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Magnesium Nutrition

Most fertility programs are designed around the macronutrients (N,P,K). In fact, when we discuss these fertility regimes we usually describe them as 150 parts per million (ppm) or 200 ppm, etc. This, of course, refers to the amount of nitrogen in solution, regardless of the analysis of the material used to make it up (i.e. 20-10-20, 15-16-17, 29-20-20, etc.). I guess phosphorus and potassium nutrition aren't that critical to plant growth or maybe we're just lazy.

The micronutrients are important too and growers frequently use a micronutrient supplement to meet crop needs. These can be pre-incorporated in to the growing media or added as a soluble application/drench. Since most of these micronutrient supplements are a "package" (containing all of the essential micro's) there is always some concern about getting too much of one and not enough of another.

Frankly, we don't see many problems with N,P,K deficiencies or toxicities. These fertilizer materials are cheap and easy to use and most crops are pretty forgiving as long as you are on the high side of the recommended rate. When it comes to micronutrient nutrition, most of the problems we see are related to pH and alkalinity. These two factors, in media and water, effect availability. When out of range (i.e. pH >7.0, alkalinity>150 ppm) deficiencies can occur regardless of the concentration of micros in the water/media. Most growers find a way to work around this problem by: acid injecting their irrigation water, using acid forming fertilizers, increasing micronutrient levels, using tolerant cultivars, etc.

Where we do see reoccurring nutrient problems is with the "secondary nutrients." This is a term used to describe calcium, sulfur and magnesium. There has been a lot of discussion lately on both calcium and sulfur. These nutrients are often present as a component of complete N,P,K fertilizers but may be required in higher concentrations for optimum plant growth. However, it's magnesium (Mg) that can give Texas growers fits during the spring and summer months.

The primary source of Mg in most fertility programs is from dolomitic lime. This material is incorporated to raise pH and increase buffer capacity. Dolomitic lime is usually used at a rate of approximately 4-6 pounds per cubic yard of mix. In many cases this is the only source of Mg provided for plant growth.

Most greenhouse crops are on the bench for a relatively short period of time (i.e. 6-12 weeks). During this cycle plants may be irrigated once, twice or in some extreme cases even three times per day. As a result, much of the incorporated dolomitic lime (Mg) may be leached from the container. This is even more of a problem on longer term crops (i.e. >20 weeks).

Magnesium deficiencies are pretty easy to spot. Most plants develop a very distinctive, interveinal chlorosis on the middle to upper leaves. However, I've seen these same symptoms from the top of the plant to the bottom. Generally speaking, this condition cannot be corrected once it occurs so be on the lookout early in the crop. In severe situations the entire plant may defoliate.

This problem is relatively simple to take care of. Supplemental applications of magnesium sulfate (Epsom salts) during the growing season. The only complication is that Epsom salts can't be tank mixed with your regular N,P,K solution. The resulting precipitate will clog up drip emitters and other pieces of irrigation equipment. Therefore, separate drench-type applications are necessary. Generally speaking one application midway through the crop should be sufficient unless excessive leaching is apparent. Some complete fertilizer products have a Mg component which makes it a little easier to use.

Table 1 presents a variety of injection ratios and the amount of magnesium sulfate required to make up a low and high rate of Mg.

Interveinal chlorosis, stunting and defoliation.

Magnesium deficiencies usually appear as a yellowing between the veins of the leaf. This situation is rarely correctable. entire plant affected.

Crops Affected:
Almost all greenhouse crops can be susceptible to Mg deficiencies.

What to Do:
Use pre-plant dolomitic lime and/or supplemental applications of magnesium sulfate.