Sizing Up Substrates

To be profitable, container production of horticultural crops requires precise control of water and fertilizers. Physical properties of horticultural substrates have been studied in order to help develop more effective irrigation and fertilization strategies. One of these physical properties–particle size–is incredibly important in substrates. The particle size distribution of a substrate influences its water holding capacity and pore size, which affect irrigation, drainage, and aeration. The pH, CEC, fertility, and stability (amount of decomposition and nitrogen immobilization by microbial populations) are also greatly influenced by particle size.

The most commonly used method to characterize the particle size distribution of horticultural substrates is to take a sample of a known mass and run it through a series of sieves of various sizes using a Rotap shaker. This method can be time consuming depending on the number of sieves being used, and does not account for the lengths of the particles being measured, or their specific shape. Pine bark mixes in particular have many irregularly-sized particles and a wide variety of particle sizes and lengths. In order to better understand, and therefore better utilize, substrates commonly used in container production, as well as to better engineer new mixes, the NCSU Horticultural Substrates Lab is taking a revolutionary approach to particle size analysis.

Using a Tyler CPA 2 (computerized particle analyzer), the lab will have the ability to count and measure, in real time, each individual particle of a substrate sample. Based on digital image processing, a high resolution digital camera scans the particles against the background of an LED lighting array with a recording frequency of over 20,000 scans per second. The scanned lines are combined by the CPA to form a data record, and the shadow projections of the particles are recorded and evaluated in real time, even down to sizes as small as 34 microns. This technology provides consistent, repeatable results, and allows the operator to see the size and shape of the individual particles being analyzed.

Using this technology, the Horticultural Substrates Lab hopes to “shape up” the accuracy and efficiency with which substrate particles are characterized and understood.

– written by Laura Kaderabek,  Department of Horticultural Science Graduate Student