Fertilizing 101: Feeding plants organically

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Spring in Kona usually means the beginning of our rainy season and the days are longer and a little warmer. Longer, warmer, wetter days mean that most plants are putting out new leaves and many are gearing up to flower or fruit. Though not all plants are awakening from dormancy this time of year in the tropics, most of them could use some nutritional help. With rain becoming more regular, this is an excellent time to apply some fertilizer that will get watered in adequately.

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Spring in Kona usually means the beginning of our rainy season and the days are longer and a little warmer. Longer, warmer, wetter days mean that most plants are putting out new leaves and many are gearing up to flower or fruit. Though not all plants are awakening from dormancy this time of year in the tropics, most of them could use some nutritional help. With rain becoming more regular, this is an excellent time to apply some fertilizer that will get watered in adequately.

For optimal health, plants require 16 different elements. They obtain hydrogen, oxygen and carbon from air and water and the other thirteen elements they derive from the soil. The three major elements for plant nutrition are nitrogen, phosphorus and potassium. The percentage of these in a packaged fertilizer is indicated by the numbers on the package. A fertilizer labeled 20-10-15 contains 20 percent nitrogen, 10 percent phosphorous and 15 percent potassium. Plants also need calcium, magnesium, iron and sulfur but in lesser amounts. Zinc, manganese, copper, boron, molybdenum and chlorine are known as micronutrients and are necessary for plant growth but needed in much smaller amounts.

Though some nutritional deficiencies have visible symptoms, the best way to determine what your plants need is by conducting a soil and/or tissue test. A University of Hawaii College of Tropical Agriculture and Human Resources publication is available online at ctahr.hawaii.edu/oc/freepubs/pdf/pnm2.pdf providing information on both tests. Call the local office at 322-4892 to find out what day to take your samples in for testing. Test results may suggest an adjustment in your soil’s pH to ensure the availability of nutrients to your plants. Most elements that plants need are available to them at a soil pH between 6 and 7. If your soil is too alkaline (over pH 7.5) or too acid (less than pH 5.5) you may only need to adjust the pH to ensure that your plants get the nutrition they need.

If the leaves on your plants are yellowing or have purple undersides, they are probably suffering from a deficiency of a major nutrient. Small, weak plants as well as poor fruit production are also signs that nutrition is lacking. Choosing the right nutritional supplement can be tricky without test results or good observational skills. If you have maintained a healthy growing environment by augmenting your soil with compost, topping it with mulch and applying compost teas to the leaves regularly, you may not need additional nutrition.

All the major, minor and micro nutrients aid plant growth in different ways. The most important element for plants is nitrogen. It promotes new stem and green leaf production. Though important for plant health, overdosing nitrogen can be problematic. Producing lush green growth can take energy away from flower, fruit and root production and can be very appealing to hungry insects. Nitrogen can also move quickly through the soil and may require frequent application.

Balancing nitrogen with other elements will ensure that the whole plant is served. Phosphorus is usually more stable in the soil. It provides strong healthy roots and is best applied in the root zone, especially when establishing new plants. Potassium helps plants produce large, healthy fruit and is best applied just before plants begin to set flower buds.

Though major nutrients are available in chemical formulations, organic fertilizers won’t burn your plants or kill soil microorganisms and are probably a better choice for fertilizing edible crops. Organic fertility is available in stores but can be cheaper if you do it yourself. Composts, worm castings, coffee grounds and green manures from legumes can all provide nitrogen. Blood meal, fish emulsion and animal manures are nitrogen sources that are available in stores. Bone meal and slow release rock phosphate are good phosphorus sources. Greensand and sea kelp can provide potassium and other nutrients.

The minor nutrients calcium, magnesium, iron and sulfur are often included in organic fertilizers. Select ones that include more elements than NPK or apply these nutrients separately. You can make your own slow release calcium from crushed egg shells. Sea kelp contains plant available iron and Epsom salts are a DIY way to add magnesium to your fertility program. Tea made from comfrey leaves is loaded with nutrients including phosphorus, potassium and magnesium as well as trace minerals and vitamins.

However you decide to feed your plants now is a good time to check local garden supply stores to see what they have available or go online to see what you can make yourself.

Tropical gardening helpline

Kathy asks: I have a large allspice tree on my property that flowers every year. It never sets berries, however. I have been told that allspice is dioecious and that I may need a male tree close by so that the female flowers can be pollinated with male pollen. Is that true and if so, how do you tell a male from a female tree?

Answer: Allspice (Pimenta dioica) does have bisexual flowers. Some of the flowering trees have few stamens and the pollen does not germinate. They are the female trees and they produce fruits. Some male trees have sterile ovules and do not bear fruit. This is why allspice is sometimes categorized as a dioecious plant and it is recommended to plant male and female trees close together to increase female tree productivity.

The trees can be sexed by close inspection of the flowers. The hypanthium, or receptacle, of the flower is sunk around the pistil in male flowers but is flat in female flowers. Though both male and female varieties will produce blossoms, it seems that only the blossoms of the female trees mature to produce berries.

Your tree may be a male. You might want to plant more trees in hopes of getting a female that will give you berries. Do know that the berries are filled with seeds. If you are not going to use every one, they will drop or be picked up by birds and produce an overabundance of seedlings either on your property or wherever the birds drop them. This prolific production of seedlings from the berries, qualifies allspice as an invasive plant in Hawaii.

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Email plant questions to konamg@ctahr.hawaii.edu for answers by certified master gardeners. Some questions will be chosen for inclusion in this column.

Diana Duff is a plant adviser, educator and consultant living on an organic farm in Captain Cook.

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