Soil Management

Soil Fertility

Soil fertility is a crucial factor in the success of sorghum yield. When managing soil nutrients for sorghum it is important to consider the soil type, organic content matter within the soil as well as soil pH. Factoring in the soil's crop history, what was planted the previous year, is crucial, especially if the previous crop was a legume like alfalfa or soybeans. Knowing the manure application history can also help you understand your soil's fertility.

Soil nutrient testing is critical in determining the amount of nutrients to add by fertilizing. Soil testing starts with proper soil sampling. If precision application of fertilizer is an option, consider grid sampling. Once soil test results are available, it is important to understand how to interpret the results to best help in maintaining good soil health.

Good Soil Fertility Begins with the Proper Nutrients

Nitrogen is most often the nutrient needed to be added to the soil for the best results. State universities and soil labs differ in the amount of nitrogen recommended to be added for a given yield. A good rule of thumb is 1.1 lb of nitrogen for every bushel of grain sorghum. The highest nitrogen use efficiency is obtained with split applications. To accomplish this, apply a portion of nitrogen preplant, then as a sidedress application approximately 25 days after crop emergence. If chemigation is possible, apply a portion of nitrogen just prior to heading.

Sorghum also needs potassium in high quantities. The amount of potassium to be added as fertilizer will vary greatly depending on the soil type and environment. Many soils are naturally high in potassium and require very little potassium as fertilizer, which is why soil testing is crucial. As a rule of thumb, the amount of potassium taken up by the sorghum plant is very similar to the amount of nitrogen utilized by the plant.

Phosphorus is the third major nutrient required by sorghum. Depending on soil type and the environment, the amount of phosphorus needed to be added as fertilizer will vary considerably. In order to not limit sorghum yield, approximately 80 lbs per acre of phosphate should be available within the soil. If large amounts of phosphorus are needed, consider banding to reduce the amount needed to be added as fertilizer. In high yielding environments or when planting on cool soils, consider applying phosphorus in a starter fertilizer.

Other nutrients that may need to be applied include sulfur, iron and zinc. Lime may be required if the pH of the soil drops below 5.6. Low pH greatly limits the availability of certain nutrients.

Water Use and Irrigation

Water sources covering the Sorghum Belt, like the Ogallala Aquifer, are being depleted at a much faster rate than they are being replenished. According to a research study by Kansas State University and the Kansas Geological Survey, if no action is taken within the next 50 years, the Ogallala will be 70 percent depleted and 40 percent of the area irrigated by the aquifer will not support a 400 gallon per minute well. In many cases, water availability and well yield are declining, resulting in reduced irrigated capacity. As water supplies decline, risk increases for growers. Many farmers are looking for an effective alternative to help sustain their water supplies and economy. Sorghum's ability to produce when other crops wither make it a smart choice in areas with unpredictable water availability.

Sorghum and Water

Sorghum has a long planting season and a diverse hybrid portfolio, offering farmers a broad window to take advantage of moisture patterns. While other crops require close to 12 inches of water to produce the first bushel, it only takes six inches of total water (soil, rainfall, irrigation) to produce the first bushel of grain sorghum. In low water settings, sorghum can produce more grain per inch than comparable grain crops.

The total amount of water needed to grow grain sorghum without limiting yield is often referred to as its evapotranspiration (ET) demand.  ET is the sum of water lost through evaporation from the soil and the water that moves through the plant and evaporates from the leaf surface, mostly through the stomata.  Sorghum's ET demand is dependent on the environment for which it is being grown. ET is largely determined by radiation, humidity and wind speed. As environmental conditions change, so does the amount of water required to grow grain sorghum. Tropic and subtropic regions will require less water for sorghum production than temperate regions, which require less water than arid and semi-arid regions.

Water use efficiency (WUE) is the amount of yield produced per inch of water used. In areas where water is in short supply, it is important to implement cultural practices that maximize WUE.  In research conducted by USDA scientists near Amarillo, Texas, WUE of grain sorghum under well-managed conditions yielded 329 pounds of grain per acre-inch of water used.  In this environment, 20 inches of total water used by the plant would be expected to produce a yield of 6,580 pounds or 118 bushels.

In the semi-arid region of the Southern Great Plains growers have changed production practices in recent years to maximize the ability of the soil to store water.  No-till and limited till practices greatly reduce the amount of water lost from the soil as evaporation while decreasing the amount of water from rainfall events that is lost through runoff.  In addition, planting in more narrow rows shortens the time the sorghum leaves need to canopy over and shade the soil which also reduces evaporation.

Efficiencies in the use of irrigation water takes into account losses during storage, transport and application. In the southern Great Plains, growers over the years have moved from inefficient furrow irrigation to low and mid elevation sprinkler heads to subsoil drip irrigation systems.