Way out West,  on the edge of land and sea, where the North American plate collides with the Pacific plate, the long dry season is upon us. Most of the Western region received well below “normal” levels of rainfall this year. In many of the West’s agricultural areas, we were down by as much as 50% below average. Regardless of whether you receive your water from surface sources such as rivers and streams, from one of our many reservoirs, most of which are supplied by snowmelt—that frozen reservoir high in the mountains—or from your own well water drawn from hidden aquifers, dry years are always an important time to reflect upon all of your growing practices, your choice of crops and how water is distributed, used and reused on your farm.

Starting with the soil, have you done all that you can to increase your soil’s water-holding capacity? This is most readily accomplished through increasing the organic matter content of your soil. While this is a slow and multi-year process, raising soil organic matter is fairly easily done through the use of compost and by incorporating crop residues directly into your soil. As you build organic matter, you simultaneously increase water-holding capacity, soil aggregate stability, the tilth or workability of your soil, and your soil’s resistance to erosion, compaction and other forms of degradation. While incorporating organic matter into your soil brings many benefits, it is also important to limit the frequency and extent of soil cultivation, because excess cultivation volatilizes organic matter, degrades aggregates, and can contribute to both compaction and crusting. Though the subject of another conversation, both deep and shallow soil cultivation need to be undertaken within an ideal soil moisture window, which for most soils is between 50-75% of field capacity. Soil cultivation out of this window is another prime contributor to wind borne erosion, aggregate degradation, crusting, compaction and plow pans.

Choice of crops is often driven by our markets and by what is adapted to our local climate or the climate possible in high tunnels and greenhouses, but if your water is in short supply, it also makes sense to consider if there any particularly thirsty, but nonessential crops in your repertoire. Perhaps more important than eliminating crops that work for you, you might ask yourself, am I delivering the right amount of water for each of my crops across their life stages? Am I providing too little water? Usually not a problem, because plants readily let us know if we are undersupplying water. Am I delivering too much water, to depths beyond the root zone of my crops at their current life stage? Unfortunately, the literature on cut flowers rarely provides enough detail about typical root depth of our crops, and certainly not across their lives from seedling to harvest. Yet this is a critical question because too much water can contribute to soil-borne pathogens such as damping off fungi in germinating crops, to the leaching and loss of precious nutrients and to inefficient use of precious water resources.

Here, I believe it is imperative to do two separate things: better understand your plants and better understand the movement of water in your soil. First, learn more about your crops and their root nature by digging up several plants across their life cycle to gain firsthand knowledge about the extent and depth of their roots. If done carefully, your plants will suffer only minor setbacks, but sacrifices will be minimal, especially in comparison to the new knowledge you gain. At the same time, it is important to understand how the water you deliver moves through your soil. This can be done through the use of a simple soil probe or soil auger, which you can insert into the soil at progressively greater depths to see how far a given irrigation set moves into your soil. For example, through sampling and extensive observation, we know that one inch of water moves approximately 8” deep when applied at 50% of field capacity. If I have shallowly rooted seedlings, one inch of water is going to be far more than required, but if I am irrigating mature stock, whose roots are 12-15” deep, then more water and/or delivery before 50% of field capacity is necessary for the health of my crop.

Assessing soil moisture can be done both by qualitative or “hand-feel” methods, and by quantitative methods, using tools such as gypsum blocks, tensiometers, time domain refractometers, and other sophisticated soil moisture measuring devices. While highly reliable, these devices can be expensive and can limit the number of possible sampling sites due to the infrastructure required. Shovels, post hole diggers, and soil probes, on the other hand, are inexpensive, highly portable and provide rapid assessment of current conditions. However, the qualitative method requires practice and consistency to achieve accurate results, and because of its subjective nature, there is some potential for inaccuracy. Along with lots of practice, one useful resource for making qualitative assessments is the USDA publication “Estimating Soil Moisture by Feel and Appearance”, available through your local NRCS office or online at: http://www.msue.msu.edu/objects/content_revision/download.cfm/item_id.483981/workspace_id.-30/FeelSoil.pdf/

This is all standard practice in agriculture, but building knowledge of soil-water interactions, the precise needs of our crops and the influence of current weather patterns, taken together, can contribute to much more effective use of resources, saving water and conserving nutrients. Mulching, repairing leaks, the use of drip irrigation, and avoiding overhead water in the heat of the day or when it is windy, are all useful and easy steps to conserve water. Really knowing your soil and your crops water needs is a more involved process, but one that will contribute to greater crop health, farm viability and sustainable use of the resources we must have to grow what we love. 

Christof Bernau

UCSC Center for Agroecology

Christof Bernau UCSC Center for Agroecology Contact at [email protected]