There comes a time when we have to let this year go and look forward to the prospects that a new year will bring. We have come off several years of lower rainfall, which had left little spare stored soil moisture; the next crop had to rely heavily on winter and spring precipitation. In recent years, target crop yields were based on the hopes of getting that winter and spring moisture. This year, the overabundance of rain in many areas has left the soil profile full to overfull. All this water provides the opportunity to significantly increase 2011 target yields by taking full advantage of this stored soil moisture. How can we best do this?
We will need to know how much stored soil moisture and crop available nitrogen (to 24 inches) is actually present. There may not be as much water there as we think. A simple moisture probe and an understanding of how soil holds water will give a good estimate of stored soil water.
The moisture probe will only go into soil if the soil is moist and stops when it hits dry soil (or rocks, hardpans etc.). To determine the depth of soil moisture, take random probes across a field and record the penetration depth at each sampling to determine an average depth of moisture. If you know the soil texture, you can now determine roughly how much moisture there is for the upcoming crop (Table 1). If you don’t know your soil texture, a good soil test that measures cation exchange capacity (CEC) and organic matter (OM) will, in most cases, allow you to estimate the soil texture for the various depths sampled.
HOW MUCH WATER IS DOWN THERE?
A sandy soil with low OM will have a low CEC, around 10, while a clay loam with high OM will have a high CEC (30+). Generally, the higher the CEC, the higher the water (and nutrient) holding capacity. From Table 1, we can see that a sandy soil holds about one inch per foot, far less moisture than a clay loam soil which can hold around 2.3 inches per foot.
As an example, if the average depth of moisture is 30 inches and the surface soil (zero to six inches) is a loam, while the lower depths ( six to 12 and 12 to 30 inches) are clay loam and clay. Then, in the top six inches there is about one inch of stored water, for the six-to 12-inch depth 1.1 inches and for the 12-to 30-inch depth about 3.9 inches for a total of six inches of stored soil water. We know from research that the first three to four inches are necessary to build the crop factory and after that, each inch of water goes towards yield. In this case, we have enough water to build the factory and at least two additional inches of water to start building yield.
SETTING YIELD TARGETS
Keeping an eye on the snow and early-spring rains will give a better approximation come spring seeding time. For example, 20 inches of snow will add at least two inches while early-spring rains will generally contribute inch for inch. For this example, let’s say we had 20 inches of snow and two inches of early-spring rain. This brings our water storage total to 10 inches of which at least six inches are available for crop yield. If we are growing HRS wheat, we know we have the potential to grow a minimum of 24 bushels per acre (bu./ac.) without any more rainfall. If our average rainfall during the growing season is 6.7 inches, we can now figure on 12.7 inches of water that will go to yield. With this assumption, we now have a yield potential in excess of 51 bu./ac. If we’re afraid that we won’t get the average rainfall and only expect four inches then we can program our N fertilizer needs for a smaller crop (41 bu./ac.) and decide to pull the N trigger at a later date (in crop) if early-season rains are more plentiful than expected.
Couple this knowledge with the soil test information gathered from this fall’s soil sampling and a fertility plan comes together that will take full advantage of the
*silt content can affect CEC calculations
Modified from New Mexico State University Climate Centre
unprecedented level of moisture in the soil. Remember that soil N is mobile, so there is a good chance there will be plenty of N at depth in many fields this spring. Therefore, we should be sampling to 24 inches this fall and spring for accurate information.
Table 1. Available Water Capacity by Soil Texture
Sandy clay loam
Available Water (Inches/Foot of Depth)