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  • Writer's pictureClay Nelson

Soil Health for Horse Pastures: Part 2 - Phosphorous

Updated: Mar 9, 2020

This is the second in a series of blog posts aimed at helping you better understand soil testing data for improved horse pasture management, healthier horses, and a better environment. In this series, we will explore traditional soil testing data like N, P, and K levels and soil pH, along with less well understood data including micro-nutrient levels and biological measures of soil health.

In Part 1, we discussed nitrogen (N), considered perhaps the most challenging soil nutrient to manage sustainably. In part 2, we will discuss the next macro-nutrient on the list, phosphorous.

Phosphorous (P)

While P is a critical soil nutrient throughout the life-cycle of grasses, it is particularly important in the early stages of their establishment and growth. This is due to the role of P in helping grasses establish a healthy root system, essential for grasses to be able to withstand the pressures of grazing horses and temperamental weather. The importance of P, however, goes beyond root establishment. It is also a key building block of proteins and nucleic acids in grasses, and improves overall disease resistance.

Unlike nitrogen and potassium, which are highly mobile in soils, P is quite immobile. This has several important consequences. For one, grasses (and plants in general) are slow in taking up any P applied to soil as fertilizer. For example, in some parts of the country it is estimated that 2/3rds of P applied as fertilizer is still in the soil as opposed to being taken up by plants. In the context of water pollution, this also means that any efforts to reduce P from soil runoff into our creeks and rivers may take years or even decades before we see any positive changes. This is just one example of how our actions today (in terms of responsible pasture management) have environmental consequences well into the future.

Where soil test results indicate P applications are needed, P can be applied to the pasture by means of organic soil amendments like manure or compost, or by means of synthetic fertilizers. In general, use of organic soil amendments is preferred, as these amendments also add organic matter back to your soil. Besides, horses are great at producing all the organic soil amendment you need (in the form of 50 lbs of manure and urine produced daily per horse).

It’s important to know the nutrient content of the manure or compost so that the correct amount can be applied. In general, the nutrient composition of horse manure, per ton, is 10 to 28 lbs of N, 6 to 14 lbs P, and 12 to 24 lbs of K. However, the exact ratio of nutrients is highly variable from one farm to the next depending on a number of factors including diet, age of the horse(s), and the type of bedding (if any) mixed in with the manure. This is why it is important to have your manure or compost tested (we can help). Also, you can’t easily change the nutrient ratio of the manure or compost you apply to your soils. This means that depending on the needs of your soils, you may reach the optimal level of one nutrient before another. Rather than over-applying manure or compost to reach optimal levels for all soil nutrients, it may be better to supplement organic soil amendments with a small amount of targeted synthetic fertilizer that can fill in any gaps. My favorite analogy is…think of organic soil amendments like the food we eat, and synthetic fertilizers like taking a vitamin supplement to address a specific deficiency.

In the case of synthetic fertilizers, all P in chemical fertilizer is manufactured from deposits of P-bearing rocks, commonly referred to as rock phosphate (for an interesting read on concerns over long-term availability of rock phosphate, check out this article). Rock phosphate is highly immobile (insoluble) in soil and not available to grasses, so manufacturers will first treat rock phosphate with acid (and sometimes heat) to produce a variety of soluble P-containing compounds. Often, other compounds, such as urea, ammonia (NH3), or potash (K) are added to add nitrogen or potassium to the mix in various ratios.

It is important to understand that while such acid-treatment of rock phosphate enables soil manufacturers to provide P in water-soluble forms, this does not guarantee that the P in the fertilizer will necessarily be available to your grasses as intended. Soil chemistry is complex, and soluble P in fertilizer is subject to fixation in soil as aluminum or iron phosphate in low pH soils, and tri-calcium phosphate in high pH soils. In other words, it’s possible that the P you apply as fertilizer never makes it to grasses unless you effectively manage overall soil health. Recall the fact presented earlier that scientists estimate that 2/3 of soil P applied as fertilizer has yet to be taken up by plants. Not exactly efficient use of your money or time.

The take home message (for now) is that soil health can’t effectively be managed by focusing on just one piece of the puzzle. You have to see the full picture. This theme will become even more evident as we proceed through future blog posts. At some point soon, we will get to the story of soil microbes and how they have co-evolved with plants to provide plants and grasses with the nutrients they need, when they need them. By focusing our pasture management efforts on keeping these soil microbes happy, we enable them to do the work for us. This is, after all, how nature worked before we came along and started applying chemical fertilizers in large quantities, but that’s a story for another day.

Other posts in this series

At Sustainable Stables, we believe that the key to healthy pastures starts with healthy soils. Our soil testing services go beyond simple macro-nutrient (NPK) and pH analysis to include micro-nutrient levels along with biological measures of soil health. Such soil data is key to developing a comprehensive and sustainable management plan to create healthy, resilient pastures. Our testing services can be done independently, or as part of our equestrian property planning and design services. Contact us today to learn more.

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