Nursery - Lime and Nursery Growing Media

I came across a good article on lime in potting media from Andrew Ristvey from the University of Maryland that I thought I would share.

Questions about liming potting medium have come up recently. This is an important issue that needs to be addressed in your nursery if you are creating your own mix. Your potting soil or substrate is not really a true soil and apart from sharing a few physical traits like providing a place for roots to grow, there is very little chemically and physically that they have in common. I have always written about the importance of monitoring your substrate for electrical conductivity and pH. But what should you be doing about pH before you place your plant in the container? This is a complicated answer. Most important is that you have a goal for what ever plants you are growing. It is often recommended that your substrate have a pH of between 5.5 and 6.2. Why so low? Without getting too complicated, the organic materials you use to make your substrate are chemically different than a mineral soil. Your plants will rely on you supplying most if not all of the 14 essential mineral nutrients so not to limit growth. The availability of many of these nutrients is determined by the pH of the substrate. The recommended pH optimizes nutrient availability. Too high a pH and nutrients like iron and manganese are not available to the roots, causing a deficiency. Too low a pH and the same nutrients can become too available, causing toxicities or preventing the uptake of other nutrients.

Adjusting the pH of your home-made substrate before planting may or may not be necessary. The most commonly used substrate amendment apart from fertilizer is lime. Lime adds calcium carbonate and dolomitic lime adds a little magnesium with that calcium to your substrate. Apart from supplying calcium and magnesium to your plants, lime can increase the pH of your substrate and most importantly will act as a buffer, reducing the potential for large pH swings. Various recommendations have been cited by those who add lime to their homemade mix, but without knowing the whole story about your nursery, a blanket recommendation is hazardous at best. First and foremost, your irrigation water must be tested to determine the alkalinity or the amount of bicarbonates and carbonates. Without this information first, no recommendation should be made. A good level is between 50 and 80 ppm (1 ppm = 1 mg/l) with a little room for play. If your irrigation water has alkalinity of 120 ppm or greater, I would not recommend the use of lime in your substrate or I would be very sparing. However if your alkalinity is much below 50, then there are other factors to consider, like what type of fertilizer you are using, what plants you are growing, and what materials you are mixing for your substrate. Pine bark usually is relatively acidic, and the fresher it is the more acidic it is. Try to insist on a well aged or composted pine bark from you supplier, when the pH will have stabilized.

Interestingly, most of the recent research has shown that liming substrates decreases growth compared to adding just micronutrients or that adding more than 5 lbs of lime per yard of substrate reduces plant growth. Another recent recommendation is the type of particle size you use. Recent studies have shown that a long lasting granular lime is better than pulverized lime (which does not last long in your container). In one study, 5 lbs per yard of granular lime had the best affect on growth of juniper compared to pulverized lime, if lime was used. However in the same study, no lime addition also proved as effective as 5 lbs of granular per yard. Still other researchers are not convinced of the effectiveness of lime. In my opinion, each nursery will need their own recommendations based on the factors listed above. Your experience is the most valuable information you can use to determine the best course of action. If you are not having pH problems, then don’t fix anything. However, if you are, then you must gather all the facts to determine your liming rates. I would recommend the use of granular lime (solely or mixed with pulverized) to give persistent and steady pH balance, but be careful of over-applying. You can contact me for more information on your situation.

Reprinted from "pH and Potting Media" by Andrew Ristvey in the July 18, 2008 edition of the TPM/IPM Weekly Report for Arborists, Landscape Managers & Nursery Managers from the University of Maryland Cooperative Extension.