Brown paper lunch bags! I finally thought of something clean I have enough of, to temporarily hold soil dug from twelve separate holes around my garden. I'm getting a soil test from the University of Massachusetts.

For years I've been adding the most compost I can to my garden, and I've also relied a lot on organic forms of nitrogen (N), phosphorus (P) and potassium (K). Plus lime to adjust the soil pH, which affects the availability of nutrients.

Till now I've checked the soil with those little test kits from the hardware store or garden center. But they only measure NPK and pH levels; I've decided to find out more. For example, humus and clay influence how readily pH will change. An additional measurement, Buffer pH, would better determine how much lime to use.

Cation Exchange Capacity measures how well the soil can hold nutrients, in the form of positively charged ions. Sand has very poor CEC, clay better, but organic material provides the best. How this capacity is filled by nutrient ions is the Percent Base Saturation. I'm particularly interested in the percent of my soil's CEC filled by potassium.

It's possible to use too much compost! It can raise potassium too high, locking up calcium, magnesium and phosphorus. My soil potassium is probably okay, but I did bring in a few truckloads of compost....

Calcium is needed more than any other element — by plants, animals and soil. The dolomitic limestone I use for pH also contributes calcium and magnesium, but I have no idea how much.

For $9 the Standard U. Mass soil test looks at all of this — and much more. For complete information, go to their page on the test or write Soil and Plant Tissue Testing / Lab West Experiment Station / 682 North Pleasant Street University of Massachusetts / Amherst, MA  01003. (Don't phone in spring, their busy season.)

Next I'll spread the twelve samples on clean paper, mix each one thoroughly, and let it air-dry. Then I mix them all together, and send one cup from that in a ziploc bag to U. Mass.

But first the soil has to thaw more: a shovel driven down yesterday stopped at about seven inches. That's almost deep enough for my samples, but the frozen layer prevents the upper part from draining, and U. Mass says don't use wet soil. Plus it would make a mess in the bags.

• The University of Massachusetts' Soil and Plant Tissue Testing Laboratory's site. In addition to much information here, you can also download complete instructions and an application form for your soil tests.
• Soil Testing Handbook for Professionals in Agriculture, Horticulture, Nutrient and Residuals Management, by Bruce R. Hoskins of the University of Maine Soil Testing Service
• Maintaining Soil Humus — also explains how you can have too much.
• Buffer pH