These next three articles will outline the sources and chemistry of nutrients when applied to the soil and the plant itself. Later articles will refer back to these chemical pathways, nutrient sources, and concepts when discussing specific fertilizers.

When fertilizing hay fields, farmers apply nutrients to the soil or to the plant itself. There are five sources of nutrients which reach the plant roots through the soil. They are the clay particles and organic matter, organic matter mineralization and microorganism decay, soil formation, atmospheric deposition and precipitation, and finally fertilizers. All nutrients reach the plant in ionic form. An ion is a charged particle, anions have a negative charge, and cations have a positive charge. These ions reach the plant in the soil solution-the soil water mixed with all the nutrients and many other compounds. The soil solution can be compared to really watered-down coffee, very low concentrations of nutrients are present at most times of the year.

A very important source for plant nutrients are the clay particles and organic matter in the soil mixture. They are constantly binding and releasing the nutrient ions. Most of the nutrients exchanged are in the cation (+) form because the clay and the organic matter have more negatively charged (-) sites than positively charged sites. The amount of positive charges that a soil can exchange is called the Cation Exchange Capacity (CEC). This is one of the reasons why crops grown on clays and peat soils are often higher yielding than those grown on sands and loams. Figure 1 below shows the different CEC levels for different soil types. These levels are almost unchangeable, except in some cases the organic matter percentage can be increased slightly when transitioning from tillage intensive row crop production to hay or pasture farming. Most nutrients that enter the soil solution will at some point attach to the clay and organic matter particles before being taken up by the plant.

Figure 1: CEC Levels of Soil Types

Organic matter mineralization and microorganism degradation also provide nutrients to plants. Organic matter mineralization occurs when the organic matter is exposed to oxygen and then breaks down into nutrients and other compounds that are available to plants. When bacteria, fungi, and other microorganisms die they release nutrients, mostly nitrogen from their proteins, into the soil solution. The most widely used sources for increasing the amount of nutrients available from organic matter mineralization is the decomposition of a cover crop and the incorporation of animal manure. In many situations these two things can contribute a significant amount of nutrition to the crop. However quantifying the amount of nutrients available from these processes can be difficult and good management is necessary to use these sources reliably. The following links below are good resources for cover crop and manure management.

Links to Cover Crop Resource web pages

SARE ATTRA Midwest Cover Crops Council Michigan State Oregon State

Links to Manure Management Resource web pages

Wisconsin Iowa State Cornell

Soil formation, the breakdown of soil rocks and minerals over time, is a very slow process and relatively small contributor to nutrients plants use. This process should not be relied upon solely for a soil fertility program.  This is why the application of some soil amendments such as rock phosphate contribute little if any value to soil fertility programs under certain climatic conditions and soil pH levels. In the Midwest and Great Plains of the US the availability of the phosphorus from rock phosphate, even when finely ground, is often delayed for years unless it is modified prior to application to a field. A greater response to rock phosphate application has been seen in tropical areas on very acid soils, with heavy rainfall conditions on oil palm, rubber, and cacao plantations

The atmosphere and precipitation also can contribute nutrients to the soil. Before the Clean Air Act this was especially true of sulfur and some other nutrients that came from the smoke stakes in the factories in the Northeast and the Midwest. Click on the link below to see change of the deposition of sulfur over time in the US. These days, China and other nations without strict air quality regulations have the only reliable source of pollution nutrition for their crops.

Sulfur Deposition Changes

Despite all of these other sources, fertilizers and other additions to the soil are the primary source of plant nutrients. Soil tests quantify collectively the nutrient base from these other sources. The difference between these levels and what the plant requires is what the farmer needs to apply to the soil in order to feed their crops.

Up next: Fertilizing Hay Crops, Part 3.