Soil Amendments
Overview and Introduction
Garden Tip
Technical Bulletin Series

Whenever installing a new plant a recommendation to amend the native soil surrounding that plant is nearly universal. This is especially true in the arid desert areas. The composition of soil in most areas where rainfall amounts are low is poor. Often high in clay and slow to percolate or drain. Sometimes very sandy and drains too fast.

Though some people may say that native plants do not need amended soil, as they tend to survive anyway, even these more durable plants will do far better when their root system is encouraged to expand when surrounded by a good friable and organically rich soil.

When the native soil is clayey and slow draining it is especially important for this soil to be amended with an appropriate formula. Plants fail and are then returned to the nursery; The most common visual appearance (above) is that of the original root ball being the shape and size of the nursery pot. When the surrounding soil is either poorly draining or very sandy and rapidly draining, this is what often happens. New roots are discouraged from growing into this very challenging soil.

Organic material like compost and other blends of planting mix, recommended to be mixed in with or replacing the native soil, is vital for helping microbes feed and grow. This then can create a living soil. Beneficial bacteria and Mycorrhizae fungi work in harmony with root systems to aid in the uptake of nutrients and moisture. Roots actually require microbial assistance when growing in soil.

Just guessing that you have high clay soil is not sufficient or good practice. There are six different categories of clayey soil and all have differing drainage rates. Which type is yours? You can ascertain the composition of your soil by performing a "Jar Test" or even a "Drainage Test". Problems from incorrect amendment of your soil will show up in future years, not necessarily the first year. If you have a large property, testing several areas will be the best idea.

Just as it is important not to overload a campfire with too much new wood, the amount of green (not fully composted) organic matter added should be in moderation. When the need arises to add high percentages of organic material (higher than 50%); then using stable organics like potting soil or peat moss is recommended. These contain much less green matter (which is needed to feed microbes) and plenty of stable organics. With excessive green organics, soil bacteria are overactive and actually take nitrogen away from the plant. As with many things; moderation is the concept that brings success.

When blending the soil for planting; a portion of any High % clay Native soil may need to be set aside (not used for planting) as the goal is to create a soil composition that favors the growth of new roots. The Garden Calculator Pro will specify what percent of the Native can be used, as well as the quantities of the other amendments needed (based on the size hole it specifies for planting).

Once, having dug the planting hole, the amendments and remaining Native soil can be mixed together and used to surround the new plants rootball. Whatever soil is placed under the new plant should be packed firmly. Avoid situations where the rootball "sinks" after being planted.

With high clay soil, simply adding some organic material will not improve drainage sufficiently. The only real way high clay soil can be "fixed" is by substantially changing the composition formula. This means eliminating some of the clay. This is why the Garden Calculator will sometimes recommend adding sand into the planting formula just as it recommends removing a portion of the native soil which is so high in clay. What is to be done with that portion of clayey soil which is not used will really depend on the homeowner's situation. Filling holes where nothing is growing - gradually putting it into the green waste container, or then some other use is - ok. Maintaining a high percent of clay in the soil surrounding the plant is not ok.

Clay is the foundation for adobe, and this can be used for making a number of useful things around the home. It does a great job of sheltering pets from summer heat. There are lots of ideas for using clay, but not for trying to grow plants. Amending soil is a very important practice for green thumb gardening.

When measuring the amount of native soil and various amendments the most common measuring tool is the shovel being used to dig and move. Remember that the amount of sand that stays on a shovel will be less than the amount of clay or peat moss as the more compact a soil or amendment is, the higher it will stack on the shovel before sliding off. So, a good practice in general is to count a shovel of compact native soil or peat moss as 1.5 to 2 shovels; as compared to sand and most planting mixes. Multiply cubic feet by 8 in order to get shovels of loose soil, and count heavy soil and amendments as 5 shovels per cubic foot. The Garden Calculator Pro-I provides mixing ratios in cubic feet (not shovels). Some shovels are bigger than others, so keep that in mind when using a shovel to measure.

As mentioned, amending high clay soil will often include the recommendation for using some sand. When you looked at the information on "The Jar Test", sand settles first and will move downward in soil, while clay settles last and moves to the surface. Though this visible separation of soil particle sizes is not always this apparent, seeing sand at the bottom and very fine particles at the top is always obvious. The best sand to use is a fine or play sand that will tend to stay mixed with the native soil for a longer time. Construction sand is often too gritty or contains too many large particles, but even then will help to open up a slow draining soil. Doing the job right is the best idea. Know just how much clay is in your soil.

The Garden Calculator Pro-I will actually specify the amount on Native soil, and each various amendment to add and mix for surrounding your new plant during the planting process. It calculates the permeability of this "NEW" soil mixture in order to provide the correct number of emitters and the irrigation run time. It is IMPORTANT to amend according to these specs! Putting in way too little or too much amendment changes the new soil mix composition to the extent that irrigation moisture will not reach the bottom of the planting hole, or in the case of sandy native soil; it may cause premature wilting of your new plant between waterings.