I add fertilizer to my irrigation solution every three of four waterings, at one-fourth to one-sixteenth of the strength recommended on the label. Nutrition is supplied only during the growing season. It is important that no growth stimulation be supplied when plants are resting, during winter or summer. After tiring of supplying nutrition for my cacti by chipping away at the rocks my granular fertilizers had become since the last growing season, no matter how carefully I sealed the containers, I use a liquid fertilizer.
It is best to buy a premium product that is supported by research. Complete nutrition for container-grown plants requires more than 10 elements necessary for proper growth. Good fertilizers are made from calcium nitrate, potassium nitrate, monobasic ammonium phosphate, magnesium nitrate, and if possible, a mix of soluble trace elements. The nitrogen present in fertilizer is usually a blend of nitrate (N03) and ammonium (NH4). Too much of the wrong type of nitrogen may cause soft growth that makes plants less resistant to disease. It is helpful if the label states that the formula is designed for soilless mixes.
Numbers on a plant food label are an international code, showing proportions of three major elements needed to support growth. The first number represents nitrogen, responsible for building tissues such as stems and leaves. The second number represents phosphorus, which orchestrates the growth of healthy root systems, flowering, and seed production. Superphosphate is the usual source of phosphorus.
The third represents potassium, which increases plant resistance to stress and disease. Potash is the usual source of potassium. Thus a fertilizer with a 5-10-5 label contains 5 parts nitrogen, 10 parts phosphorus, and 5 parts potassium.
These major elements should be supplemented with minor elements called trace elements. They are called minor not because they are unimportant, but because they are required in very small quantities. Nevertheless, they are necessary for plant health. Trace elements include magnesium, iron, manganese, copper, zinc, boron, and molybdenum. Many newer and more expensive fertilizers contain these trace elements in a soluble sulfated form that will not precipitate out and be lost. Look also for a product with buffers to help manage soil pH.
When selecting a fertilizer, consider choosing one that includes sulfur. Generally, plants rely on irrigation water for sulfur, but most water either is deficient in sulfur or has too much. Slow-growing plants such as cacti can exist at a sulfur level of 10-20 parts per million; 20-30 parts per million is required by most plants. Only about 20% of the water supply of the United States contains sulfur in proper concentrations for healthy plants. Soilless mixes contain very little sulfur. Gypsum or superphosphate will provide a quick fix when added to a soil mix, but they are only helpful for a short time since sulfate ions are leached out in a few weeks. Avoid buying elemental sulfur because it will do more damage than good. Elemental sulfur does more harm than good; it rapidly lowers soil pH and damages or kills plants. If you question the sulfur content in your water supply, either send a sample to a testing laboratory or ask your water district supplier. Diagnosing sulfur deficiency is difficult because the symptom in cacti is similar to that of nitrogen deficiency, stunted growth.
Plant nutrients are soluble salts that can accumulate until a potting mix becomes harmful. Soluble salt buildup is dealt with by leaching. Leaching should be performed with each watering. Apply enough water so that it drains from the bottom of the pot. This means applying 15-20% more water than the container can hold. Surplus soluble salts are carried away in the excess water. Even with frequent leaching it is good to repot cacti.
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