Clay soil in Kensington area of San Diego

Question from Bill:
I am a new gardener to Kensington and have discovered that my soil is very heavy clay down to about 18 inches and then it appears there is hardpan under that. Lots of rocks in the clay part of the soil. What is the best method of dealing with this soil situation in this area? I am looking at planting a new lawn and renovating the flower beds which had some very mature plantings in them. The mature plants have been removed. Thank you for any assistance you can provide.

Answer from Pat:
First let me say that there are many very fine gardens in Kensington. Roses seem to grow there remarkably well, and my experience is that, contrary to what many people think, roses thrive in clay soil as long as it drains. Clay soil has mineral richness and it retains moisture better than sand. I have also noticed that some gardeners who have opted to grow drought-resistant plants In Kensington have raised the level of the ground by creating earth mounds held up with huge boulders and this has been a successful way to overcome problems with poor drainage if any existed.

One practice that I caution you strongly against is not to remove all the rocks. A rocky soil, especially a clay soil that is rocky often drains well. Rocks aid drainage and add mineral richness. If you remove all the rocks you may cause the soil to stop draining altogether and it might also become a great deal less productive. A story was told 2000 years ago by Pliny the Younger about a farmer of the Ubii tribe of Europe who removed all the rocks from one corner of his field and after that nothing would grow there. The message here is never to monkey with the structure of your soil and that means don’t try to take out all rocks or add clay to sand or sand to clay. These are very bad things to do. Instead, amend your soil with organics.

Not all clay soils suffer from bad drainage. The rocks may be the very thing that made it possible for the gardener who lived there before you to raise plants successfully and to maturity. Also whereas a buried layer of hardpan is usually a negative factor, without testing the drainage you can’t know if the hardpan is just another layer of a different soil or if it is indeed a layer with no drainage. So the first thing to do is to test the drainage (as described in the box on page 40 in my book.) First dig a hole as if for planting a tree or shrub, then fill it with water and let the water drain out. Then fill the hole a second time and this time put a yardstick across the top with the straight edge touching the top of the water in the hole. If the water drains out at the rate of 1/4 inch per hour drainage is sufficient for planting in the hole. Always work a generous amount of gypsum into the bottom of every planting hole in clay soil.

Also, I always recommend adding gypsum to clay soil when the lack of drainage is due to alkalinity. Gypsum will not cure drainage problems caused by compaction, but working gypsum into clay soil may help improve drainage and cannot harm the soil. Follow package directions regarding amounts to use. Gypsum used to be sold in powdered form and if so you can apply enough as if a light snow has fallen, but most bagged gypsum is now in pellet form and if so it doesn’t look like snow. You simply have to read what it says and work accordingly.

Gardeners have various approaches to soil improvement. I have actually known of cases where the gardener had bulldozed poor soil into piles and then hauled away several truckloads, replacing with two or three feet of top soil. But in most cases this is not necessary and certainly not where you live where a good garden grew there before. A far better plan is to amend the entire garden with organic amendments and gypsum working it as deeply as possible and including breaking up the bottom layer so that a hard horizon between two layers no longer exists. I would purchase a truckload of well-rotted compost and work this compost plus the gypsum into the entire top layer of your soil and plants should grow fine. By compost I mean a product that is appropriate for digging into the ground as amendment. Aged manure would be fine if you could find a good clean source. The problem with manure is it sometimes contains salt and this is not good in clay soils since it will not wash out easily. Do not use a product that is designed to be used as mulch. Mulch is raw unrotted woody material and is made for putting on top of the soil. If you dig it into soil, it will rob nitrogen from the soil in order to rot and this can kill plants. Organic soil amendment (compost) can be combined with soil without causing harm. Also add organic fertilizers.