Stainless Steel Weight Hopper and Mixer used to blend custom mix fertilizers. 

Bulk Fertilizer

Fertilizer analysis use at least a 3 number sequence, like 15-10-10 to identify as a percentage the amount of nitrogen, phosphate, and potassium (potash).  Nitrogen is typically associated with plant growth above the ground, phosphate with root growth, and potash with plant health and resistance.   For more information about a specific ingredient please follow these links:  Nitrogen,  Phosphorous, and  Potassium.

Liebig's Law of the Minimum was an agricultural principle developed in 1840 can be applied where plant growth is controlled by the least available nutrient. 

Each plant has a certain proportion of nutrients that it needs.  When one of the nutrients is lacking, the growth process cannot proceed any further without more available nutrients.  Water is very often the limiting nutrient.  Of the three N-P-K macronutrients, nitrogen is the limiting nutrient at least initially because it is water soluble and will not remain in the soil.  Nitrogen also does not have a natural source without significant plant decay or the cutting or grazing of legumes.  Not part or Liebig's Law, but temperature is also a determinant. 

For a specific crop please see this manual:

www.soiltest.vt.edu/PDF/recommendation-guidebook.pdf

For more information about the industry, please see:  www.tfi.org/

Soil Samples

Soil Sample recommendations for fertilizers are give in pounds of actual plant food units and can be confusing.  Lime recommendations are often given in tons per acre or pounds per 1000 sq feet. 

A Conversion Explained:

If a recommendation was for 100lb of nitrogen per acre, and you choose to use 33.5% ammonium nitrate you would need 300lb of actual product per acre (more exactly 298.51lb).  The conversion is Actual pounds of product needed = Target Plant Food Units in lb divided by the percentage of actual units multiplied by 100% or (100lb/33.5%)*100% = 298.50746lb.

This article explains the soil test results:

pubs.ext.vt.edu/452/452-701/452-701.html

Tissue Samples

Can be used to better explain deficiencies in plants.  Some nutrients like magnesium for example, can become increasingly unavailable to plants as the level of potassium (K) increases and can be predicted by a soil test.  While the first flush of grass growth in the spring is often deficient in Mg, a soil test can show a sufficient amount of Mg.  Complicated soil interactions and recognizing a deficiency are easier to assess when you have both a tissue and a soil sample. 

You may also like this article:

www.southernstates.com/articles/tissue-sampling-plant-health.aspx