Use Compost Wisely

April is the month that draws a gardening crowd. Soil temperatures will soon be 50 degrees F, and there is still time to plant all the spring crops. I deal with a lot of people who are already involved or want to be involved in the organic gardening approach. Many say that for the sake of their family's health they don't want pesticides on their gardens and lawns. They know the facts: pesticides are toxic, they are designed to kill, and they continue to compromise human health. For anyone switching to the organic approach, it is advisable to spend some time learning the principles and guidelines involved in the organic discipline. There are many good books on organic gardening and much information available on the web. If you have questions, please do not hesitate to call or send me an email.

When we think of going organic, we automatically think in terms of adding compost to our gardens. The average soil in Arkansas has 1.25% organic matter, and a desirable level in the garden is 5%, so compost is generally needed. The nutrient quality of the compost will only be as good as the material that went into the pile. It can vary a lot. Decomposed cotton burs are exceptionally high in potassium, and too much of that kind of compost can upset desirable mineral ratios in the soil.

How much compost to add depends on the soil's current organic matter level. It is best to think in terms of building the soil to a good organic matter and nutrient level, and then doing a maintenance program. As a guide for starting new garden beds, I recommend double-digging garden beds at least one time. When double-digging the first time, add 40 gallons of compost per 100 square feet to the lower layer (10-20-inch) and 20 gallons per 100 square feet to the to the top 10 inches. While building to the 5% organic matter level, add another 20 gallons per 100 square feet for each new crop. With this plan, in 3 or 4 years you will have a crumbly, sponge-cake like soil, vastly different from the original. Once you get to the 5% organic matter level, and you get the right amounts and ratios of minerals, and have good biological activity, the compost can likely be cut to 20 gallons per 100 square feet per year.

A yard of compost is about 200 gallons and is enough for 270 square feet for the initial double-digging phase. For subsequent applications, a yard will do about 1000 square feet.

Do not add excessive amounts of compost to your garden. Do not build a garden bed with compost and vermiculite or sand only. Unfortunately, I have seen gardens where this has been done. Nitrogen levels are too high and the plants are highly vegetative, but yield little. In addition those high nitrate plants attract insects, and the fruit produced is bound to be high in nitrates (not good). Excess compost can also bring the P and K to excessive levels. If that happens, back off on the compost and be sure the other nutrients are at the proper amounts and ratios. I use the Arkansas soil tests reports to look at pounds of nutrients per acre available, but depend on other sources for making organic recommendations.

Increased organic matter in the soil also improves the soil tilth (soil structure), and makes for a lighter,
fluffier, and easy-to-work soil that we all desire. Perhaps the most noticeable characteristic of increased organic matter is the increased ability of the soil to hold water. The following table demonstrates just how important the organic matter is for holding water. "Humus" is the organic matter that has been digested by microbes.

Table 1 -- APPROXIMATE WATER HOLDING CAPACITY OF HUMUS

(Note: 1 inch of water yields 28,000 gallons/acre or 643 gallons for 1,000 square feet.)

HUMUS LEVEL------SOIL DEPTH------WATER HOLDING CAPACITY

-----percent------------------inches---------------------inches of rain

-------1--------------------------7---------------------------------.36

-------2--------------------------7---------------------------------.72

-------5--------------------------7--------------------------------1.80

------5-------------------------21--------------------------------7.40

The other important values of organic matter include its ability to provide nutrients and buffer the soil against pH changes. The organic matter also serves to hold nutrients in the soil. This is especially important in sandy soils. High organic soils are generally darker in color and can warm up faster in the spring. High organic matter soils have higher biological activity, which helps to keep the plants healthy.

It is wise to get a soil organic matter content analysis to check the effectiveness of your soil building activities. This organic matter test is done separately, so take an extra pint of soil when you get your other soil test done by the Extension Service. The regular soil test is free, but the organic matter test now costs $6.00.

Building the soil organic matter content requires a long-term commitment. Expect major improvement over the first few years, and then generally a slower process for the next 5-10 years. Remember to get soil tests and add minerals to the soil, where needed, at the same time you are adding the organic matter. It all works together, and eventually you reach a healthy, well-balanced soil that gives you tasty, nutrient-dense produce.