<strong>Q:</strong> I tested my soil and found it was completely deficient in phosphorus. That’s probably why my peppers didn’t do anything last year. How do I add phosphorus to it? I can’t find any phosphorus in the garden center.

<strong>A:</strong> Let’s talk about soil testing first. Home soil-testing kits can’t tell you a lot about your soil, usually just a good guess of how much nitrogen, potassium and phosphorus is in the soil and its pH. I say good guess because when these soil-test kits are compared to lab results, they usually are off quite a bit.

Farmers have their soil tested in labs, and it makes more sense for them to soil test because they are growing a single crop that has very specific needs. Your home garden is a menagerie of plants with varying needs for minerals and nutrients, as well as pH levels.

Another problem with soil testing is the sampling system used and its fallibility. If you sample in one place, the tests might be inaccurate for other areas of the garden. If you collect in several places and combine the soil samples, it is not accurate for any given area. The depth matters, too. The most reliable results will be down in the root zone, and yet most soil amendments affect only the top few inches of soil.

Nitrogen can’t really be measured because it moves in and out of soil so readily that testing is useless. Most of it comes from thunderstorm activity. Potassium (K) can be measured, but similar to phosphorus, it might not be “available,” and therefore unreadable, in cold soil or soil with a high pH. A false reading and the unnecessary addition of phosphorus could be more damaging than a lack of it.

Phosphates are being taken out of most fertilizers because their overuse is causing algae blooms in open waterways, thanks to runoff from farms and lawns. The best way to regulate phosphorus in your soil is to provide decayed material. Phosphorus is taken up by living things and returned to the soil in dead things. So, compost is king when it comes to phosphorus.

It’s the same with nitrogen: You can boost it naturally with organic matter. When you add it chemically, in the form of fertilizers, it can overwhelm the roots and cause mushy, tasteless tomatoes; a soapy taste in carrots; and “hairy” root crops. All plant nutrients depend on soil pH to work well, too. If the pH is too low, they are too available, and if it is too high, they are bound up. That being said, the basic pH of your soil is hard to change. If you garden on sandy, acidic soil, it’s better to grow things that appreciate it.

Most garden plants like soil pH to be between 6.0 and 7.5. (You can find charts online that give you the individual preference of each plant.) If you use compost and chopped autumn leaves and have a healthy earthworm population, you have done about all you can to create healthy soil for plants to thrive.

Finally, soil tests will not reveal pesticides, toxic compounds, microbial activity, water requirements, compaction or soil structure. All of these things are mitigated by and remedied by the addition of natural material to build that “chocolate cake” soil most plants love.

<strong>Q:</strong> Please tell me again why my hawthorn tree defoliates every summer. It was some kind of disease that is carried by evergreens, right?

<strong>A:</strong> Yes, sort of. The disease is cedar apple rust, and it is a disease cycle that requires two hosts. There actually are three variations of it in our area, and they affect apples, crab apples and hawthorns. The fungus begins on cedars (Juniperus virginiana), and the spores travel to specific members of the rose family (apple, crab apple, hawthorn, etc.), where they land and complete their life cycle. The spores create round fungal spots that eventually cause the leaf to drop.

It is not a fatal disease, but it can leave the trees looking awful, and it does weaken them to some degree. You can spray them, or have them sprayed, to minimize the damage, but it also might be worthwhile to look around and see if you can find the affected cedars.

The cedars/junipers we are talking about are the old-fashioned, upright kind. (Don’t confuse them with arborvitae.) They have pointy needles, and the infection usually can be spotted most easily in the spring, when the fungal spores form bright orange masses on the inner branches. If you find them, you can cut them out.

Trees that are in this group ideally should not be grown near upright junipers, but practically speaking, that would require an impressive amount of cooperation among neighbors. You might be able to get together and look for the spore masses on cedars/junipers and remove them, but the neighbor might not be willing to get rid of the plants, so the infection likely will return. Also, the spores can travel up to a quarter-mile.

Resistant varieties are available among apple/hawthorn trees. Check the Morton Arboretum for a list, or talk with your local nurseryman about his experience with them. In any case, if you already have susceptible trees, cleanup and disposal of fallen leaves is very important in controlling the cycle.