases below 6.0, H⁺ cations occupy more cation exchange sites forcing K⁺ cations to leach down through the soil profile. For corn and soybean plants with approximately 80% of their root mass being in the top foot of soil, this can create a K plant availability problem. Any combinations of the dynamics listed below can create an environment where the use of a K-based starter would be of benefit:

Soil nutrient dynamics can be complex.  Your FS Crop Specialist is trained to understand your individual field dynamics. Consult them for further understanding.