Orginally published in the N.E.P.G.A. Newsletter - March, 1995
Calcium and the Soil
Calcium is an extremely important element for growing giant pumpkins. Calcium ions attach themselves to clay and humus particles in the soil and help make other essential elements available to your plant. Limestone is the most common form of applied calcium and is used to increase the pH of acidic garden soils. A pH of 6.5 to 7.0 is a good pH range, however, record setting pumpkins have been grown in soils ranging from 6.0 to 7.8. If your soil is on the high or low side of the acceptable range, you may start to have problems with certain essential elements. Chemical reactions take place that tie up essential nutrients into compounds that are not easily water soluble. A plant nutrient must be in solution for a root hair to take it up. Calcium carbonate (limestone) is not readily water soluble so how does it enter the plant? One way that calcium becomes available is when organic matter in the soil decomposes giving off carbon dioxide. The carbon dioxide reacts with water in the soil and produces a mild form of carbonic acid which slowly dissolves the limestone. Once dissolved, the calcium ions can attach themselves to clay and humus particles and enter the roots of the plant. if you have a soil that is low in pH, it takes a while for newly applied limestone to transform into an available form of calcium. Just adding limestone does not insure that you will immediately have adequate levels in the pumpkin.
Calcium and Plant Physiology
Calcium is not only important for cation exchange, it is also an essential element in the structure of the pumpkin plant. Calcium enters the root hairs and forms an organic compound called calcium pectate. Calcium pectate produces strong cell walls and is found in the middle lamella which is the cementing layer between plant cells. Calcium is carried throughout the plant by the transpiration stream. Therefore, plant organs that have stomates (pores), such as mature leaves, will transpire more than organs that do not have stomates, such as fruits and flowers. In general leaves tend to have more calcium than a fruit would have. If you have a leaf that has adequate levels of calcium, you can still have a calcium deficiency in a large pumpkin.
Calcium and Disease Resistance
Adequate calcium provides for strong cell walls. Strong cell walls are less prone to disease. Calcium deficiences in pumpkins may result in cracking, blossom end rot, internal breakdowns or post-harvest rot. Many studies have shown increased disease resistance with elevated calcium levels, although some of the studies involve other vegetables such as potatoes, tomatoes, cucumbers and the like. I believe the benefits will also apply to the pumpkin. Calcium levels should make up 1 to 2 percent of leaf tissue and 3 tenths of one percent of the fruit. Since most of us will not be monitoring calcium levels with tissue analysis, the practical thing to do is to elevate calcium with the following guidelines.
How to Apply Calcium
Rule number one is to never apply limestone without a soil analysis. If your soil has a pH of 7 or higher, you should not be applying limestone. So how can you get more calcium into the ground without raising the pH? One way is to use agricultural gypsum (calcium sulfate). It looks like limestone and is applied in the same way. Another way, and probably the best way, is to use calcium nitrate. Unlike the other forms of calcium, calcium nitrate is water soluble and is taken right into the plant.
Joel Holland has been using calcium nitrate for two reasons. He likes more calcium but also is interested in the nitrate form of nitrogen which is easily taken up by the roots. In cold spring soil amoniacal and other forms of organic nitrogen are not easily converted to nitrates so why not start with nitrate nitrogen for immediate plant uptake. Another calcium deficiency problem is a phenomenon called blossom end thinning. Many of you know how dangerously thin a pumpkin gets on the blossom end with splits and soft spots quite common there. It is not unusual to have a pumpkin 9 inches thick on the stem end and one inch thick on the blossom end. Again, calcium not being easily mobile in a plant or fruit would explain why levels decrease as you travel further away from the entry point (the stem). I will not say that lower calcium levels in the blossom end are solely responsible for thin blossom end walls. Certainly genetics play a role in this too, however, if there is a possibility that thinner walls can result from low calcium levels it seems wise to try to boost calcium levels in the plant. One interesting study for next fall would be to conduct a tissue analysis calcium levels on a pumpkin that suffers blossom end split and compare calcium levels on a healthy pumpkin. Where can you get calcium nitrate and how do you apply it? Calcium nitrate is not yet a household term in many garden centers. You will find it at greenhouse suppliers or at a large farm fertilizer outlet. There are two forms of calcium nitrate, agricultural grade and greenhouse grade. The former is generally found in 50 pound bags and is less expensive. This type is meant to be broadcast on the ground. This type is coated and is n( suitable for using in hose end sprayers or fertilizer injectors, for it has a gummy residue that plugs up filter~ and sprayers. The greenhouse grade dissolves with no residue and can be used the same way that you use Peters or Miracle Grow. A 25 pound bag of this costs twice as much as a 50 pound bag of the other, however, it is cleaner and easier to handle. One tablespoon per gallon would be a typical application rate for pumpkins and could be applied the same way you presenfly apply your water soluble fertilizers. Keep in mind that calcium nitrate has 15% nitrogen so don't over do it.
Calcium nitrate is not the magic bullet on the path to the first 1,000 pound pumpkin, but it does make sense to incorporate it into your fertilization program. You will have better quality fruit with greater disease resistance and better post harvest shelf life.
Hydro Agri Corporation Technical Bulletins Dr. Wade Elmer
Connecticut Agricultural Experiment Station
Joel Holland, Puyallup, Washington
Dr. Salvador Locascio, University of Florida, Gainseville