What do the numbers on fertilizer sacks indicate?
The “big three” nutrients show on bags are nitrogen, phosphate, and potash which provide N, P, and K elements. They are the ones declared on a bag of fertilizer. If you see the combination of 5-10-5, it means that the bag contains 5% nitrogen, 10% phosphate, and 5% potash by weight. These aren't in a pure form but with other material.
How do each of the “big three” fertilizer elements help a plant?
1) Nitrogen is an essential element of chlorophyll that helps a plant produce starches and sugars. Since nature seldom provides it in sufficient quantities, fertilizers are used to augment what is naturally available.
If a plant is deficient in nitrogen it will be small, the older leaves turn from green to yellow, then finally brown but remain attached to the plant. Stalks are slender and few new ones develop.
2) Phosphorous is associated with cell division. It contributes to stiff stems, flowering, and seed production. In acid or alkaline soils it becomes unavailable to plants so it is important to manage the pH of the soil for efficient uptake as well as providing adequate amounts of supplemental phosphate. Fertilizers bags show the amount of phosphorus content in terms of percent rock phosphate (P2O5) for historical reasons.
Phosphorus deficiency causes the whole plant to be dwarfed. The foliage is a dark. dull green, and often the stems turn purple. Areas between the leaf veins also can become purple, while the edges turn yellow. Loss of lower foliage follows.
3) Potassium is essential for movement of the sugars in the plant, the utilization of nitrogen, root development, and it also hastens maturity. It can be depleted over time in soils as plants grow.
A potassium deficiency results in the leaves becoming mottled with yellow between the veins, then turning brown and dropping. Plant growth will be stunted. Potassium in fertilizers is measured in potash form (K2O).
What is the best fertilizer for perennials and annuals?
The best fertilizer is the one that provides just enough of the elements lacking in the soil and no more. A balanced fertilizer—one with no zeros in the analysis like 15-5-10—will usually work.
Should I buy organic fertilizer?
The “big three” elements in organic fertilizers can be highly complex and are unavailable to plants until they have been broken down into simpler forms by soil organisms. Since these organisms are partially dormant at temperatures below 60 degrees and grow progressively more active up to 90 degrees they are not very effective in early spring. However, some add organic matter, an advantage not provided in synthetic or mineral fertilizers.
In either type of fertilizer the plant nutrients must be in a usable form before they can be used by the plant. This process of conversion happens naturally based on environmental conditions for both types, but organic materials are usually slow to release.
The choice is also driven by one's philosophy toward gardening.
My friend has horses and will give me manure. What should I know about using it in the garden?
It can be a valuable soil amendment. One caution: Many owners use pesticides to control insects or parasites, or feed hay that has may have been treated with an herbicide, so manure should be well composted in a separate pile before being applied to the soil. Cow manure can be a valuable soil amendment if allowed to rot in a compost heap during the winter in preparation for spring use. Check out: http://smallfarms.oregonstate.edu/sfn/f09Herbicide In the Eastern Sierra we tend not to have issues with local animals.
Remember that un-composted manure can carry human pathogens. You should take measures to ensure that plenty of time passes before harvest after using raw manures.
When should one apply fertilizer?
For permanent plantings such as trees, shrubs, evergreens or perennial flowers , fertilizer may be applied in the fall or spring with equal success. For annual flowers or vegetables it is recommended that fertilizers be applied just prior to planting.