Fertilizers

Marijuana Horticulture

by Jorge Cervantes

The goal of fertilizing is to supply the plants with the proper amount of nutrients for vigorous growth, without creating toxic conditions by overfertilizing. A 2-gallon container full of rich, fertile potting soil will supply all the necessary nutrients for the first month of growth, but the development might be slow. After the rots have absorbed most of the available nutrients, more must be added to sustain vigorous growth. Unless fortified, soilless mixes require fertilization from the start. I like to start fertilizing fortified soilless mixes after the first week or two of growth. Most commercial soilless mixes are fortified with trace elements.

Marijuana’s metabolism changes as it grows and so do its fertilizer needs. During germination and seedling growth, intake of phosphorus is high. The vegetative growth stage requires larger amounts of nitrogen for green leaf growth, and phosphorus and potassium are also necessary in substantial levels. During this leafy and vegetative growth stage, use a general purpose or a grow fertilizer with high nitrogen content. In the flowering stage, nitrogen takes a backseat to potassium, phosphorus, and calcium intake. Using a super bloom fertilizer with less nitrogen and more potassium, phosphorus, and calcium promotes fat, heavy, dense buds. Cannabis needs some nitrogen during flowering, but very little. With no nitrogen, buds do not develop to their full potential.

Now we come to the confusing part about the guaranteed analysis of commercial fertilizer mixes. Federal and state laws require nutrient concentrations to appear prominently on the face of the fertilizer packages, even though the accuracy of the values is dubious.

Do you think the N-P-K numbers on the label give the percentages of nitrogen, phosphorus, and potassium? Well, yes and no. The scale measures nutrients with different scales. Nitrogen is listed as total combined elemental. Most hydroponic fertilizers break nitrogen into slow-acting nitrate (NO3) and ammonium (NH4). Phosphoric anhydride (p2O5) is listed as the form of phosphorus, but this figure understates phosphorus content by 44 percent. It gets worse! The balance (56 percent) of the phosphorus molecule is comprised of oxygen. Twenty percent P2O5 is 8.8 percent actual phosphorus. potassium (K) is listed in the potash form of potassium oxide (k2O) of which 83 percent of the stated value is actually elemental potassium.

The rest of the mineral nutrients are listed in their elemental form that represents the actual content. Most often, the mineral elements used in the fertilizer formulas are listed in chemical compounds on the label. Look at the fertilizer labels to ensure that the elements, especially trace elements, are chelated and readily available for root absorption. Also, be careful about having too much sodium in your water / nutrient solution. The sodium will block potassium and several other nutrients, causing deficiencies and slow growth.

Nutrients in the United States are measured in parts-per-million (ppm_), even though they are expressed as a percentage concentration on the label. The ppm scale is simple and finite – almost. The basics are simple: one part per million in one part of 1,000,000, so divide by one million to find parts per million,. To convert percentages into ppm, multiply by 10,000 and move the decimal four (4) spaces to the right.

Fertilizers are either water-soluble or partially soluble (gradual-release). Both soluble and gradual release fertilizers can be organic or chemical.