Budgeting is fundamental to BMP for nutrients. First, the budget must take into account the amount of nutrients a grower expects the crop to take up and, subsequently, leave the system in the crop biomass. This amount will vary among crop species as well as among levels of productivity within the same species. For example, a corn crop that yields 100 bushels/acre (5600 lbs) will export (meaning that nutrients leave the field in the harvested portion of the plant) approximately 80 lb/acre of N in the grain and 60 lbs/acre of N in the stover (which is above-ground biomass that is not grain and includes stalks/stems and leaves). If the corn crop were to yield 80 bushels/acre, those numbers would be reduced by 20%. Compare that with an iceberg lettuce crop that yields 40,000 lbs/acre. This will export approximately 80lbs/acre of N from the field, all in the above-ground biomass (since the whole above-ground portion of the plant is harvested). How does one figure such numbers out? There is information available for prominent crops via extension services and other online tool. However, it is also possible to estimate these numbers by multiplying the concentration of a nutrient by the quantity of biomass that contains that concentration. (For example: Corn grain contains about 1.4% N at harvest. Therefore, for a 3 ton/acre crop, the amount of N leaving the field in the grain is 6000lbs x 0.014 = 84lbs/acre N.
In order to anticipate the amount of nutrients likely to be exported from the field in the crop biomass, a grower must consider in advance what a reasonable yield goal is for the crop s/he is growing. If the grower has had previous experience with the crop at the same location, this is often a good guide. Also, trying to get a general idea of typical yields for the crop and region in question can be an important step. This information might be gained by consulting with other growers, with a professional crop consultant, and/or a university extension agent, such as the UCANR Statewide Integrated Pest Management Program, the UC Vegetable Research and Information Center, the UC Manure Management Crop N Uptake Calculator and the UCANR Soil Fertility Management Guide for Fresh Market Tomato and Pepper Production. It is important that the yield goal not be a “yield wish”. Fertilizing for a crop yield that is not attainable in a given context (due to inherent biophysical and/or management constraints) is a very easy way to over-budget the fertility needed and create an opportunity for nutrient pollution in connected water bodies.
No crop will use fertilizers with 100% efficiency. In fact, 60-70% efficiency is generally as good as can be accomplished, and many of the most common crops grown in California are estimated to have much lower average efficiencies. The reasons are that 1) plants are often in competition for nutrients with the micro-biota in the soil and 2) nutrient losses via the movement of water and gas are an inherent part of a dynamic, productive biological environment. However, applied fertility that goes unused by a given crop can still be incorporated into the plant-soil system by using cover crops, rotating with crops that have distinct root systems and nutrient uptake patterns, and by other management practices that are soil building. A fertile soil with a high nutrient supplying capacity can compensate for a fertilizer deficiency in the short to medium-term. Conversely, a less fertile soil may require more applied nutrients than the above ground portion of the crop will use in order to account for the fertilizer use inefficiency and the low nutrient supplying capacity of the soil. For this reason, soil fertility testing is an important part of determining the right amount of nutrients to add. However, interpretation and application of soil tests varies greatly from crop to crop and across environments.
SOURCE: California Agricultural Water Stewardship Initiative