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Law of the Minimum

Thursday, June 23rd, 2011

Community contributed by Glenn I. Teves, County Extension Agent

We have over 140 soil types in Hawaii, more than any place in the world, and each soil is a little different: from the sandy coastal soils to the young lava soils of Mauna Kea. The composition of our soil is based on age, rainfall, soil particle size and parent material. Annual rainfall statewide can range from a few inches to over 400 inches, and older, more eroded soils and those in high rainfall areas are usually more weathered, and lack certain nutrients, including potassium, calcium and magnesium. Ho`olehua soil is highly weathered, so calcium and magnesium are in short supply and must be added, but there’s usually adequate potassium, the last number on the fertilizer bag, because the parent material has lots of it.

Phosphorus, the middle number on the fertilizer bag, is another limiting nutrient and is why many use 10-30-10 as a pre-plant fertilizer. The availability of phosphorus can also be affected by soil pH, the level of acidity or alkalinity of a soil. The soil pH should range from 5.5 to 6.5 for most crops. If the pH is too low or acidic, phosphorus will be locked up in the soil structure and is not be available to the plant. Also, compounds such as aluminum and manganese are at toxic levels in low pH soil, and can kill plant roots. However, a few plants prefer acid soils, including sweet potato, gardenia, azalea, camellia, and tea. The red color of the Ho`olehua soil is due to high iron, but is in a form that’s not available to plants.

In areas of very low rainfall, such as Kalamaula, the soil is rich and probably needs little in the way of additional nutrients. In all soils, we usually assume nitrogen is lacking and should always be added. Horse, pig, poultry and cattle manure are good sources of nitrogen fertilizer, but should be aged and not fresh. Old-time farmers use to make manure teas, manure mixed with water, and fed plants small amounts at each watering to grow beautiful crops of lettuce and other leafy vegetables, for example. If your soil is low in certain nutrients, the veggies you eat from that ground will also be low in those same nutrients.

Growers need to know what’s missing in their soil, and one way is by collecting a soil sample. To take a soil sample, take handfuls of soil from a depth of 2-4 inches from many spots on your lot or garden and mix it together in a bucket. Remove about two cups and put it in a sturdy plastic bag. Bring it to our office between 8:30 a.m. and 3:30 p.m., Monday through Friday, and be sure to fill out soil sample forms. There’s a $12 fee for each sample and a check is preferred. We’re located next to the Ho`olehua Post Office. With the high cost of fertilizer, it pays dividends to know exactly what’s missing in your soil and how to correct. And gardening without a soil sample is like baking a cake without a recipe.