Sunny Slope Orchard

Sunny Slope Orchard
In the coast range foothills overlooking the Sacramento Valley

Tuesday, September 18, 2012

Checking Soil's Vital Signs

We are finally getting some cooler mornings, the sun's path is moving quickly south and we're occasionally treated to beautiful cloudy sunrises, all signs that Fall is approaching and it’s time to prepare the orchard soil for the wet season. Our annual load of compost is ordered and I will soon spread it in the orchard along with any needed amendments, then sow the cover crop seed. Just like our body chemistry, soil chemistry needs to be balanced in order to maximize plant vigor and disease resistance. So each September I pull soil samples and send them off to an agricultural testing lab for analysis. The results will give a snapshot of chemical balance in the soil and point out amendments that might be beneficial to the cover crop and fruit trees.

Of course a perfectly balanced, rich soil alone does not guarantee good plant health since there can be many other problems like plant disease, poor drainage, harmful nematodes, fungi, root eating rodents, or aggressive competing roots from nearby non-crop trees. I occasionally sample soil for others, and in fact in some cases have seen the most obvious reasons for their poor plant health to be invasive roots from nearby landscape trees as well as severe gopher damage to roots. Those problems need to be dealt with, but a balanced soil will give plants the best chance of coping with most other stresses as well as maximizing nutrient content and flavor of the food harvested from that soil.

Soil testing probe
Taking soil samples
For meaningful results soil samples must be correctly taken, and to spot trends over time they should be taken at the same time each year and sent to the same lab. I use A&L Western Agricultural Labs in Modesto which provides excellent service. Of course once you have the results you need to figure out what they mean, and soil balancing is as much art as science. A&L will provide broad recommendations if requested. Another option is to order their test through Peaceful Valley Farm Supply and order the soil test package that comes with the booklet, "Understanding Your Soil Analysis Report." I send my samples directly to A&L and interpret the results myself using several books and publications along with observations from over the years.

The soil test report contains measurements of available major nutrients, the proportions of certain positive ions, pH, soil organic matter content, the presence of salts, and for the comprehensive test we order, trace mineral amounts. Before amending soil to correct imbalances there are several important points to remember:

-If a soil is too acid or alkaline (pH too low or high), some nutrients become unavailable to plants. Generally calcium in the form of lime is used to raise pH in acid soils and sulfur is added to lower pH in soils that are too alkaline. However there can be other reasons for a pH problem and just adding calcium or sulfur to soils already high in those elements may cause other problems.

- Higher nutrient levels are not necessarily better. If the level of one element is too high it can "tie up" other elements making them unavailable to plants. So the aim of fertilizing should generally be to balance nutrients rather than assuming that "more is better."

- Trace elements are critical to plant health and balanced soil chemistry. Just as our bodies need very tiny amounts of vitamins along with macro amounts of carbohydrates, proteins etc., elements like boron, copper, manganese, and zinc need to be present in single digit parts per million amounts for balanced and healthy soil life.

- Building a high organic matter content should be a top priority. Organic matter (decaying plant and animal residue) is the engine of soil health. It improves soil structure, aeration, drainage and water retention, it increases soil biological activity and thereby provides for the slow steady release of nutrients to plants, and it tends to buffer nutrient levels so they naturally come into balance. But as with other amendments the type and amount of organic matter is important. Too much raw high carbon material like wood chips or straw mixed into the soil use up nitrogen as they break down, causing a shortage in the crop. So well decomposed compost is best.

- It can take years to increase organic matter content, and tilling is the enemy of organic matter, causing it to quickly oxidize and disappear. Three steps to building soil organic matter are:
  1. Park the tiller.
  2. Never leave a bare soil surface, instead always keep either a crop or cover crop growing, or cover the ground with compost or mulch when between crops.
  3. Before planting the garden mow the cover crop closely, then run a stirrup hoe down the row to slice off roots just below the surface without inverting the soil. Then plant and apply more mulch or compost.
Aggregation or tilth are terms used to describe the physical condition of soils that are porous, high in organic matter, and crumbly, all important qualities of a healthy soil. Our untilled soil has by now developed the texture of an oatmeal cookie, bound together loosely by root hairs and the excretions of biological life but easily crumbling in the hand.