Magnesium (Mg) is the 'engine' of photosynthesis. It is responsible for moving phosphorus within the plant and activating enzymes. Tomatoes and petunias act as indicators: they react more sharply than other crops to a lack of this mesoelement. The danger of magnesium deficiency lies in the fact that it is often mistaken for disease or nitrogen deficiency, leading gardeners to treat the plant for the wrong issue.
1. Identifying the Problem: Symptoms in Tomatoes and Petunias
Unlike nitrogen, magnesium is a mobile element. When it's in short supply, the plant begins to redirect resources from old leaves to the growth point. This is why the first signs always appear on the lower tier of leaves.
Typical signs of magnesium chlorosis:
- Interveinal chlorosis: leaf tissue between veins turns yellow, pale, or white, while the veins themselves remain bright green.
- Spotting: tomatoes may develop brown or purple spots on lower leaves, and leaf edges may curl upward.
- Brittleness: leaves become fragile and drop prematurely.
- Small flowers: in petunias, bud coloration becomes pale, and blooming is sharply reduced.
Agronomist's Secret: pH Impact and Antagonism
Magnesium deficiency is often 'false.' It occurs in acidic soils (pH below 5.5), where magnesium shifts into an unavailable form. It is also vital to remember antagonism: an excess of potassium (e.g., from overusing wood ash) blocks magnesium absorption. If you have overfed your petunias with potassium to encourage blooming, expect magnesium starvation.
2. Causes of Deficiency
Besides pH levels, magnesium availability is affected by soil structure and weather conditions. In light sandy soils, magnesium is easily leached away by heavy rain or frequent irrigation.
Risk Factors:
3. Replenishing the Deficiency: Emergency Aid
If you notice signs of chlorosis, you must act quickly. Foliar feeding is the most effective method, as magnesium is absorbed through the leaf 5-8 times faster than through the roots.
Products and Dosages:
- Magnesium Sulfate (Epsom Salt): 15-20 g per 10 liters of water. Spray on the leaves in the evening.
- Potassium Magnesium (Kalimagnesia): suitable for soil application (20 g per sq.m) if a long-term solution is needed.
- Magnesium Chelate: the most expensive but most effective form for emergency resuscitation of container petunias.
For better results, add a small amount of urea (5 g per 10 L) to the magnesium sulfate solution. Nitrogen acts as a 'transport' agent, helping magnesium penetrate the leaf tissues faster.
Prevention and Mulching
To avoid nutritional 'swings,' it is important to maintain stable soil moisture. Mulching with organic matter helps retain magnesium in the upper fertile layer and prevents leaching. A regular pH test once a season is a mandatory procedure for those growing demanding tomato hybrids and trailing petunias.
When planting petunias in containers, add dolomite lime to the substrate—it serves as a natural, long-acting source of both magnesium and calcium.
Magnesium chlorosis is not a death sentence but a call to action. Timely spraying with magnesium sulfate can restore a healthy look to plants within 3-5 days. Keep an eye on the potassium-magnesium balance, and your tomatoes will reward you with juicy fruits, while petunias will offer lush clouds of flowers.
