# Bill’s Forage Files: Fertilizing Hay Crops, Part 1: Soil & Tissue Testing, Planning A



## ZB (Apr 1, 2008)

Now that most of the harvest is in for the year it's time to start planning next year's fertilizer program. There are a lot of products and claims on the market, but one thing everyone can agree on is that hay crops, whether they are alfalfa, cool season grasses, warm season grasses, other legumes, or alternative crops, require the farmer to apply additional nutrients. *The foundation of nutrient management for any crop starts by providing a sufficient level of nutrients throughout the growing season to the crops. Figure 1 below illustrates this concept. Farmers typically start a fertility program by managing N, P, K, and pH levels. *Most extension publications and research has been focused here, but other nutrients can be beneficial and are required in some situations.

*Figure 1: Concentration of Nutrients in Plant Tissues*

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_Step 1: Assess Current Levels of Fertility by Soil and Tissue Testing._

Soil and tissue testing are essential tools to measure the levels of nutrients in the ground and what is actually present in the plant. Soil tests are most often taken once in a rotation or the fall prior to a new crop (every 2-4 years) and form the backbone of soil fertility program. Tissue tests can be taken throughout the growing season, but usually aren't taken until after the seeding year. They are especially useful for detecting nutrients in the Critical Range in Figure 1 above. When extreme differences are present throughout the hayfield, a combination soil and tissue tests should be taken from the good and bad areas of the field in order to solve the problem. Forage farmers can do this because their crops' growth cycles are restarting after each cutting. Row crop farmers' crops are often past the point of no return by the time a deficiency is detected by tissue testing. When growing an annual forage that is usually a grain crop and only harvested once, such as corn or soybean silage, soil tests prior to planting will be the better strategy to manage nutrients.

Soil Sampling Technique

A constant sampling method representing the major different areas of the field is key to a successful soil sample. Outlier areas that do not represent large areas of the field, such as field edges, low wet spots, wheel tracks, and sites where manure or lime was stockpiled , should be avoided when sampling. Soil sampling should also not be taken right after an application of fertilizer, manure, or lime. Most soil samples are taken in the fall after harvest, but spring or summer sampling can be used occasionally if needed.

A farmer should identify different management zones of 10-15 acres or less within the field prior to soil sampling. A management zone has the same slope, soil type, and similar past management. *Many row crop farmers have gone beyond simple management zones and take grid samples in areas typically around 2.5 acres in order to apply variable rates of fertilizers, seeds, herbicides, etc.* However in most hay fields, management zone sampling is able to provide enough information for the hay grower to improve their fertilizer management without spending a lot of money on soil samples.

Within each management area, 10-20 soil cores are required in order to accurately determine the soil contents. More accurate measurements are seen when 20 cores are sampled. Take the samples in a zig-zag pattern across the management zone to a depth of 6-8 inches, the plow layer depth. In pastures and no-till fields 2-3 inch samples should also be taken for surface pH. Combine the samples into a small pail, mix well, and then fill the soil sample bag ½ to ¾ full. The bag should be labeled with the field, number of the management zone, and the farmer's name. Sending the samples off to lab soon after sampling will be more accurate than leaving the bags on the dash of the truck or on the shop bench.

_Authors Note: The subject of measuring soil conductivity and it's implications for forage fertility will be discussed in a later article. _

Tissue Sampling Technique

The growth stage of the plant is important when tissue testing. For example in an alfalfa field at bud to first flower, only the top 6 inches should be sampled. Alfalfa that's lying in a windrow or swath should have the whole plant sampled for analysis. Grasses should be sampled prior to heading -newest fully developed leaf. Similar to the soil test procedures, tissue tests should be taken from different management zones, 20-30 plants or parts of plants are the standard recommendation.

_Step 2: Select Timing & Method of Application_

Before a farmer selects the _method_ of nutrient application they need to determine the _timing _of application. Three time frames are available for nutrient application onto forage crops. First a farmer can apply the nutrients prior to planting. They can incorporate them into the field with tillage or apply by knifing, injection, etc. under reduced tillage management. For example the required amount of lime is often applied and tilled into the soil at least 1 year prior to alfalfa planting in a conventional tillage system, while a no-till farmer would apply a smaller amount of lime every couple of years throughout the rotation.

The second time a farmer would consider applying nutrients is during the growing season. Potentially providing the nutrients closer to when the plant is demanding them is the biggest advantage of in-season application. Top-dressing and foliar applications are the most widely used in-season application methods. Top-dress applications of granular N, P, K, and other nutrients that are required in large quantities is a sound practice when fertilizing hay fields. Foliar applications require large amounts of water to be transported and is best suited to apply small quantities of micronutrients, humates, and other materials to the hay crop.

Finally a farmer can apply nutrients after the growing season is completed. Most often this is an additionally top dressing of lime, or nutrients for the next growing season. It is often the most convenient time to apply fertilizers, but a significant amount of fertilizer can be lost in certain situations with certain fertilizers and this practice needs to be managed to limit nutrient loss over winter.

_Step 3: Select a Source of the Required Nutrient._

By and large farmers will buy the least expensive source of fertilizer for each nutrient, unless there is a measurable benefit from spending more money on a more expensive product. However it's the _chemistry_ of each specific nutrient, not just the _price _that influences how profitable an application is to a hay crop. This will vary depending on the timing of application discussed above. Evaluating each specific nutrient based on their availability to a hay crop, how they interact with methods and timing of application, optimal levels in soil and tissue tests, their effects on forage quality and disease and insect management, costs, and other topics will be examined in detail in the rest of this series.

If you have a specific interest or question regarding fertilizing hay crops post a comment or send an email to [email protected] and I will try to address your questions in the future articles.

Next up *Bill's Forage Files: Fertilizing Hay Crops, Part 2: The Chemistry of Soil Fertilizers*

Resources for Soil and Tissue Sampling:

Iowa State, Missouri, Penn State, Texas, Kentucky, Pacific Northwest, Wisconsin


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