HOW SOIL PHYSICAL PROPERTIES AFFECT PLANT FERTILITY

Achieving a landscape with dark-green foliage and with colorful inflorescence is not a matter of good luck. It requires a basic knowledge of plant sciences, an understanding of how plants grow and a familiarity with the manner of root absorption of essential minerals.

Have you every wondered how essential minerals or nutrients in the soil reach the plant roots? In other words, do nutrients dissolve in the soil moisture and move to the roots in the solution as plants transpire? Or, do the nutrients move by diffusion to the roots, or must the roots intercept the nutrients as the roots grow through the soil and come in contact with the nutrients?

The answer to the above questions are important. By knowing how nutrients become available, you can recommend and use techniques to optimize plant growth, establishment and appearance. Also for the environment, it is also important to know how to use fertilizers properly in order to avoid contamination of ground water by excessive amounts of nutrients.

In addition, you need to know how to make the nutrients which are in the soil available to the plants. For some elements, the addition of larger amounts is not as beneficial as rendering what is in the soil more available. For example, iron is very plentiful in the soil. In fact it is the fourth most abundant element in soil. The average concentration in soil is 5.0%. Potassium is about half this. Its average concentration is 2.6%. However, iron is a trace nutrient. Only about 0.01% iron on a dry weight basis typically is found in leaves whereas potassium averages 2%. Unfortunately, iron availability is a continuing problem in common Western soils because ordinary soil conditions terribly limit its availability. With better soils, many fertility problems would be greatly diminished.

How Nutrients Get To the Plant Roots

Nutrients get to the roots apparently by all three methods mentioned above but generally a different method for different nutrients. The highly soluble nutrients such as nitrate, sulfate and calcium move to the roots as solutes in the water. This method of nutrient movement is termed Mass-Flow.

Other nutrients bind to clay and silt particles. The dissolved concentration is low but the reserve supply to replenished what has been assimilated by the roots can be quite high. Beneficial aspects of these nutrients are a very low leachability and a low salinity in the soil moisture. Due to soil binding, fertilizing the soil surface is not nearly as effective as tilling the nutrients into the soil to be near the feeder roots. On soils with good physical structure, low compaction and good infiltration, surface binding of nutrients will not be a problem to the same degree. Examples of nutrients which move by diffusion are potassium, phosphorus and some of the micronutrients.

Other nutrients are so insoluble in the soil that the plant has to use ore-processing techniques to absorb them. Roots of dicots can release acids that dissolve nearby elements. Monocots produce specialized chelating compounds for dissolving iron.

Effect of Soil Conditions on Nutrient Uptake

Plants require metabolic energy to absorb and transport nutrients. The energy comes from respiration of organic compounds in the roots. The process consumes oxygen. Thus soils need to be well aerated. Excessive irrigation lowers the oxygen content of soil by filling the pores with water. Also vigorous root growth for exploring a large volume of soil is important for good nutritional status. Soil compaction not only lowers the oxygen content in soil, but also limits the rate of root growth due to the firmness of the soil.

The average soils in California do not possess physical properties for excellent plant growth. Fortunately however, excellent soils can be created with Complete Green polymeric soil conditioner known as PAM/Soil Drain especially if the fertility is also good. Soil testing or plant tissue testing should be performed to access the nutrient status. If necessary, needed nutrients can be added. With good fertility and good physical soil properties, an excellent landscape is more of a sure thing.